TECHNICAL FIELD

[0001] The present invention relates primarily to an antenna with a low physical profile and a particular radiation pattern.

[0002] In one particular (albeit non-limiting) example application, the antenna can be placed in or on the surface of a road, a driveway, or the like, and can be used to perform radio- frequency identification (RFID) with RFID capable tags (RFID tags) which are located on the front and/or the back of passing vehicles. In this application, the antenna would be a part of a RFID reader which is operable to communicate with RFID tags. Preferably, the RFID tags will be located on the vehicles' license plates. (Or more specifically, for vehicles which have a license plate on the front and the rear, a RFID tag will preferably be placed on one or both of a said vehicle's license plates, or for vehicles which have only one license plate, a RFID tag will preferably be placed on the single license plate). Vehicle licence plates (and hence the RFID tags) may be mounted to the vehicle centrally, or at an off-centre location, relative to a vehicle's widthwise direction. Also, there will often be variation in the position/trajectory of travelling vehicles; for example within a given lane one vehicle may be travelling quite close to the left- hand side of the lane whereas another vehicle may be travelling closer to the right-hand side (or a vehicle may even intentionally travel between or across multiple lanes in an attempt to avoid detection). Consequently, although the tags on vehicle licence plates may sometimes pass directly over the RFID reader (and its antenna), often the tag on a vehicle may pass to one side or other of the RFID reader. This can have a significant impact on RFID read performance. However, the present invention may be well suited to accommodate such issues.

[0003] It is thought that embodiments of the antenna may be particularly suited for use with vertically polarised RFID tags. The use of the antenna for communicating with vertically polarised RFID tags may help to facilitate, for example, a RFID tag read environment which is directionally independent (at least in a plane parallel to the surface of the ground on/in which the antenna is located) and which may thus help to reduce the number of required antennas (the antenna count) and the associated system complexity in applications involving detection of RFID tags on vehicles.

[0004] However, it is to be clearly understood that the invention is not necessarily limited to use with vertically polarised RFID tags. In fact, no particular limitations are to be implied from any of the example applications or uses mentioned above or discussed below. Thus, the antenna could potentially be used in a wide range of other areas and/or applications as well. By way of example, rather than being used in "on-road" or "in-road" applications for detecting RFID tags which are placed on the front and/or back of vehicles (or on the vehicles' license plate(s)), the antenna could instead potentially find use in side and/or overhead placements to read/communicate with RFID tags on vehicles, or on goods or products which are moving past the antenna (e.g. goods or products being carried past the antenna by a machine, or on a conveyor, etc).

[0005] Nevertheless, for convenience, the antenna will hereafter be described with reference to, and in the context of, the above application where the antenna is used with vertically polarised RFID tags which are located on vehicle license plates.

[0006] It is to be clearly understood that mere reference herein to previous or existing devices, apparatus, products, systems, methods, practices, publications or any other information, or to any problems or issues, does not constitute an acknowledgement or admission that any of those things, individually or in any combination, formed part of the common general knowledge of those skilled in the field, or that they are admissible prior art.

SUMMARY OF THE INVENTION

[0007] In one broad form at least (and there may be other possible forms evident to those skilled in the art from the disclosures herein), the invention relates to an antenna for a communication device, wherein:

the physical construction of the antenna includes a number of parts, one of the parts is a substantially planar ground plane, and the size of the antenna in a direction perpendicular to the ground plane (i.e. perpendicular to the plane of the ground plane) is less than the largest dimension of the ground plane (e.g. less than the largest of the diameter or length or width, etc, of the ground plane, as applicable); and

the antenna's radiation pattern extends further in a direction parallel to (the plane of) the ground plane than it does in a direction perpendicular to (the plane of) the ground plane.

[0008] In the broad form of the invention just described, the dimension of the antenna (that is to say, its size) perpendicular to the ground plane is less (preferably much less) than the largest dimension (diameter, length, width, etc) of the ground plane. Often, when the antenna is in use, it will be positioned such that the ground plane is oriented horizontally (e.g. on or parallel to the actual ground, which is part of planet earth) and such that the other parts of the antenna extend vertically upward relative to the ground plane. Accordingly, for convenience (albeit without limitation to the scope of the invention or the orientations in which the antenna may be used), the dimension (i.e. size) of the antenna perpendicular to the ground plane may be thought of as the "height" of the antenna (even though this "height" dimension may not always be vertically up and down, depending on the antenna's orientation in a given installation). Thus, the height of the antenna should be less (preferably much less) than the largest dimension of the ground plane. Putting this another way, the antenna should have what may be termed a "height-restricted" configuration, in that the antenna's height should be less (preferably much less) than the diameter or length or width, etc, of the ground plane. In some embodiments, the height of the antenna (i.e. the antenna's dimension perpendicular to the ground plane) may be less than half the largest dimension of the ground plane, and preferably less than a quarter of the largest dimension of the ground plane. In fact, in some preferred embodiments, the height of the antenna may be approximately 10%-20% of the size of the largest dimension of the ground plane.

[0009] Also, in the broad form of the invention described above (as has been explained), the antenna's radiation pattern should extend further in a direction parallel to the ground plane than it does in a direction perpendicular to the ground plane. In other words, the antenna's radiation pattern should extend further in a direction parallel to the plane of the ground plane than it does in the antenna's "height" direction. The reason for this will be explained below.

[0010] It is envisaged that there may be embodiments of the invention in which the antenna's configuration (i.e. the nature of its physical construction) makes the antenna a type or form or species of dipole or monopole antenna. In some such embodiments in particular, specifically where the antenna might be described as a type of (or a variation of a) monopole antenna, a part of the antenna's physical construction (i.e. another part in addition to the ground plane) may be a monopole element. The monopole element may have two ends, namely a first end and a second end. Thus, the monopole element may extend between its first end and its second end. The first end of the monopole element may be positioned near, but not in connection with, the ground plane. The second end may be positioned further from the from the ground plane than the first end such that the monopole element is oriented perpendicular to the ground plane.

[0011] Furthermore, in some of these embodiments, yet another part of the antenna's physical construction may be a substantially planar top load element. This top load element may be connected to the monopole element on the second end of the monopole element, such that the plane of the top load element is substantially parallel to the plane of the ground plane. In these last-mentioned embodiments which include a top load element, the antenna might be referred to as a type or species of "top loaded monopole" antenna. It is thought that the inclusion of a top load element connected on the second end of the monopole element (i.e. the end of the monopole element that is further away from the ground plane) may help to restrict the amount of radiation emitted by the antenna in a direction perpendicular to the ground plane. In other words, the inclusion of such a top load element may help to ensure that the antenna's radiation pattern extends further in a direction parallel to the ground plane than it does perpendicular to the ground plane (i.e. in the antenna's "height" direction). Also, the inclusion of a top load element may help to maintain radiation efficiency without increasing the "height" of the antenna. This may, in turn, assist with allowing the antenna to have a "height-restricted" configuration.

[0012] In some embodiments where the monopole element is positioned perpendicular to the ground plane on one side of the ground plane (as described above), a dielectric material may also be provided on the opposite side of the ground plane from the monopole element. A layer of (typically unbroken) conductive material (which may perform a shielding function) may also be provided on the opposite side of the dielectric material from the ground plane. It should be noted that this optional dielectric material, and likewise the optional (shielding) layer of conductive material, may be used in a range of different embodiments. Therefore, for example, one or both of these may form part of an antenna in embodiments like the one described with reference to Figure 24 below, or like the ones described with reference to Figure 25 or Figure 27 below, etc.

[0013] In some particular embodiments where the monopole element is oriented perpendicular to the ground plane and on one side of the ground plane, the ground plane may be thin and circular. That is, the ground plane may be shaped, for example, like a thin disc, or a thin circular plate, or the like. The monopole element may have a short, wide (i.e. squat) cylindrical shape, and the location of the monopole element relative to the ground plane may be such that a cylindrical axis of the monopole element (or a hypothetical extension of such an axis) would extend through the centre of the circular ground plane. Furthermore, the top load element may also be thin and circular. In other words, the top load element may also be shaped, for example, like a thin disc, or a thin circular plate, similar to the ground plane, although the top load element may not be (and often will not be) the same size as the ground plane. Where the top load element is circular, as just described, the location of its connection to the monopole element may be such that the cylindrical axis of the monopole element would extend through the centre of the circular top load element.

[0014] Suitably, the antenna may be (configured so as to be) operable for use with a radio signal having a predetermined wavelength (λ). In embodiments such as those described in the previous paragraph (i.e. where the ground plane and top load element are circular, and the monopole element is cylindrical, etc), the diameter of the ground plane may be approximately 2/5 the signal wavelength (2Λ/5). Also, the distance between the ground plane and the top load element (in a direction perpendicular to the ground plane) may be approximately 1/16 the signal wavelength (A/16). Furthermore, the diameter of the monopole element (i.e. the dimension of the monopole element parallel to the ground plane) may be approximately 1/12 the signal wavelength (A/12). And yet further, the diameter of the top load element may be approximately 1 /3 the signal wavelength (A/3). [0015] It should be noted that, even though the relative size proportions of different parts of the antenna are described in the previous paragraph with reference to exact fractional sizes (exact fractions of signal wavelength (λ)), nevertheless these relative size proportions are approximate only. That is to say, an antenna is still considered to conform to these relative size proportions, even if the actual relative size of different parts of the antenna (or some of the parts) is not exactly in proportion with (or varies somewhat from) the relative ratios expressed above. In fact, it is envisaged that, in different embodiments of the invention, the actual relative sizes of different parts of the antenna will vary to some extent from the exact relative proportions given above. Reasons for this will be discussed below. Nevertheless, for the avoidance of doubt, an antenna is still considered to conform to the relative size proportions above even if the actual relative size of different parts of the antenna (or some of them) is not exactly in proportion with (or varies somewhat from) the relative ratios expressed as exact fractions of signal wavelength (λ) above. It should also be noted that some dimensions and/or other design attributes of the antenna may be independent of the signal wavelength (λ).

[0016] By way of illustrative example, it is anticipated that antennas in accordance with some particular embodiments of the invention may be operable with signals of wavelength (λ) around 350 mm or lower, or in other words with signals of frequency around 860 MHz or higher (although no limitation whatsoever is to be implied from this as to the possible ranges of frequencies/wavelengths with which the antenna of the invention might potentially be used) . Some antennas in accordance with embodiments of the invention may operate with signals around 1 GHz (approximately). In any case, for an antenna in accordance with an embodiment of the invention which is operable to operate with a signal wavelength of approximately 350 mm, and wherein parts of the antenna conform to the approximate relative size proportions discussed above, the antenna should consequently be sized to fit within a housing having an overall outer diameter of 180 mm or less and an overall height/thickness (i.e. perpendicular to the overall diameter) of 40 mm or less.

[0017] In embodiments having a top load element connected to the second end of the monopole element (i.e. connected to the end of the monopole element which is furthest from the ground plane), the antenna's physical construction may further include a plurality of pole elements. Each pole element may have one end connected to the ground plane and an opposite end connected to the top load element. Hence, each pole element may extend perpendicularly between the ground plane and the top load element. Each pole element may also have a thin elongate shape (e.g. an elongate cylindrical shape, or an elongate rectangular prism shape, or the like), and the location of each pole element may be outwards (radially outwards if the ground plane is circular) from the centre of the ground plane. In some preferred embodiments of the antenna which incorporate such pole elements, there may be four pole elements located an equal distance from, and at equally spaced locations around, the monopole element, and in some particularly preferred embodiments the spacing between the axial centres of the pole elements may be approximately 1/7 the signal wavelength (A/7) (although again this should be treated as approximate only - see above). Suitably, one or more (or possibly all) of the pole elements may be hollow. This may help to enable, for example, electronics or other equipment, which may be positioned on the opposite side of the top load element from the ground plane, to be connected with electronics or a power source, etc, located on the opposite side of the ground plane from the top load element. Hence, things like electrical connectors, cables, fibre-optic lines, etc, may extend through the hollow inside of one or more of the pole elements in order to connect such electronics, etc.

[0018] Referring again to embodiments of the invention which might be considered to be forms (or variants) of monopole antenna (i.e. embodiments which incorporate a monopole element, possibly amongst other things), in such embodiments the ground plane of the antenna may incorporate one or more slots which extend part way through, or all the way through, the thickness of the ground plane. The slots may also extend into, or all the way through, the dielectric layer adjacent the ground plane (if present). It is thought that these slots may function to help compensate/account for the ground effect, including the "near ground" effect". The "near ground effect" is the ground effect caused by the ground (which is part of planet Earth), or by the surface on which the antenna is mounted, in the immediate vicinity of the antenna (e.g. within about 6 m or about one typical vehicle length from the antenna). This "near ground effect" (i.e. the ground effect from the "near ground") in particular may be highly variable and even dynamically variable (i.e. subject to change with time and/or due to changes in conditions, etc). This is discussed further below.

[0019] While discussing the ability of the present antenna to help compensate/account for the ground effect, and especially the near ground effect, it is useful also to pause to emphasise certain other/related points which are important insofar as the present antenna and its operation in intended applications are concerned. A first point is that, when an antenna in accordance with the present invention is used in, for example, a RFID vehicle detection and/or identification application, the antenna is effectively being used in a way that may be considered generally similar or analogous to an antenna in a RADAR transmitter/sensor. Indeed, as explained below, RADAR essentially involves a radio signal that is first transmitted by a sensor; that radio signal is then reflected by the object to be observed, and the reflected signal is received and interpreted by the sensor (e.g. for the purpose of detecting the presence of the object, and/or its location and/or movement relative to the sensor, etc). In the case of RFID, a signal may be emitted by an RFID reader (which includes an antenna in accordance with the present invention), and a "reflected" signal may then be sent back from e.g. an RFID tag on a vehicle, back to the RFID reader. In RFID, both of these signals (i.e. both the signal emitted by the RFID reader and also the "reflected" signal sent back from the RFID tag to the RFID reader) can be modulated to carry information/data (this modulation of data onto the signals is at least part of what distinguishes RFID from traditional RADAR wherein the signals are unmodulated). In other words, in RFID, information can be modulated onto the signal emitted by the RFID reader such that information is sent from the reader to the tag, and similarly information can be modulated onto the signal sent (reflected) by the RFID tag such that information is sent back from the tag to the reader. Where there is this kind of two-way data exchange, and specifically in RFID vehicle identification applications, the exchange of information may be used to perform (and in fact this may be what makes it possible to perform) the identification (i.e. ID detection/recognition) of a specific vehicle. Still referring to RFID, alternative arrangements or situations may also be possible where the signal emitted by the RFID reader and the "reflected" signal sent back from the RFID tag to the RFID reader, or one of them, are unmodulated, such that there is therefore no two-way data exchange like that just described above. However, even in this alternative case where the signal emitted by the RFID reader and/or the "reflected" signal sent back from the RFID tag to the RFID reader is/are unmodulated, nevertheless the signal sent by the RFID tag, which is still received and interpreted by the reader, may still be used for vehicle detection, among other things. Indeed, when such a reflected signal, which is sent (reflected) back from an RFID tag, is received by the reader, this signal (even if it is an unmodulated signal) may immediately signify the presence of a RFID tag (and hence a vehicle) within the read range of the reader (although which specific vehicle it is - i.e. the specific vehicle identity/ID - may not in this case be determinable, at least not from the signal sent by he RFID tag alone). Furthermore, the way the said signal changes with time (i.e. the way the signal which is sent from the RFID tag and received by the reader changes with time, even if it is an unmodulated signal) may be used (interpreted by the reader) to ascertain information about the (non-identified) vehicle in addition to merely its presence. Indeed the location and movement of the vehicle - e.g. its distance or position relative to the reader, its speed (and possibly direction) of travel, etc - may possibly be determined. It will be appreciated that this last unmodulated- signal scenario is somewhat more akin to traditional RADAR.

[0020] Another point that should be emphasised is that, whilst antennas in accordance with the present invention, when used in e.g. RFID vehicle detection and/or identification applications, may be used in a similar or analogous way to traditional RADAR antennas (see above), nevertheless at the same time the region within which antennas in accordance with the present invention may need to operate, and the required transmission ranges, radiation pattern shapes, and even the physical position of the antenna (and hence the physical location in which, and from which, the antenna's signal is transmitted) may all be vastly different to antennas used in conventional RADAR. Indeed, for reasons explained in detail below, antennas in accordance with the present invention will often need to be located at ground level, typically on or in the surface of the ground (i.e. on or in the surface of planet Earth) - e.g. on or in the surface of a road. So, the antenna will generally need to be configured to be positioned at (and such that its signal radiation is emitted from) ground level on planet Earth. This is very different to conventional RADAR wherein traditional RADAR antennas are almost always located well above ground level, typically at least 2 wavelengths above the ground (i.e. the height from which a conventional RADAR antenna operates is generally at least twice the wavelength of the RADAR signal it transmits). Accordingly, traditional RADAR antennas are generally not required to accommodate for much (if any) change in signal transmission propagation conditions due to the "near ground effect". Rather, for them, the effect on signal transmission propagation caused by planet Earth may often be assumed negligible or at least constant, e.g. regardless of time and/or position variant changes in weather or ambient conditions or ground conditions. This is very different to the antenna in the present invention which must operate on/in the ground and where the effect on signal transmission propagation caused by the ground on/in which the antenna is located (especially the near ground) can change drastically both between different locations and also dynamically at the same location (i.e. signal transmission propagation conditions can change drastically with time even at a single location, e.g. with changes in surface conditions due to surface water vs dry, wet soil vs dry in the vicinity, the presence or absence of metal or other conductors in the road base, substances of different conductivity like paint or oil on the road, etc). Appropriate configuration and/or tuning of the slots discussed above and below may be what (or at least this may be part of what helps) enables the antenna to accommodate these effects.

[0021] Furthermore, traditional RADAR antennas generally have a very focussed/directional radiation pattern intended to transmit over large or very large transmission distances (typically in a broadcast manner). So, not only are conventional RADAR antennas normally positioned well above ground level, but they have narrow focussed/directional radiation patterns and transmit over large distances (i.e. they operate in what is often termed the far field - a.k.a. the Fraunhofer region). In contrast, antennas in accordance with embodiments of the present invention may (and typically will) need to transmit over and within a range that is very much closer to the antenna, possibly even within the antenna's radiating near field a.k.a. Fresnel region. Furthermore, antennas in accordance with embodiments of the present invention may (and typically will) need to provide a radiation pattern that is non-focussed, and which extends further in a direction parallel to the plane of the ground plane than it does in a direction perpendicular to the plane of the ground plane. The shape of the antenna's radiation pattern is discussed in detail elsewhere herein. By way of illustrative example only, in embodiments of the antenna that are configured to operate with signals of frequency around 1 GHz (and hence with signal wavelengths of about 300 mm), the antenna, which is part of an RFID reader located on/in the road surface, may be used to (so to speak) "radar" detect and identify one or more vehicles within a radius of about 5 or 6 m around the antenna, where the RFID tag(s) on the vehicle(s) is/are at or below a height of about 2 m. Note: this example is given to provide context only and should not be considered limiting in any way. [0022] In embodiments of the invention which incorporate a monopole element and one or more slots in the ground plane, as discussed above, the ground plane may in fact incorporate one or more sets of slots. Each said set may comprise a plurality of slots with each slot in a set being approximately parallel to other slots in that set. The slots in each set may also be spaced apart from one another in a direction extending away from the monopole element. Furthermore, in some embodiments, two or more sets of slots may be provided. The slots in each set may be substantially arc-shaped, and the arcuate length of the respective slots within a set may increase with increasing distance from the monopole element. A specific embodiment of an antenna incorporating sets of slots, as just described, is discussed further below. It is to be noted that there is no limitation in terms of the way the slots in the ground plane may be formed. For example, the slots may be formed by cutting or otherwise removing material from the ground plane. Alternatively, the slots might be formed by adding additional material to the ground plane in some places such that slots are formed in between thickened areas of the ground plane. In another possible alternative, the ground plane may be provided with a "corrugated" or "rippled" configuration (the corrugations or ripples might extend in one or more shapes or directions) and areas of the ground plane which are recessed (i.e. troughs, as distinct from raised/elevated areas/peaks) may serve as the slots. A range of other possibilities may also be evident to those skilled in the art. Everything that has just been said about the way in which the slots in the ground plane may be formed also applies equally to the optional formation of slots in a dielectric layer (if present) and/or in a shielding layer (if present).

[0023] As explained above, in antennas according to the present invention, the antenna's radiation pattern should extend further in a direction parallel to the plane of the ground plane than it does in a direction perpendicular to the plane of the ground plane. In some preferred embodiments, including those which incorporate a monopole element (possibly amongst other things), it will preferably be the case that the antenna's radiation pattern extends further in all directions parallel to the plane of the ground plane than it does in a direction perpendicular to the plane of the ground plane. In such embodiments, and where the antenna incorporates one or more slots in the ground plane, the arrangement of the one or more slots in the ground plane may be operable to (i.e. the slots may, because of their particular arrangement) help provide at least approximate symmetry of the antenna's radiation pattern about an axis or a plane that is perpendicular to the plane of the ground plane. The arrangement of the one or more slots in the ground plane may also be operable (i.e. the slots, because of their particular arrangement, may function) to help accommodate/compensate for the potentially dynamically variable ground effect, as discussed above.

[0024] In some particularly preferred embodiments, the antenna's radiation pattern may have a generally toroid-like shape. Even more preferably, the antenna's radiation pattern may have a "dropped doughnut" or "toroid on the ground" shape which, compared to the shape of a regular toroid, is lower/flatter in the toroid's axial direction and extends more broadly radially (i.e. similar to the shape of a doughnut that has been dropped flat onto the ground and thus flattened somewhat). One such radiation pattern shape which is considered to be especially desirable/advantageous is discussed further below.

[0025] Embodiments of the invention have been described above in which the antenna's configuration (i.e. the nature of its physical construction) makes the antenna a type or form or species of dipole or monopole antenna. However, it is also envisaged that there may well be embodiments of the invention that do not fall within this general category. By way of example, embodiments may be provided in which

the antenna (again) has a substantially planar ground plane, and the size ("height") of the antenna in a direction perpendicular to the ground plane is less than the largest dimension of the ground plane;

the antenna's radiation pattern (again) extends further in a direction parallel to the ground plane than it does in a direction perpendicular to the ground plane; and furthermore another part of the antenna's physical construction is an inverted F antenna element (I FA element) which is shaped like a sideways capital letter "F".

[0026] In such last-mentioned embodiments, a single long side of the F-shaped IFA element, which would be the vertical part of the "F" if the "F" were in a normal upright orientation, may be oriented parallel to the ground plane. Also, the F-shaped IFA element may have two "prongs" depending from the single long side, namely a second prong which would be the lower of the two if the "F" were in a normal upright orientation and a first prong which would be the upper of the two if the "F" were in a normal orientation. The second prong may extend vertically downwards from partway along the single long side to near (or through), but not in contact with, the centre of the antenna's ground plane. The first prong may extend vertically down from one end of the single long side and connect to the antenna's ground plane slightly to one side of the second prong.

[0027] In some particular embodiments in which the antenna incorporates an IFA element as discussed above, the ground plane may be thin and rectangular. In such cases, the ground plane may incorporate one or more slots which extend part way through, or all the way through, the thickness of the ground plane. The ground plane may actually incorporate one or more sets of slots, each said set comprising a plurality of slots with each slot in a set being approximately parallel to other slots in that set, and the slots in each set being spaced apart from one another in a direction extending away from the location of the connection between the ground plane and the second prong of the IFA element. In some specific embodiments, two or more sets of slots may be provided, the slots in each set may be substantially arc-shaped, and the arcuate length of the respective slots within a set may increase with increasing distance from the second prong of the IFA element. A specific embodiment of an antenna incorporating sets of slots, as just described, is discussed further below. (It should also be noted that the slots in the ground plane described in this paragraph could also extend into, or through, the thickness of a dielectric layer adjacent the ground plane (if present).)

[0028] In embodiments of the antenna incorporating an IFA element, the antenna's radiation pattern should preferably extend further in at least two opposed collinear directions (i.e. forward and backward along a single linear axis) parallel to (the plane of) the ground plane than it does in a direction perpendicular to (the plane of) the ground plane. Where this is the case, and where the antenna incorporates one or more slots in the ground plane, the arrangement of the one or more slots in the ground plane may be operable to cause the antenna's radiation pattern to extend in the said at least two opposed collinear directions. The arrangement of the one or more slots in the ground plane may also (again) be operable (i.e. the slots, because of their particular arrangement, may function) to accommodate/compensate for the potentially dynamically variable ground effect.

[0029] In embodiments of the antenna which incorporate an IFA element, another part of the antenna's physical construction may be a substantially planar top load element (or cap). The top load element (cap) in these embodiments may be located on top of the single long side of the F-shaped IFA element, such that the (plane of the) top load element is substantially parallel to the (plane of the) ground plane.

[0030] In some particular embodiments in which the antenna incorporates an IFA element and a top load element, as discussed above, the ground plane may be thin and circular. The top load element may also be thin and circular, and the top load element may be positioned with an edge of its circumference above the second prong of the IFA element and with its diameter aligned with the linear axis of the single long side of the IFA element. In these particular embodiments, the ground plane may (again) incorporate one or more slots which extend part way through, or all the way through, the thickness of the ground plane (and/or into or through the thickness of a dielectric layer (if present)). And the arrangement of the one or more slots in the ground plane may (again) be operable (i.e. the slots, because of their particular arrangement, may function) to accommodate/compensate for the potentially dynamically variable "near ground" effect. Also as above, one or more sets of slots may be provided in the ground plane, each said set comprising a plurality of slots with each slot in a set being approximately parallel to other slots in that set, and the slots in each set being spaced apart from one another in a direction extending away from the location of the connection between the ground plane and the second prong of the IFA element. Two or more sets of such slots may be provided. The slots in each set may be substantially arc-shaped, and the arcuate length of the respective slots within a set may increase with increasing distance from the second prong of the IFA element.

[0031] In antennas which conform to the present invention (i.e. in embodiments of the antenna such as those described above, or indeed in other in embodiments), the antenna may comprise a first antenna operable for radio communication, and a second antenna may also be provided for data communication in addition to the radio communication performed by the first antenna. One way in which this may be achieved, in embodiments of the invention which incorporate a top load element, is for the top load element be provided with an elongate slot therein. The elongate slot may function as the second antenna. (That is, the second antenna may be a slot antenna.)

[0032] As mentioned above, controlling electronic equipment associated with the antenna may be located on the opposite side of the ground plane (and also on the opposite side of the dielectric layer and/or conductive shielding layer) from the other resonant (radiating) parts/elements of the antenna. This may help to shield the electronics, and the resonant parts/elements of the antenna, from one another. Therefore, for example, in the embodiments of the invention discussed above which incorporate a monopole element, controlling electronic equipment associated with the antenna may be located on the opposite side of the ground plane (etc) from the monopole element. Similarly, in the embodiments of the invention discussed above which incorporate an IFA element, controlling electronic equipment associated with the antenna may be located on the opposite side of the ground plane (etc) from the IFA element.

[0033] Antennas which conform to the present invention (i.e. antenna embodiments such as those described above, or indeed other possible embodiments) may be operable for use as an antenna of a radio frequency identification (RFID) reader, and the RFID reader may be operable to communicate with RFID capable tags mounted on vehicles (e.g. cars, buses, trucks, motorcycles, etc). Preferably, the RFID tags may be mounted on vehicles' licence plates. The RFID reader may be configured to be positioned on, or partly in, or in, the surface of a road, driveway, carpark, or the like, on which the vehicles travel. Alternatively, the RFID reader may be configured to be positioned to the side of, or above, where the vehicles travel (e.g. at the roadside or on an overhead or over-road gantry, etc). The RFID tags may preferably be linearly polarised, and where this is the case the antenna (and the RFID reader with which the antenna is associated) may be correspondingly linearly polarised. Even more preferably, the RFID tags and the antenna may both be vertically polarised. It should be noted that all references herein to polarisation refer to polarisation of the electric field (E-filed) and not polarisation of the magnetic field (H-field).

[0034] Following on from the above, it will be evident to those skilled in the art that the invention, in another possible broad form, may reside in (or provide) a RFID reader including an antenna of the kind described above and wherein the RFID reader is operable to communicate with RFID capable tags mounted on vehicles. And again, as has been explained, the RFID tags may be mounted on vehicles' licence plates, and the RFID reader may be configured to be positioned on, or partly in, or in, the surface of a road, driveway, carpark, or the like, on which the vehicles travel.

[0035] In either of the forms of the invention describe so far (i.e. in the "antenna" form or the "RFID reader" form), the power of the antenna and the RFID tags, respectively, may be such that the beam shape associated with the antenna's radiation pattern extends approx. (or at least) 5 or 6 m in all directions around the antenna in a plane parallel to the antenna's ground plane. Where this is the case, the majority of the beam shape associated with the antenna's radiation pattern may also be contained within the region 0-2 m perpendicularly to one side of the antenna's ground plane (the same side as the monopole, or I FA element).

BRIEF DESCRIPTION OF THE FIGURES

[0036] Preferred features, embodiments and variations of the invention may be discerned from the following discussion entitled "Background, Motivations, and Description of Invention Embodiment(s)" which provides sufficient information for those skilled in the art to perform the invention. The discussion given under the heading "Background, Motivations, and Description of Invention Embodiment(s)" is not to be regarded as limiting the scope of the preceding Summary of the Invention in any way, and it will make reference to a number of Figures as listed below.

[0037] Note that several of the Figures contain reference numerals identifying particular features or things depicted therein. Many of these reference numerals are also referred to in the discussion below. The way in which specific reference numerals in the Figures are referred to in the discussion below is that, for example, reference numeral 1 appearing in Figure 8 will be referred to as "8-1", and likewise reference numeral 6 appearing in Figure 21 will be referred to as "21 -6", etc.

[0038] Figure 1 , Figure 2 and Figure 3 together help to illustrate the importance of beam width and direction in successfully reading (communicating with) a RFID tag.

[0039] Figure 4 is a plot of the radiation pattern (including the 3 dB beam width) for a directional narrow beam antenna.

[0040] Figure 5 is a schematic representation of a typical construction of a patch antenna.

[0041] Figure 6 illustrates, from the side, the use of a RFID reader/antenna located on an overhead gantry to read a RFID tag on a vehicle windscreen and/or license plate. [0042] Figure 7 illustrates a RFID overhead reader and a RFID side reader scenario, when viewed in a direction opposite to the direction of travel of the vehicles in the depicted lanes of the road.

[0043] Figure 8 illustrates the travel path of a windscreen-mounted RFID tag on a vehicle within an overhead RFID reader beam.

[0044] Figure 9 illustrates factors that contribute to create non-linear variation of the signal between an overhead RFID antenna and a windscreen-mounted RFID tag.

[0045] Figure 10 shows a vehicle license plate mounted within a cavity to protect it from damage.

[0046] Figure 1 1 illustrates the travel path of a vehicle's front and rear license plate within an overhead RFID reader beam.

[0047] Figure 12 helps to explain the required (or at least a desirable) beam shape for a RFID reader antenna placed in/on the road.

[0048] Figure 13 illustrates, inter alia, the impact of a short following distance between, on the one hand, a classic patch antenna beam shape, and on the other hand, a flat antenna beam shape, as emitted from an in/on-road reader.

[0049] Figure 14 illustrates one possible example of a vertically polarised horizontal slotted upright antenna.

[0050] Figure 15 illustrates the radiation pattern of the vertically polarised horizontal slotted upright antenna in Figure 14.

[0051] Figure 16 illustrates how the ground effect can effect the direction of maximum gain in a radiation pattern.

[0052] Figure 17 illustrates antenna beams which are pushed upwards due to a conductive ground effect.

[0053] Figure 18 illustrates the beam shape, in free space, of a hypothetical/idealised upright half-wave dipole antenna.

[0054] Figure 19 illustrates the read-zone for a RFID enabled vehicle license plate.

[0055] Figure 20 schematically illustrates the orientation of a RFID tag (which is mounted on a vehicle license plate) within the read-zone of an in/on-road RFID reader. [0056] Figure 21 illustrates the effective read-zone for a RFID tag which is on a vehicle license plate when read using a RFID antenna of the kind provided by the invention.

[0057] Figure 22 illustrates example uses of the kind of antenna provided by the invention, and the resulting effective read-zone, in different scenarios.

[0058] Figure 23 illustrates the desired radiation pattern for an antenna of the kind provided by the invention.

[0059] Figure 24 illustrates an antenna in accordance with one possible embodiment of the invention which is suitable for use as an on/in road RFID reader antenna. The antenna has a top loaded monopole configuration with a round and periodically slotted ground plane. This configuration has been found to achieve a radiation pattern as illustrated in Figure 23.

[0060] Figure 25 illustrates an inverted F antenna with a square periodically slotted ground plane, which has been found to achieve the radiation pattern shown in Figure 26, and where the direction of maximum gain is up and down the road, somewhat similar to the antenna in Figure 14.

[0061] Figure 27 illustrates an antenna in accordance with another possible embodiment of the invention having an inverted F antenna (IFA) configuration with a round, periodically slotted ground plane, and which has been found to achieve a radiation pattern that is somewhat similar to the one depicted in Figure 23. This antenna also has a slot in the top load cap which may provide an additional antenna which may help to allow or facilitate data communications using WiFi. The radiation pattern for this additional/cap antenna is shown in Figure 28.

[0062] Figure 29 illustrates possible desirable placements of a RFID reader antenna (or the placement of the device which houses the RFID reader and its antenna) on or in a road surface.

[0063] Figure 30 illustrates the approximate general dimensions of the antenna in the particular possible embodiment in Figure 24.

BACKGROUND, MOTIVATIONS, AND DESCRIPTION OF INVENTION EMBODIMENT(S)

[0064] RFID technology, particularly passive backscatter UHF RFID technology, as described by ISO/IEC 18000 part 6, is thought to be suitable for use in vehicle identification. Hence, the term RFID (and like terms) herein includes passive backscatter UHF RFID technology, as described by ISO/IEC 18000 part 6.

[0065] By way of further explanation, the present invention may be useful for all types of RFID, and may also be useful for RF communications between vehicles and a roadside unit. In the case of passive backscatter UHF RFID, the benefits may be much particularly significant (even disruptive in nature) in that they may (at least help to) overcome several limitations associated with current commercially available electronic vehicle identification systems that use passive backscatter UHF RFID. For this reason, the term "RFID" herein may be thought of as referring primarily (although not necessarily exclusively) to passive backscatter UHF RFID. ISO/IEC 18000 part 6 type C might be considered to be the current de facto standard for passive backscatter UHF RFID technology.

[0066] Passive backscatter RFID is, in fact, similar in some ways to RADAR (the term "RADAR" actually originated as an acronym of RAdio Detection And Ranging). RADAR essentially involves a radio signal transmitted by a sensor that is then reflected by the object to be observed and the reflected signal is interpreted by the sensor. In the case of RFID, the signal emitted by the RFID reader, and the "reflected" signal (i.e. the signal sent from the RFID tag back to the RFID reader), are modulated to carry information from the reader to the tag, and from the tag to the reader, respectively. In some cases, for example ISO/IEC 18000 part 6 type D, it may be that only the tag modulates information on the reflected signal from the tag to the reader. This can be the case when the tag is energised by the signal from the reader.

[0067] For RFID generally, the beam shape of a RFID reader (in three-dimensional space) may be defined as the locus of points at which a RFID tag receives enough energy from the reader to switch on and communicate intelligently with the reader. This is generally a sharp, but moving edge due to the nature of digital electronics and electric field propagation. Normally, the beam shape closely follows (or is closely related to) the radiation pattern of the reader antenna. It is for this reason that RFID systems are often designed with only the reader antenna radiation pattern in mid. However, in fact (as will be discussed further below), the ability of a RFID reader to transmit enough energy for a RFID tag to be switched on, for the RFID tag to decode modulated information from the reader (if necessary), and for the RFID tag to then modulate a response on the signal it reflects back to the reader (the "reflected" signal), depends significantly on the radiation pattern, orientation and environment of both the RFID reader antenna and the RFID tag antenna. Tag sensitivity refers to the minimum signal power at a specific locus (in the air) at which a given tag switches on. This sensitivity can be influenced by a number of factors including chip power levels, tag antenna radiation pattern, tag construction and tag orientation.

[0068] It should also be appreciated, at this point, that regulatory requirements often place limitations on the maximum energy or power of radio frequency transmissions in a given area (i.e. limitations on the allowable amount of "power in the air"). This often has a major influence on the efficiency of RFID (and like) systems. These power limitations can mean that it is not always possible (or allowable) to simply increase the transmission power (e.g. the power with which a signal is emitted from a RFID reader) in order to increase the range or distance at which a RFID tag can "power up" and communicate intelligently with the RFID reader. It should also be noted that it is generally considered desirable that a RFID reader should be able to read a tag if the tag is "switched on" by the signal from the reader, as in backscatter RFID systems.

[0069] For best performance in systems involving communication between a RFID reader and a RFID tag, the reader and tag, respectively, should preferably each be positioned and oriented in the direction of maximum power/sensitivity of the other. Figure 1 illustrates the radiation pattern (in free space) of a high gain, directional antenna of a kind typically found in (or used in) current/conventional RFID systems.

[0070] Note that in Figure 1 , and likewise in Figure 2 and Figure 3, the reader antenna's radiation pattern is shown in two dimensions only. Of course, the reader antenna's radiation pattern actually projects from the reader antenna (which is located on the very left-hand edge in Figure 1 ) in three-dimensions. In other words, the radiation pattern in Figure 1 , if represented in three dimensions, would actually extend into and out of the page as well. Therefore, it will be understood that Figure 1 , Figure 2 and Figure 3 all actually show the antenna radiation pattern in cross-section, in a vertical plane which extends through the centre of the antenna radiation pattern in the direction of maximum gain, with the said plane viewed from one side.

[0071] In Figure 1 , Figure 2 and Figure 3, the dark red (indicated as 1 -1 in Figure 1 ) signifies the maximum power, as allowed by the relevant applicable regulations, and at the other end of the spectrum the blue (indicated as 1 -3 in Figure 1 ) signifies almost zero power at that locus. (Recall that "tag sensitivity" refers to the minimum signal power at a specific locus (in the air) at which a tag switches on.)

[0072] Figure 2 and Figure 3 both illustrate a RFID tag positioned in front of the RFID reader of Figure 1. Figure 2 and Figure 3 both show the RFID tag's radiation pattern, represented in both cases by solid black lines, superimposed on the radiation pattern of the reader antenna of Figure 1 . In Figure 2, the RFID tag is located further away from the reader than in Figure 3. However, in Figure 3, the RFID tag is oriented at an angle (approximately 45°) relative to the reader, whereas in Figure 2 the tag is oriented directly "face on" to the reader. Hence, in Figure 2, the RFID tag's radiation pattern (and hence its beam) points directly towards the reader, whereas in Figure 3 the RFID tag's radiation pattern (and hence its beam) points somewhat upwards toward a point above the reader. As those skilled in the art will appreciate, the tag in Figure 2 is therefore more likely to be read than the tag in Figure 3, even though the tag in Figure 3 is closer to the reader. These Figures also help to illustrate the importance, in terms of read performance, of not just the reader radiation pattern and tag radiation pattern, but also of "angles of read", and furthermore the significance of the antenna's aperture. The meaning and importance of the antenna aperture will be discussed further below. [0073] Focussed (i.e. high gain, directive, narrow beam) antennas have become a de facto standard for RFID use, largely because they are thought to reduce radio noise by focusing the radiation pattern to the area of intended read. Figure 4 plots a typical radiation pattern for such an antenna. In other words, Figure 4 is a plot of the radiation pattern for a focussed antenna. The radiation pattern plot in Figure 4 is, in effect, a representation of the antenna's "directivity" ("directivity" is the way the antenna's gain varies with direction).

[0074] In Figure 4, the radiation pattern is plotted in terms of dBi. Those skilled in the art will recognise that, in the field of antenna design, dBi is shorthand for dB(isotropic) and signifies the directional gain of an antenna compared with a hypothetical isotropic (point) antenna which distributes energy uniformly in all directions. The indicated 3 dB beam width in Figure 4 may be considered to be the "aperture" of the antenna. From an RFID perspective, a 3 dB reduction at a specific locus means a 50% reduction in energy (i.e. a 50% reduction in the "energy in the air") at that locus. This means that, even if the distance from the reader remains constant, the amount of energy that is available to switch on a tag located from the reader in a direction on the edge of the reader's 3 dB beam width is only half of the amount which is available to switch on a tag located in the reader's direction of maximum gain (which is at 90° in Figure 4). Furthermore, the amount of energy that is available to switch on a tag located outside the reader antenna aperture (i.e. outside the 3 dB beam width) will be less than half of that which is available in the reader's direction of maximum gain, and with further movement outside (away from) the reader antenna aperture the amount of energy reduces rapidly. Hence, a tag which is located in the direction of (or on the edge of) the reader's 3 dB beam width (i.e. a tag which is on the periphery of the reader antenna aperture), or a tag which is outside the reader antenna aperture, needs to be closer (possibly much closer) to the reader in order to be switched on as compared to a similar tag located in the direction of the reader antenna's maximum gain. Thus, the antenna in Figure 4 is most effective (i.e. its RFID read range is greatest) for a tag located in its aperture (i.e. within the 3 dB beam width).

[0075] The significance of the antenna aperture for focused antennas, as described above, is that, generally, unwanted signals (noise) from outside the antenna aperture are (often naturally) filtered from a signal that is reflected from a tag that is in the antenna aperture. It should be noted that Figure 4 actually relates to an antenna design that is a conventional patch or parabolic design, which is one form of focused antenna currently used in e.g. point to point communications and RFID.

[0076] In any case, Figure 4 and the associated discussion above help to illustrate generally how focused antennas reduce radio noise by focusing the radiation pattern to the area of intended read, and thus why focused antennas have become a de facto standard for use in conventional RFID systems. However, as discussed below, such focussed (i.e. high gain, directive, narrow beam) antennas may not be well suited for use in the applications considered herein, specifically given the drastic RFID read range reduction away from the direction of maximum gain (and also when considering issues such as the predictability of the orientations of both the reader and the tag on the vehicle/license plate).

[0077] Figure 5 is a schematic representation of a typical construction of a patch antenna. Recall that a patch antenna is one conventional form of focused/directional antenna. It is also important to note how the construction of such a conventional patch antenna impacts on the antenna's radiation pattern. The antenna beam (and its overall radiation pattern) points perpendicularly away from the ground plane (i.e. it points vertically upward relative to the ground plane in Figure 5). For UHF patch antennas as currently used in vehicle identification, typically the ground plane has an area of more than 300 mm x 300 mm. At this point it should be noted that, in the context of the present discussion, the "ground plane" of an antenna should not be confused with the "ground" which is part of planet earth.

[0078] RFID has become (and is continuing to become) increasingly popular, and its use is becoming more common/widespread, including in the identification of vehicles on the road. Overhead readers on gantries, as depicted in Figure 6 and Figure 7 for example, seem to have become a de facto RFID reader deployment standard for free-flow multi-lane in-traffic vehicle identification. Gantry based RFID reader infrastructure is, however, complex and typically very time consuming and extremely expensive to deploy and maintain. The result is that RFID vehicle identification is predominantly used only in revenue earning and cost saving applications; for example in freight logistics, toll and congestion charging, and the like. These applications are, however, generally loyalty based, meaning that they rely on compliance by users. Such current RFID vehicle identification systems are generally not well suited to cope with, or adapt to, situations where a person takes steps to prevent detection of their vehicle, or to cause misdetection (incorrect identification) of their vehicle. Current RFID vehicle identification systems are also typically of a closed loop nature (which means that all elements in the systems are specified and regulated by a single entity).

[0079] The high cost of current RFID infrastructure (in particular, gantry based overhead RFID reader infrastructure) is one of the major factors currently inhibiting wider deployment of RFID, especially for compulsory and/or regulatory identification of vehicles (e.g. for law enforcement purposes) in an open loop manner (as posed to closed loop). As an example of what is meant by open loop, authorities in one European country should preferably be able to read and verify all vehicles (the RFID tags thereon) travelling on that country's roads, including vehicles visiting from other countries and where the RFID tags on other countries' vehicles may have been issued by separate authorities.

[0080] A requirement for compulsory RFID tags on all vehicles, complemented by more cost efficient, and easy to deploy and maintain RFID reader infrastructure, may help to provide vehicle movement, identity and demographic information which is currently desperately needed, not just for law enforcement purposes (e.g. for identifying traffic infringements, enforcing vehicle registration, etc), but also for purposes such as effective traffic and road planning, road network operations and management, etc. The information needed for these purposes is currently not readily available (if at all), or where it is available (to some extent) it is often incomplete and obtained from various (often incompatible) sources using expensive and convoluted technologies and methods. The proposals discussed herein may help (or may be used) to provide RFID reader infrastructure which is more cost efficient and easier to deploy and maintain. And this, together with a requirement for RFID tags on all vehicles, may in turn help to facilitate the generation of more complete, accurate and immediate vehicle and traffic movement, identity and demographic information as needed for the range of traffic and transport related purposes discussed above (potentially even with one kind of RFID reader/sensor being suitable for use in multiple different applications).

[0081] In normal conditions, bearing in mind radio properties, interference, the need for data loss retries, etc, it is thought that vehicle identification using passive UHF RFID requires approximately 80 ms to reliably exchange 512 bits of identification data. In this regard, 512 bits of data is thought to be enough to identify a vehicle and perform a rudimentary offline verification of that identity. A vehicle travelling at 36 km/h will travel 0.8 m in 80 ms, and a vehicle travelling at 180 km/h will travel 4 m in 80 ms. Therefore, for the purposes of the present discussion, 4 m of vehicle travel will be used as the minimum exposure required to reliably read a RFID tag on a moving vehicle. Those skilled in this area will recognise that this is based on an assumption of a maximum vehicle speed of 180 km/h, which seems reasonable given that vehicles will very rarely (if ever) travel faster than this on public roads.

[0082] Given that most current road/traffic related RFID systems utilise overhead (e.g. gantry based) RFID reader mounting, it follows as a consequence that the most typical location for the placement of RFID tags on vehicles (for use in such current overhead reader systems) is in the windscreen of the vehicle (usually on the inside of the windscreen, near the driver's rear- view mirror). Currently, the general thinking in the industry is that the placement of a RFID tag in (or on the inside of) the vehicle windscreen is easy, such that RFID tag installation is typically left for vehicle owners to perform themselves. It is also thought that such "self-installation" allows for sufficiently reliable RFID tag installation with the placement/location/orientation of the tags being sufficiently consistent across all vehicles. However, as discussed further below, this assumption, namely that the placement/location/orientation of RFID tags is consistent across all vehicles and vehicle types, may be incorrect or at least inappropriate. In any case, read ranges of more than 15 meters have been reported for certain current commercial RFID tag/reader systems. These measurements are, however, based on (or taken in) ideal conditions. Realistically, reliable read ranges are often less than 8 meters.

[0083] Figure 8 illustrates the travel path 8-3 of windscreen-mounted RFID tags (such as windscreen-mounted RFID tag 8-2) within an overhead RFID reader beam 8-4. The vertical width of the tag travel path 8-3, which extends from approximately 1 m above the ground to approximately 2 m above the ground, exists due to the fact that RFID tags will be positioned at different heights in different vehicle types. For example, a RFID tag installed in the windscreen of a large truck will typically be higher (closer to 2 m) above the ground than a RFID tag installed in the windscreen of a low-slung sports car (which may be closer to 1 m above the ground). It must also be noted that, for different vehicle types, the vehicle windscreen orientation varies from approximately vertical (as found on e.g. trucks and buses) to almost horizontal (as found on vehicles like sports cars). Hence, the orientation of the RFID tag antenna when installed on the inside of different vehicles' windscreens can vary from approximately vertical to almost horizontal (and this is in addition to the possible variation in the height of the RFID tag placement for different vehicle types discussed above). The reason this is important is because of the significant influence relative antenna position and orientation can have on read performance, as discussed above.

[0084] In Figure 8, the reader antenna 8-1 is placed 6 m above the road, which is a typical road clearance height. Considering the range of possible windscreen angles (and hence the range of possible RFID tag antenna orientations), the possible variation in tag height within travel path 8-3, and general RFID technology read performance limitations, in the scenario in Figure 8 a minimum read range of approximately 6.5-7 m is required to read a windscreen- mounted RFID tag reliably. This minimum 6.5-7 meter (approx.) read range is depicted in Figure 8 by the shape of the RFID reader antenna's effective beam 8-4, which (in this two- dimensional, cross-sectional representation) has a "70° sector" shape with a radius of 6.5-7 m (in Figure 8 the radius shown slightly less than 7 m). Also, considering that buses and trucks are likely to travel at lower speeds than passenger vehicles, and normally the tag in their windscreens is placed higher, this reduces the effective read range requirement (possibly to below 6 m) for them.

[0085] The scenario in Figure 8 is well within the limits of what can be achieved with RFID, given RFID technology read performance limitations and the geometry imposed by the locations of the RFID reader 8-1 and RFID tags 8-2 in Figure 8. The reason why the scenario in Figure 8 is well within the limits of what is possible can be appreciated from the fact that, in Figure 8, the minimum required tag travel path/distance 8-5 (which is 4 m long for reasons discussed above) easily fits within the effective beam shape 8-4 of the reader antenna 8-1 . Hence, for vehicles travelling at even the assumed maximum speed (180 km/h), the vehicle's windscreen-mounted RFID tag 8-2 will be within the effective beam shape 8-4 of the RFID reader 8-1 for (more than) enough time to be reliably read.

[0086] However, even though current RFID systems (which utilise overhead RFID reader infrastructure) may be capable of reading windscreen mounted RFID tags, like in the scenario depicted in Figure 8, nevertheless the windscreen as a location for vehicle RFID tag placement is thought to be unsuitable, for reasons discussed below, as is the use of overhead readers/gantries.

[0087] Stationary measurements support the theoretically superior read performance achieved by windscreen mounting RFID tags where overhead RFID readers are used, as compared to the case where RFID tags are mounted on vehicle license plates and read by the same overhead RFID readers. This is not surprising given that, for overhead RFID reader placements, windscreen mounting the RFID tag places the tag higher and closer to the reader, as compared to mounting the RFID tag on the license plate. In fact, stationary measurements of windscreen mounted RFID tags read by overhead readers indicate a close to 100% read performance where static influences and expenses are negated. However, for existing real RFID operations involving windscreen mounted RFID tags read by overhead readers (i.e. actual real world RFID system implementations with overhead readers reading windscreen mounted RFID tags in traffic applications) read performance is actually less than 98%, and this equates to a surprising and unacceptably low tolerance of (at least) one potentially missed vehicle identification instance in every 50 vehicles. (It will be appreciated that this level of read-reliability is unacceptably low if the system is to be used for, for example, law enforcement such as identification of traffic infringements, enforcement of vehicle registration, etc.) The 98% read- reliability figure above also seems to drop further as vehicle speed and traffic density increase.

[0088] As mentioned above, it is thought that there are inherent problems associated with both (i) the use of windscreen-mounted (or likewise headlamp-mounted) RFID tags, and (ii) the use of overhead RFID readers, for the purpose of vehicle identification, especially in open-road free-flow applications for regulatory compliance purposes. For instance, as mentioned above, in relation to (ii), overhead RFID readers, gantry based RFID reader infrastructure is complex, very time consuming and extremely expensive to deploy and maintain (the significance of this problem in particular must not be underestimated.).

[0089] In relation to (i), the use of the windscreen (or headlamp) as the location for the placement of a RFID tag on a vehicle, it must be recognised that such placement of a tag (in a windscreen or a headlamp) leads to a number of factors that significantly affect RFID read reliability, including due to the properties of the glass/plastic used in the windscreen/headlamp and also due to vehicle body materials and shape. It is also important that, as alluded to above, RFID tags when installed in windscreens (or headlamps), are mostly installed by unskilled persons, and this results in a high inconsistency of tag placement location/orientation. [0090] For windscreen mounted RFID tags, and also headlamp mounted a RFID tags, metal body parts of a vehicle can deform/distort/complicate the radiation pattern of the RFID tag's antenna. For instance, the vehicle's metal body generally surrounds the RFID tag antenna and tends to generate a mutual-coupling effect that distorts the antenna properties both in radiation features and signal fidelity. And the windscreen/headlamp glass/plastic, due to both its composition and thickness, often displays an uncertain dielectric variance and may even act as a radio shield as a result of tinting and/or hardening. (In worst-case scenarios, these factors may even cause the vehicle overall to operate, in effect, as a Faraday cage surrounding the RFID tag, thus preventing or severely inhibiting communication between the RFID tag and a RFID reader). The use of the windscreen (or headlamp) as the location for the placement of a RFID tag on a vehicle can therefore lead to issues that randomly and unpredictably affect read performance. Actually, there is even more to this, as discussed further below.

[0091] Figure 9 illustrates certain factors that contribute to create non-linear variation of the signal between an overhead RFID reader antenna and a windscreen-mounted RFID tag, including as a result of movement of the vehicle. More specifically, Figure 9 illustrates the direct communication path 9-4 between the overhead RFID reader antenna 9-1 and the windscreen- mounted RFID tag 9-2, together with a number of multi-path factors which contribute to create signal nonlinearity associated with the direct communication path.

[0092] Firstly, movement of the vehicle towards the overhead gantry on which the reader 9- 1 is mounted causes a decrease in the length of the direct communication path 9-4, as indicated by 9-5. The decreasing length of the direct communication path 9-4, resulting in a Doppler shift, in fact, changes as a trigonometric tangent function of the angle of the communication path (the signal path length ^¾ tan(a) where a is the angle of the direct communication path), and this, in turn, results in a squared tangent Doppler shift for the reflected signal carrier wave. This effect might perhaps be handled, at least to some extent, by using short data packet lengths allowing rapid communication timing synchronisation. Use of such short packet data lengths may be effective when the signal received is predictable in behaviour and of singular source. However, in addition to the non-linear decreasing length 9-5 of the communication path 9-4 and the effects this causes (just described), the metal surfaces and edges of the vehicle body (especially the bonnet) act as near-perfect reflectors causing a multitude of other near-perfect (but slightly out of phase) reflected communication paths 9-3. The multiple reflected communication paths 9-3 (which are inherently unpredictable due to varying vehicle windscreen and body shapes/configurations, and also bearing in mind that each of these reflected paths 9-3 is also subject to communication path length decrease and the issues associated therewith) combine to result in an overall/net communication signal to the RFID reader 9-1 that incorporates the multiple variable signals, each having an exponential tangent (i.e. a highly nonlinear) Doppler shift. This results in a communication signal with unpredictable signal distortion, but where the distortion is similar in nature to the real communication signal (this can be worse, and more difficult to filter out, than unrelated background noise or the like). This is especially true for RFID tags with limited power and processing capabilities. The occurrence of these highly damaging permutations on the signal is dependent on relative tag-reader orientations, tag placement, vehicle construction, vehicle velocity and other reflectors (e.g. other vehicles) in close proximity. It should be clear that the alleviation or reduction of such non-linear multi-path problems is very difficult to achieve using windscreen/headlamp mounted RFID tags, because many factors such as vehicle body configuration/construction, tag placement, etc, cannot be controlled, and the above problems are especially difficult to alleviate when the vehicle is moving at high velocity.

[0093] The alternative placement of a RFID tag on or in a metal plate on a vehicle (for example, and preferably, on/in a license plate of the vehicle) may help to largely avoid the radio influences of the vehicle (like those discussed above). A metal plate on a vehicle (such as the vehicle's license plate) can also have a highly consistent shape/construction (i.e. the shape/configuration of the metal plate will typically vary very little, if at all, from vehicle to vehicle). Such a metal plate (license plate) can also function as a ground plane which largely shields the antenna beam from the reflective effects of the rest of the vehicle structure. This is especially true where the metal plate (preferably a license plate) on/in which the RFID tag is mounted is itself mounted in such a way that a clear line of sight is maintained to the plate (so that there are no (or few) intervening reflectors/reflections between the RFID tag and the RFID reader). Importantly, in most jurisdictions, the applicable governing legislation requires vehicle license plates to be installed in such a way that they can be clearly seen (i.e. such that there is a clear line of sight to the license plate). This thus makes the vehicle license plate a particularly suitable placement location on a vehicle for a RFID tag if the tag is to be reliably read by a RFID reader. (As has been mentioned, for vehicles which have a license plate on the front and the rear, a RFID tag may preferably be placed on one or both of a said vehicle's license plates, and for vehicles which have only one license plate, a RFID tag may preferably be placed on the single license plate.)

[0094] Figure 10 shows a licence plate mounted within a cavity (i.e. within the channel section of a metal girder) to protect it from damage. This mounting does not obstruct the reading of the plate by a human, but an overhead RFID reader would likely have problems reading a RFID tag installed on such a plate (due to the shielding effect of the section of the metal girder that extends out above the plate). This is another example problem associated with the use of overhead RFID readers for vehicle identification. (Note that the license plate, and also the vehicle registration information, have been obscured in Figure 10 for privacy reasons.)

[0095] Figure 1 1 illustrates the travel path 1 1 -3 of a vehicle's license plate, where the license plate has a RFID tag thereon (making it a "RFID plate" 11 -2), within an overhead RFID reader beam 1 1 -4. Figure 1 1 actually illustrates that a vehicle may have a RFID plate 1 1 -2 mounted on the front and/or the back thereof. Similar to Figure 8, the vertical width of the tag travel path 1 1 -3 in Figure 1 1 , which extends from approximately (just above) ground level to approximately 1 m above the ground, exists due to the fact that RFID plates 1 1 -2 may be positioned at different heights (i.e. different distances off the ground) on different vehicle types. For example, a RFID plate 1 1 -2 installed on a large truck will typically be higher (closer to 1 m) above the ground than a RFID plat 1 1 -2 installed on a low-slung sports car (which might be as little as 20 cm or less above the surface of the ground).

[0096] However, unlike the situation for RFID tags installed in vehicle windscreens/headlamps (where the vehicle windscreen/headlamp orientation can vary from vertical to almost horizontal), vehicle license plates generally display little (if any) variation from vehicle to vehicle in terms of the orientation (angle) at which they are installed. Vehicle license plates are typically required to be installed vertically, such that the plane of the license plate is perpendicular to the direction of travel of the vehicle. Consequently, RFID tags on/in vehicle license plates (hence "RFID plates" like the plates 1 1 -2 in Figure 1 1 , and the antennas thereof) generally have highly consistent orientation from vehicle to vehicle, even across different vehicle types. Hence, the orientations of the antennas of RFID tags, when the tags are installed on/in vehicle license plates, will generally vary very little, even across different vehicle types. The importance of this should not be underestimated given the significant influence that relative antenna orientation can have on read performance (see above).

[0097] The influence of the vehicle body on a license plate tag may be related to the size of the metal (conducting) background to the plate, which may be small for a sedan and larger for a bus, as an example. A larger conducting background may have a general effect of making the antenna aperture more narrow and perpendicular to the plane of the background. This can have an overall negative effect for a gantry placed reader, but a positive effect for an in/on road reader; see Figure 12 and the relevant discussions below.

[0098] In Figure 1 1 , the reader antenna 1 1 -1 is again placed 6 m above the road. Considering the possible variation in RFID plate height (tag height) within travel path 1 1 -3, and given general RFID technology read performance limitations, in the scenario in Figure 1 1 a minimum read range of around 7.5 m is required to read the plate tag reliably. This minimum 7.5 m (approx.) read range is depicted in Figure 1 1 by the shape of the RFID reader antenna's effective beam 1 1 -4, which (in this two-dimensional, cross-sectional representation) has a "30° sector" shape with a radius of around 7.5 m (the radius of the beam 1 1 -4 extends to midway between the 7 m and 8 m arcs from the location of reader 1 1 -1 ).

[0099] The scenario in Figure 1 1 is on the edge (i.e. it is approaching the limit) of what can be achieved with RFID, given RFID technology read performance limitations (within spectrum regulations) and the geometry imposed by the locations of the RFID reader and RFID plates (tags) in Figure 1 1 . The reason why the scenario in Figure 1 1 is approaching the limits of what is possible can be appreciated from the fact that, in Figure 1 1 , the minimum required tag travel path/distance 1 1 -5 (which must again be at least 4 m long for reasons discussed above) only just fits within the effective beam 1 1 -4 of the reader antenna 1 1 -1. Hence, for vehicles travelling at or near the assumed maximum speed (180 km/h), the vehicle's license plate mounted RFID tag (RFID plate) 1 1 -2 will be within the effective beam 1 1 -4 of the RFID reader 1 1 -1 for only just enough time to be reliably read (or possibly, due to the possible influence of communication distorting/inhibiting factors, the vehicle's RFID plate 11-2 may not be within the effective beam 1 1 -4 for quite long enough, in which case a reliable read may not be, or may not always be, possible).

[00100] The fact that the scenario in Figure 1 1 is on the edge of what can be achieved with RFID should not be interpreted as a downside or problem associated with the choice of the vehicle license plate as a placement location for RFID tags on vehicles. On the contrary, as explained above, there are numerous reasons why the vehicle license plate may be a highly advantageous location for the placement of RFID tags on vehicles. Hence, the fact that the scenario in Figure 1 1 is on the edge of what can be achieved with RFID is mainly an indication of the problems associated with mounting RFID readers in overhead locations (such as on over- road gantries and the like). The difficulties highlighted by the scenario in Figure 1 1 therefore clearly indicate that an alternative RFID reader placement location is required, especially where the vehicle license plate is chosen as the location for the placement of RFID tags on vehicles. An in/on-road location is thought to be a more preferable placement location for a RFID reader if vehicles' RFID tags are on or part of the vehicle license plate.

[00101] Sensors which are positioned on or in the road (e.g. as opposed to located on overhead gantries or the like) have previously been proposed and trialled. However, previously their use has been avoided due to issues associated with, for example, difficulties in achieving safe access for personnel for maintenance of the on-road or in-road sensors, the potential for damage to the integrity of the road surface due to the placement of the sensor in the road, the undesirable necessity for (at least partial) road closures for installation, repair or maintenance of the sensors, etc. In/on-road sensors also need to deal with road vibrations, wheel impact shocks and on road fluids, dirt, contaminants, etc. Nevertheless, it is thought that an appropriate structure for an on-road or in-road sensor, which alleviates or at least reduces these problems (or which can cope with them acceptably), is feasible. For instance, sensor size, format, construction, power provision and communications facilities may be selected and combined in a manner to minimise the impact on the road and the time to install the device. At the same time the design of the sensor may ensure durability and ease of maintenance of the device on/in the road. Note that full details of the overall sensor design (e.g. the design of the sensor unit, its housing, construction, etc) to meet the above requirements are not critical to the present invention, and therefore will not be discussed herein in detail.

[00102] As mentioned above, an in/on-road location is thought to be a preferable placement location for a RFID reader, especially if vehicles' RFID tags are on or part of the vehicle license plate (which is also thought to be highly preferable). For one thing, where in/on-road RFID readers are used and RFID tags are located on vehicle license plates, the multi-path problem (discussed above with reference to Figure 9) may be largely alleviated since the only real reflectors which might reflect a signal between the in/on-road RFID reader and an on-plate RFID tag are the road itself and other vehicles in an adjacent lane. The road is a weak reflector which tends to scatter the signal (rather than produce the much more problematic near-perfect, but slightly out of phase, reflections typically associated with the vehicle bonnet etc for windscreen mounted tags). And adjacent vehicle multi-path reflections typically display a close to linear Doppler shift which can be filtered relatively easily.

[00103] Figure 12 illustrates a desirable radiation pattern, and hence a desirable beam shape 12-4, for a RFID reader antenna 12-1 which is placed in/on the road. More specifically, Figure 12 illustrates a cross-section of the said desirable beam shape 12-4, in a vertical plane which extends parallel to the vehicle's direction of travel and through the centre of the antenna radiation pattern, with the said plane viewed from one side. It will be noted that the beam shape 12-4 is quite low (relative to vehicle height) and long/wide (relative to travel direction). Contrast this with the radiation pattern 13-2 on the right hand side in Figure 13 which is a radiation pattern for a conventional directional (and upward-pointing) patch antenna.

[00104] The RFID tag 12-2 is placed in or on the vehicle's front and/or rear license plate resulting in a potential tag travel path 12-3 which is typically the space between about 200 mm and about 1200 mm above the road surface. In other words, depending on the type of vehicle (e.g. car, truck, bus, motorcycle, etc), its license plate, with the RFID tag thereon, will typically pass through this region 12-3 which is approximately 200 mm-1200 mm above the ground as the vehicle passes the reader. The bulk of the space inside the effective beam 12-4 of the antenna 12-1 in Figure 12 (i.e. the bulk of the space within which the RFID tag on the vehicle license plate will receive sufficient energy to "switch on" and communicate with the RFID reader) is within this plate tag travel path 12-3. This is why this beam shape 12-4 is thought to be highly suitable.

[00105] Those skilled in the art will appreciate, from Figure 12, how an in/on-road placement may also alleviate or at least reduce read issues associated with short following distances, tailgating, etc. In Figure 12, the illustrated gap between the depicted vehicles is 4 m, which corresponds to quite severe "tailgating", especially at moderate-high vehicle speeds. Nevertheless, even with such close vehicle proximities, a given vehicle (and it's RFID plate tag 12-2) will be within the beam 12-4 and visible to the reader 12-1 for sufficient time to achieve a reliable read.

[00106] Whilst an in/on-road location is considered to be a preferable placement location for a RFID reader for the reasons discussed above, and especially if the location of RFID tags on vehicle is on or part of a vehicle license plate (which is also thought to be highly preferable), nevertheless in/on-road RFID readers do also present certain challenges.

[00107] Figure 13 illustrates, on the right-hand side thereof, the radiation pattern (and hence beam shape) 13-2 associated with a reader having an upward-pointing conventional patch antenna. Figure 13 also illustrates, on the left hand side, a low antenna radiation pattern (beam shape) 13-3 as emitted from a form of in/on-road reader 13-1 having an antenna like those discussed below. (Note that the radiation pattern/beam shape 13-3 on the left-hand side in Figure 13 is the same as the radiation pattern/beam shape 12-4 illustrated in Figure 12.)

[00108] The metal surface under a vehicle can act as a reflector, and it is close to the reader antenna 13-1 . This may result in a blinding energy reflection which, in the case of an upward- pointing conventional patch antenna (or any other kind of upward-pointing focused, narrow beam antenna), will be very high (as indicated by the amount of depicted energy within the region 13-6 in Figure 13). An antenna with a radiation pattern which possesses low radiation in the vertical direction, especially directly above the antenna (like the radiation pattern 13-3 on the left in Figure 13) may help to reduce this reflected blinding energy substantially (this is illustrated by the substantially lesser amount of energy within the region 13-5 in Figure 13, as compared with the amount of energy in the region 13-6 above the patch antenna).

[00109] Thus, Figure 13 illustrates that even though an in/on-road location is a preferable placement location for a RFID reader, it is also the case that focused, narrow beam antennas (as conventionally used in other RFID applications) may be inappropriate for use in this application, due to the possibility for a blinding reflection from the underside of a vehicle. Accordingly, it would appear that an antenna with a radiation pattern having an overall low, flat shape would be preferable.

[00110] A low, flat shaped antenna radiation pattern could possibly be achieved by turning a directional antenna (like the one illustrated in Figure 5) on its side. However, this would result in a physical structure on the road which might be (typically) approximately 300 mm tall and 300 mm wide. Such a physical structure is obviously not feasible for use on the road as it would obstruct traffic and would likely be destroyed by the first vehicle to collide with it (not to mention the damage caused to the vehicle, potential accident injuries, etc). Thus, simply turning a directional antenna on its side in order to achieve the desired radiation pattern may not be an option.

[00111] Hence, there would appear to be a need for an antenna with a radiation pattern having an overall low, flat shape and where the antenna's physical construction also has a low profile (i.e. a low physical height) so as to be suitable for use in on-road or in-road applications. Hereafter, reference to a "height-restricted" antenna should be interpreted as a reference to an antenna which meets, at least, this last-mentioned configuration criterion (i.e. a "height- restricted" antenna is a reference to an antenna that has a low profile physical structure, or in other words, a low physical height). Of course, for reasons discussed above (and discussed further below), a height-restricted antenna which also has an overall low, flat shaped radiation pattern may be particularly desirable.

[00112] Low-profile antenna structures may be achieved by using, for example, upright slotted antenna designs. Another advantage of these may be that only one antenna is required to create radiation up and down the road. Figure 14 depicts an example of an upright slotted antenna and Figure 15 illustrates its radiation pattern. Importantly, it should be noted that the antenna in Figure 14 is merely one example of an upright slotted antenna, and the term slotted antenna really defines an entire category of antennas. The particular example upright slotted antenna depicted in Figure 14 uses an upright slotted radiator 14-1 which has a generally forward and backward pointing radiation pattern, and the forward and backward pointing portions of the radiation pattern are each (individually) somewhat similar to the radiation pattern of a patch antenna (see Figure 15). A reflector 14-2 is used to push the radiation to one side, and at the same time functions (to some extent) to neutralise the ground effect. However, it is thought that this type of antenna, although simple to construct, may not be suitable for use in the particular applications discussed herein due to its inability to cover the desired read zone. Further explanations relating to the read zone are given below.

[00113] Antenna design and wave propagation analysis, including taking into consideration ground effect, have been investigated in the past. However, for the most part, such previous investigations have focused on low-frequency and long-distance communications, and antenna designs suitable for this. Therefore, the ground effect has generally been considered/analysed at a macro scale (e.g. considering the effects soil, water, grass, forest, etc, can have for communication over large distances). However, in the context of the present discussion, which involves the use of RFID (including ultrahigh frequency (UHF) RFID) for vehicle detection in road/traffic applications, communication distances are comparatively much shorter; almost always less than 8 m and typically around 5 or 6 m. Also in the present context, for reasons discussed above, the RFID reader will preferably be located on/in the surface of the road. Accordingly, within the range of communication distances currently under consideration (typically 8 m or less and usually around 5 or 6 m) the ground will always be the surface of a road. In other words, if the RFID reader is placed on/in the road surface, the ground within a communication distance radius from the RFID reader (i.e. within at least 8 m or less and usually around 5 or 6 m) will usually be constituted by the road surface (and normally/often nothing else). This "close ground" or "near ground" has determinable, but highly changeable radio properties. Antenna design taking into consideration such "near ground" effect (i.e. the ground effect caused by this "near ground") has not been widely investigated in the past. It is thought to be preferable that the variation in road dielectric properties in different weather conditions should also be taken into account, as discussed in the Summary of the Invention section above.

[00114] Placing a low profile upright antenna on the ground, or just below the ground surface, will result in a changing of the direction and shape of the antenna's radiation pattern. Typically the direction of maximum gain will change from a zero angle (i.e. along the ground surface) to a higher elevation angle. That is, the direction of maximum gain will be directed (at least somewhat) up/away from the surface - see Figure 16 for a far field radiation pattern.

[00115] The ground effect arises due to a change in the material(s) which the antenna's radiation propagates through or reflects from. For example, the ground (or more specifically in the present context, the road and its base) demonstrates various dielectric properties (permittivity and conductivity). This is due to the materials used in the road's construction, and also due to moisture, the latter of which is highly variable/changeable and uncontrollable. The typical impact of a conducting ground effect is to push the direction of maximum gain upwards. Figure 17 illustrates, for the situation where a patch antenna is on its side so that its beam extends horizontally, the way the radiation pattern of the patch antenna is pushed upwards because of the ground effect. This effect may be present where, for example, metal reinforcing is present in the road and/or conductive fluids (e.g. water due to recent rainfall) are on or in the road surface. In Figure 17, the reader antenna 17-1 (a patch antenna pointing horizontally) is placed on the road. The direction of maximum gain 17-3 is pushed up, in this case to -30 degrees. A narrow aperture beam shape, like the one identified as 17-4 in Figure 17, does not provide enough energy in the vehicle's license plate/RFID tag potential travel path 17-2. In other words, for the narrow aperture beam 17-4, there is not enough of the plate tag travel path 17-2 within the beam (and hence the vehicle's RFID tag will not be within the beam for long enough) for a reliable read of the vehicle's RFID tag to be achieved. The beam aperture, it might be thought, could be widened to increase the amount of the plate tag travel path 17-2 that is within the beam. This possibility is shown by the wider beam aperture 17-5 in Figure 17. However, those skilled in the art may realise that this illustrated possibility (i.e. 17-5 in Figure 17) is perhaps idealistic (and perhaps not realistic) because the ground effect would likely actually push such a directional radiation pattern further away from the road (meaning that the amount of the plate tag travel path 17-2 which is within the beam may not actually increase very much). [00116] The way in which the direction of maximum gain of an antenna is pushed upwards by the ground effect can also be of benefit, and embodiments of the present invention may take advantage of this, in relation to issues associated with radio noise and interference. As those skilled in the art will appreciate, the use of multiple radio transmitters (and the antennas thereof) which are operating on the same or similar frequency and located nearby one another can (in general terms) give rise to interference. That is, the transmitters/antennas can interfere with one another (often referred to as "cross talk"). One way to overcome or reduce this problem is to ensure that radio transmitters/antennas that are located near one another operate on different frequencies (or ideally frequencies that are mathematically orthogonal to one another so as to prevent additive or subtractive interference). However, in many applications (like RFID for example) the number of frequencies that are actually available for use (according to the applicable regulations governing spectrum allocation) can be very limited. Also, the frequencies themselves that are actually available may not meet the desirable condition of orthogonality. In such cases, it can instead be necessary for respective transmitters/antennas which are operating on the same/similar frequency to be located sufficiently far away from one another to prevent or sufficiently minimise cross talk. However, the way the direction of maximum gain of an antenna is pushed upwards by the ground effect may help with this problem (at least to some extent). This will be explained with reference to Figure 16. In Figure 16, the direction of the reader antenna's maximum gain is labelled 16-1 and it can be seen that this is elevated (approximately 30° in this example) relative to the horizontal ground. On the right-hand side in Figure 16, the lines 16-2 represent the reader antenna's aperture. The amount of energy from the reader that is available to switch on a tag located from the reader in a direction outside the reader antenna aperture is (as explained above) much less than the amount that is available for a tag at the same distance from the reader but located in a direction inside the reader's aperture, and the further outside the aperture the less energy is available. In fact, in the example in Figure 16, the amount of energy (as depicted by the plotted locus) in the direction approximately along (or just above) the ground is between -20 dB and -30 dB, meaning that the amount of energy from the antenna in this direction is drastically less than in directions that are within the antenna's aperture. What this indicates in practice is that there may be little or no signal (i.e. little or no energy emitting from the reader antenna) along or very close to the ground. Also, because the antenna's direction of maximum gain is elevated upwards "into the sky", there will usually be very little energy reflected back downwards towards the ground (assuming no overhead bodies to cause reflections or like). Taken together, these two factors (i.e. little or no energy along the ground and little energy reflected back downwards) suggest that if multiple reader antennas are placed on/in the ground, then there may in fact be little (if any) interference between them, because one reader antenna is unlikely to receive or be affected much (if at all) by the signal of nearby antennas.

[00117] One type of antenna having a low profile physical structure, specifically an example upright slotted antenna, was described above with reference to Figure 14. Another type of antenna is a dipole antenna. As summarised in Wikipedia for example, a conventional dipole antenna consists of two conductive elements such as metal wires or rods, which are usually bilaterally symmetrical. The driving current from the transmitter is applied, and for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feedline to the transmitter/receiver is connected to one of the conductors. The most common form of dipole antenna, referred to as a half-wave dipole antenna (or simply a half-wave dipole), has two straight rods or wires oriented end to end on the same axis, with the feedline connected to the two adjacent ends. Dipole antennas in general are resonant antennas, meaning that the elements serve as resonators, with standing waves of radio- frequency current flowing back and forth between their ends. As mentioned above, the most common form of dipole antenna is the half-wave dipole, an in a half-wave dipole each of the two rod elements is approximately 1/4 of a wavelength long, so that the whole antenna is a half- wavelength long (hence the name "half-wave" dipole).

[00118] Figure 18 illustrates the theoretical radiation pattern of an upright half-wave dipole antenna in free space, with the antenna located at the centre of the depicted radiation pattern. This perfect/around doughnut (or "toroid") shaped radiation pattern provided by a theoretical half-wave dipole antenna seems intuitively suitable for use in the road vehicle identification application presently under consideration, especially if the centre point of the dipole antenna were to be at the ground/road surface (assuming any effects caused by the road and the surface of the road can be ignored). Such an upright dipole antenna with its "doughnut" beam shape would be directionally independent in the plane of the surface of the road. That is to say, if such an upright dipole antenna with a doughnut beam shape were to be used in a RFID reader on a road for identifying passing vehicles having license plate mounted RFID tags, the vehicles' RFID tags would read equally well regardless of what radial direction the vehicles approach the reader from. In other words, regardless of the direction of travel of a given vehicle as it moves over/past the RFID reader (and some vehicles may approach/pass the reader in different directions vis-a-vis others) nevertheless the vehicle RFID tags will be read equally well. This may be beneficial, say, at cross roads where vehicles may pass the antenna from a variety of directions. It may also be beneficial when rapidly deploying the antennas/RFID readers as no alignment of the antenna is required, only appropriate positioning and spacing (the last - spacing - is necessary only where multiple antennae/sensors are used).

[00119] The use of an antenna with a toroid shaped radiation pattern, and the benefits/advantages this can provide, may actually be somewhat contrary to conventional thinking. As has been discussed previously, conventional thinking in fields related to radio communications and RFID has generally been that focused/directional antennas are preferable, and that the antenna should be oriented with its radiation pattern (and in particular its direction of maximum gain) pointing in the direction of likely/intended read. Thus, in a vehicle identification application for example, conventional thinking would suggest that a directional antenna should be employed with the antenna's narrow beam/radiation pattern pointed in the direction of vehicle travel (e.g. up and/or down the road). However, continuing to refer to the vehicle identification application example, a significant difficulty is that vehicles often may not travel precisely in line with or along the direction of intended read. For instance, a vehicle may travel to one side or other of the reader, or it may pass the reader at an angle relative to the reader antenna's direction of maximum gain, such that the vehicle (and the associated antenna/tag) does not move directly into or along the reader antenna's direction of maximum gain. In such situations, the antenna/tag on the vehicle must be read from the side and/or at an angle, and as has been explained, the amount of energy required to read the tag in such situations may be higher (compared to an ideal "face on" read), even if the tag is actually quite close to the reader. In this regard, one of the main benefits of a toroid shaped radiation pattern is that it is inherently non-directional (in a horizontal plane). In other words, for an antenna with a toroid shaped radiation pattern, the antenna's maximum power extends in all horizontal directions around the antenna, and this can help to significantly reduce the problems discussed above.

[00120] A dipole antenna emits linearly polarised energy/radiation. Consequently, if a dipole antenna is used as the antenna for a RFID reader, this in turn requires any RFID tags which are to be read by the RFID reader (such as RFID tags on vehicle license plates) to "reflect" a signal (or produce a modulated reply/response signal) with the same polarisation. However, in most RFID systems and technologies that have previously been developed (e.g. for use in logistics), polarisation is not predictive or fixed. Reflections also change the direction of polarisation. Therefore, it has previously been considered preferable, in the field of RFID, to use circularly polarised antennas (due to the ability to better cope with unpredictable polarisation) rather than linearly polarised antennas.

[00121] However, as explained above, a vehicle license plate (including a RFID tag (and it's antenna) thereon) is highly predictable in terms of its construction, mounting and configuration, and the impact of reflections from surrounding reflective bodies is likely to be low. Accordingly, contrary to the case for previous RFID applications (where circular polarisation was considered preferable), in the road vehicle identification application currently under consideration, linear polarisation (as emitted by dipole antennas) is thought to be potentially suitable. In fact, linear polarisation of the reader and tag antennas, which may preferably be vertical polarisation (see below), may provide additional benefits. One such benefit is that any component of a noise signal having different polarisation (e.g. any component of the noise signal having horizontal polarisation, where the reader and tag antennas are vertically polarised) may be more easily (or even naturally) filtered out. Another potential benefit of linear polarisation is that the efficiency of energy utilisation may be improved (as there may be zero polarisation mismatch, e.g. between the reader and tag).

[00122] It is thought to be feasible to provide a RFID vehicle license plate (i.e. a vehicle license plate having mounted thereon (or incorporating) a RFID tag) wherein the RFID tag incorporates a slotted antenna which is linearly (preferably vertically) polarised. In other words, such a tag antenna may emit linearly polarised energy/radiation, and preferably the energy/radiation emitted by the tag antenna may be vertically polarised. The reason vertical polarisation may be preferable is because, if the RFID reader antenna is a kind of dipole antenna oriented upright, the reader antenna will emit vertically polarised energy/radiation. Thus a RFID vehicle license plate wherein the antenna thereon is also vertically polarised may be an appropriate match for a RFID reader incorporating an upright dipole antenna (located at an "on-road" or "inter-road" level).

[00123] However, whilst dipole antennas (as a general category of antenna) may well be suitable for use in RFID readers in the present application, a simple half-wave dipole antenna (being the most common form of dipole antenna) may not be the most suitable or ideal. Rather, the form of antenna used may be a type or species of dipole antenna (i.e. the antenna proposed for use in the present context might be said to fall into the general category of dipole antennas or dipole-like antennas), however it may not be merely a simple half-wave dipole antenna. Instead, the antenna may be an adapted form of dipole antenna, or a variation or modification of a traditional dipole (or dipole-like) antenna configuration. Importantly, as well as being an adapted or modified form of dipole antenna, the antenna should also be a "height-restricted" antenna. (Recall that the term "height-restricted" herein refers to an antenna that has a low profile physical structure, or in other words, a low physical height.) Furthermore, for reasons discussed above, the antenna should be configured to provide a low, flat radiation pattern.

[00124] Figure 19 illustrates, for one possible scenario, the read-zone for a vehicle equipped with a RFID enabled license plate. The RFID plate travel path in Figure 19 is 4 m wide with the read-zone starting at 5 m before the reader antenna and ending at 5 m beyond the reader antenna (the reader in this instance is located in the centre of the road lane at the marked 0 m point). The space from 1 m before to 1 m beyond the reader antenna is excluded from the read- zone in an attempt to reduce the blinding effect of radiation reflection (discussed above with reference to Figure 13), and also because of angled-read problems that may arise in this region, especially for vehicles (and the plates thereof) which are moving near the side of the lane (rather than down the centre of the lane directly in line with the reader).

[00125] Figure 20 is a schematic representation of what is depicted pictorially in Figure 19. Thus, Figure 20 shows the license plate/RFID tag orientation within the read-zone (detect area) of an in/on-road RFID reader. The typical values for the parameters in Figure 20 (as per Figure 19) are: L = 1 m, Lx = 4 m, Ly = 2 m and 200 mm≤ h≤ 1200 mm.

[00126] Figure 21 illustrates the effective read-zone 21 -5 for a RFID tag 21 -4 located on a vehicle license plate, as read using an in-road RFID reader 21 -1 with an adapted/modified and height-restricted form of upright dipole antenna. The required read-zone 21 -7, based on the travel path 21 -3 of the vehicle, covers the typical lane width of (2Ly) 4 m and the required 4 m in-beam travel path (Lx). (Hence, the required read zone 21 -7 in Figure 21 corresponds to the read zone/detect area depicted in Figure 19 and Figure 20.) The RFID reader's (wide and flat) "dropped doughnut" shaped radiation pattern (this being a highly preferable shape for the radiation pattern) is represented in Figure 21 by the circle labelled 21 -2, however it will be understood that this beam shape 21 -2 (which is represented as large a circle in Figure 21 ) is actually a dropped-doughnut-like or squashed-toroid-like radiation pattern preferably having a shape approximating the one shown in Figure 23. In any case, the RFID reader's radiation pattern 21 -2, with a face-on read range of approximately 6 m, combined with the effect of the angle of read 21 -6 on the plate's RFID tag 21 -4, results in the illustrated effective beam shape 21 -5 (or effective read zone 21 -5). The effective beam shape (read zone) 21 -5 is the area in which a RFID tag which is on/in a vehicle license plate will receive enough power from the RFID reader 21 -1 to be switched on and effectively reflect a modulated signal. As shown in Figure 21 , the effective read zone 21 -5 is roughly "figure 8"-shaped, with the centre of the figure 8 located at the position of the RFID reader 21 -1 and the two lobes of the "figure 8" on either side thereof in the direction of vehicle travel. (It should of course be recalled that the RFID reader's antenna, being an adapted/modified and height-restricted form or variation of dipole antenna 21 -1 , is non- directional and therefore the orientation of the "figure 8" shaped effective read zone 21 -5 - i.e. in line with the vehicle's direction of travel - arises due to the geometry of the required read zones 21 -7, and the convergence of the "figure 8" lobes near the reader arises due to angle of read issues. These factors concerning the orientation of the "figure 8" shaped effective read zone 21 - 5 are therefore not a result of the design/configuration of the antenna 21 -1 itself).

[00127] Figure 22 illustrates example uses of a RFID reader equipped with an adapted and height-restricted dipole antenna 22-1 (and which provides a doughnut shaped radiation pattern, as in Figure 21 ), or multiple such RFID readers, with the resulting effective read-zone 22-2, in different read scenarios. The potential travel path of a license plate RFID tag 22-3 is indicated (indicated as 22-3 and also coloured blue in Figure 22), based on where a vehicle may physically drive, on each different type of road. All road lanes in these examples are 3 m wide, which is average for many road lanes. A bi-directional (single carriageway) narrow road 22-4 that is approximately 6 m wide can be covered with a single reader which will read vehicles in both directions (this is the example in the top left of Figure 22). This is because the plates will typically be placed in the centre on the front and back of a vehicle, meaning that the 4 m wide read-zone will actually cater for a 6 m wide vehicle travel zone (this is ignoring motorcycles and the like). A road with a shoulder, or a wide shoulder, 22-5 (the presence of the shoulder increases the width of the area in which a vehicle can travel) may however often require two readers (as illustrated in the top-middle example in Figure 22). A four lane single direction road with shoulders 22-6 may require three readers (as illustrated in the lower left example in Figure 22). A road crossing of two narrow roads 22-7 could potentially require only one reader (which is why this is illustrated in the example on the right-hand side in Figure 22); although a crossing of a narrow road with a road having wider shoulders may require two readers.

[00128] The example scenarios in Figure 22 help to illustrate that, for example in applications such as law enforcement applications, one or more RFID readers should be deployed on or in the road such that it is not possible (or at least it is very difficult) for a vehicle to avoid detection (i.e. avoid having its license plate RFID tag read by one of the readers). So, for example where the RFID readers are used in law enforcement applications, it should not be possible for a vehicle to easily avoid detection by "driving around" the reader by skirting around (and not entering) the reader's detection zone. On the other hand, there may be situations where a RFID reader is intentionally placed so as to only cover a portion of the road. As one possible example of this, in traffic or road planning applications, it may be important to gather information about the number or density of vehicles travelling in a particular lane (or lanes) of a given road, but not all lanes, e.g. at particular times. In this situation, the RFID reader might be placed so as to only detect vehicles travelling in the lane(s) in question, but so as not to detect vehicles travelling in other lanes.

[00129] From the above it will be understood that an antenna which provides a radiation pattern as illustrated in Figure 23 (or similar to this) is desirable for use in RFID readers which are to be used in "on-road" or "in-road" placement locations in vehicle identification applications. Such a radiation pattern concentrates the maximum power in the zone where a RFID tag on a vehicle license plate is most likely to travel, which is typically 8 m or less from the antenna. (This was also explained above with reference to Figure 12.) This radiation pattern is also directionally independent in relation to the travel of the vehicle, with a low level of power directly above the antenna (this last is important for reasons discussed above).

[00130] Figure 23 actually shows the calculated radiation pattern of a particular adapted/modified and height-restricted form/variation of dipole antenna (i.e. an antenna which is adapted/reconfigured compared to a conventional half-wave dipole antenna to have a low- profile or low-height physical structure but so as still to provide an overall radiation pattern shaped like a dropped-doughnut as shown), and which is placed in or on the road. Note that this radiation pattern is quite wide and flat (approximately toroidal and similar to the shape of a doughnut that has been dropped flat onto the ground and flattened somewhat). In terms of being height-restricted, the antenna should preferably be small enough - more preferably less than 50 mm tall and less than 300 mm in diameter - so as to be easily installed in or on the surface of a road. This antenna configuration should preferably also be such as to neutralise ground and surface effects which may occur.

[00131] It should be clear from the above that a RFID reader, incorporating an adapted form or variant of dipole antenna (being vertically polarised), which is operable to be placed in or directly on the surface of the road, which has a low physical profile, and which meets the radiation pattern shape requirements outlined above, may be highly desirable in the context of the presently discussed application involving reading RFID enabled vehicle license plates.

[00132] Those skilled in the art might suspect that a monopole antenna may be able to provide the presently desired radiation pattern. Indeed, a monopole antenna might be said to be an adapted type of (or a variant of or a special case of) dipole antenna. Indeed, as summarised in Wikipedia for example, a monopole antenna is a type of antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over a conductive ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the lower end of the monopole and the ground plane. One side of the antenna feedline is attached to the lower end of the monopole, and the other side is attached to the ground plane (which is often the Earth). This is slightly different to conventional half-wave dipole antennas which, as described above, generally consist of two identical rod conductors, with the signal from the transmitter applied between the two halves of the antenna. Nevertheless, like other conventional dipole antennas, a monopole antenna is a resonant antenna in that the rod functions as a resonator with standing waves of radio-frequency current flowing back and forth between its ends. The most common form of monopole antenna is the quarter-wave monopole, in which the rod length is approximately 1/4 of a wavelength of the radio waves (hence the name quarter-wave monopole).

[00133] However, whilst those skilled in the art might suspect that a monopole antenna such as a quarter-wave monopole may be able to provide a radiation pattern that is desirable for the vehicle identification applications discussed herein, unfortunately such standard monopole antennas are generally too tall (i.e. they don't meet the "height-restricted" requirement). Also, current knowledge on monopole antennas relates mostly to long distance communications where the bottom half of a conventional vertical dipole is replaced by a conducting ground plane (of sufficient size to approximate an infinite ground plane). This is especially the case for lower frequency, long distances transmission applications, and where earth may be approximated as an infinite conducting ground plane. The ground plane in the kinds of applications discussed herein will, however (as discussed above), generally be close to the antenna and imperfect and changing in nature.

[00134] Figure 24 illustrates the configuration of an antenna in accordance one particular embodiment of the invention that is thought to be particularly suitable for use in the vehicle identification applications discussed herein. The antenna in Figure 24 is actually a form of adapted/modified monopole antenna. The parts of this antenna, as labelled in Figure 24, include:

- a circular ground plane 24-3 (note that the circular ground plane 24-3 is slotted, as discussed below),

- a dielectric layer 24-4 of the same shape and located immediately beneath the ground plane (note that the inclusion of this dielectric layer 24-4 is optional, albeit preferable, and there may also be a (again optional) layer of conductive material (not shown in Figure 24) on the opposite side of the dielectric layer 24-2 from the ground plane - this layer (if present) will often be non-slotted such that it forms a non-slotted ground shield),

- an upright cylindrical monopole 24-2 (which in this case is in the shape of a short, squat cylinder) oriented vertically relative to the horizontal ground plane and perpendicular to the centre of the ground plane (note that the lowermost first end of the monopole 24-2 does not actually connect with or contact the ground plane 24-3; rather the monopole 24-2 hangs above the ground plane (suspended from the top load 24-1 ) such that the lowermost first end of the monopole 24-2 is separated from the upper surface of the ground plane 24-3 by a small gap (this gap is visible in Figure 30),

- a number (in this case four) of shortening poles 24-5, each extending vertically upwards from the ground plane, and the shortening poles 24-5 are all located at an equal radial distance outward from the monopole (i.e. they are all at an equal distance from the central vertical axis of the antenna) and with equal spacing around the monopole (note that the shortening poles form what might be termed a "bird cage" configuration), and a circular top load plate (a.k.a. simply "top load" or cap) 24-1 , which in this embodiment is of slightly lesser diameter than the ground plane 24-3, and which is mounted on top of the shortening poles 24-5 such that the top load 24-1 is parallel to, but spaced vertically above, the ground plane (and as mentioned above the monopole 24-2 is connected to, and effectively "hangs" from, the underside of the top load 24-1 ).

[00135] Note that the dielectric layer 24-4 (if present), the ground plane 24-3, the monopole 24-2 and the top load 24-1 are all mounted such that their circular centres coincide with the antenna's central vertical axis.

[00136] In Figure 24, the top load 24-1 is shown transparently. However this is merely so that other components (e.g. the monopole and the shortening poles etc) can be more easily made out. In practice, it is expected that the top load 24-1 will be made from a conductive material such as high-grade conductive copper or the like (and as such the top load 24-1 will likely be opaque). Similarly, other parts of the antenna including the monopole 24-2, the shortening poles 24-5, the ground plane 24-3 and the non-slotted ground shield (if present) will also likely be made from conductive materials, although there is no necessary requirement for them all to be made from the same material. Materials that may be suitable for these conductive parts of the antenna, and the issues associated with the selection of appropriate materials, will be familiar to those skilled in the art and therefore need not be discussed in detail. In any case, the invention is by no means limited to by any particular materials.

[00137] However, it should be noted that whereas most of the parts of the antenna will be made from conductive materials, as discussed above, the solid dielectric layer 24-4, which (if present) is located beneath the ground plane, should be made from a suitable solid material having appropriate dielectric properties. Possible candidate materials might include plastics, polymers, ceramics, some metal alloys or oxides, etc. In any case, any solid dielectric material known by those skilled in the art to be suitable (and preferably with ordinary permittivity and zero (or low) conductivity) may be used for the dielectric layer 24-4 (if present). The solid dielectric layer (and the material chosen for the formation of the dielectric layer) may also help to improve the mechanical strength of the antenna overall, for example if the ground plane is formed from a thin (and consequently flexible or insufficiently rigid) layer of conductive copper.

[00138] It should also be noted that, although not solid physical parts of the antenna, the space above the ground plane 24-3 and below the top load 24-1 , and the space radially to the outside of the antenna between the antenna and a cavity in which the antenna is located (see Figure 30), and in particular the air located in these regions, are also very important as this too has dielectric properties which significantly affect the performance of the antenna.

[00139] Electronics associated with the antenna (e.g. electronic chips, circuitry, cabling, etc, which may form part of the RFID reader, its controller, etc) should preferably be mounted (or otherwise located) vertically underneath the ground plane 24-3 (or beneath the dielectric layer 24-4 and/or ground shield (if present)). This is so that these electronics are shielded from the antenna by the ground plane 24-3 (or by the ground plane plus the dielectric layer/ground shield), and so that the antenna is shielded from the electronics. However, in addition, it may also be possible for further electronics and/or a further antenna to be provided. (An additional antenna might be used for non-RFID purposes, for example Wi-Fi communication or the like which is in addition to the main antenna's RFID function.) If present, such further electronics and/or additional antenna may be positioned on top of (or otherwise vertically above) the top load 24-1 . If positioned above the top load 24-1 , such electronics and/or further antenna may be shielded from the main antenna by the top load 24-1 . Note that Figure 24 does not depict any further electronics or additional antenna (it merely depicts the main antenna). [00140] As mentioned above, in the antenna depicted in Figure 24, the circular ground plane 24-3 is actually slotted. More specifically, the ground plane 24-3 contains periodic slots. In other words, the ground plane 24-3 is a periodically slotted ground plane. In this particular embodiment, there are four distinct sets of slots. In each set, there is a number (six) of arcuate slots. Each slot is shaped like a short arc oriented concentrically with the ground plane's circumference. And in each set the respective slots are radially spaced equally from one another, and each set extends radially outwards from the position of one of the respective shortening poles 24-5. In each set, the arcuate length of the individual slots becomes greater as the radial distance from the centre of the antenna increases. Each set of arcuate slots is separated from the adjacent set of slots by a solid, un-slotted portion of the ground plane 24-3. It should also be noted that the slots, in this embodiment, are cut (or otherwise formed to extend) through the thickness of the ground plane, although in other embodiments the slots might be merely indented into the surface of the ground plane without extending through the full thickness of the ground plane. In any case, in the depicted antenna, the slots do not extend into (or at least they do not extend all the way through) the thickness of the dielectric layer 24-4 (although the slots may do in other embodiments). Importantly, the number, the relative shape, the relative size, the relative depth (into the ground plane and/or the dielectric layer), the relative position, etc, of the slots may be varied in order to alter the performance of the antenna (i.e. these things may be varied in order to "tune" the antenna). In fact, the ability to alter the configuration of the slots is one of the important ways in which an antenna of this kind may be tuned. In general terms, the function of the periodic slots in the ground plane is (it is thought) to help ensure uniformity of the antenna's radiation pattern for the desired read zone, and also to help to negate (or minimise) the variable ground effect. The slotted ground plane is also thought to help reduce the antenna return-loss but without requiring undesirably large increases in the ground plane size/dimension. In other words, it is thought that the use of a slotted ground plane may help to reduce return-loss whilst also allowing the ground plane size/dimension to remain sufficiently small/compact. The actual way in which the configuration of the slots is varied, and the affect different changes in slot configuration may have in terms of antenna performance, is outside the scope of the present disclosure.

[00141] Importantly, the radiation pattern of the antenna in Figure 24 is a highly desirable (possibly near perfect or near ideal) "dropped doughnut" or a "toroid on the ground" shape, as depicted by Figure 23. Accordingly, the particular antenna in Figure 24 provides a radiation pattern of a shape (shown in Figure 23) which is thought to be highly desirable/beneficial/functionally suited for RFID readers which are to be used in "on-road" or "inroad" placement locations in vehicle identification applications. By way of further explanation, the shape of the antenna radiation pattern depicted in Figure 23 is still generally "toroid" like. However, compared to say the quite-high/near-spherical toroid shape of the radiation pattern in Figure 18, which is the radiation pattern of an ideal half-wave dipole, the shape of the radiation pattern in Figure 23 (which is the radiation pattern for the antenna depicted in Figure 24) is generally lower and flatter. That is, it is slightly "squat" or "squashed" in the vertical direction, and this is actually thought to be desirable/advantageous because it means that the antenna's energy extends generally more in the horizontal plane (in all directions) and less in the vertical direction (which means the antenna's beam may extend further outwards horizontally but there may also be less "blinding" from the underside of vehicles etc due to the comparatively lesser amount of energy directed in the vertical direction). Basically, the shape of the radiation pattern is similar to that labelled 12-4 in Figure 12 and 13-3 in Figure 13, which is thought to be highly desirable/beneficial/functionally suited for RFID readers for reasons discussed above.

[00142] As mentioned in passing above, the positioning of the shortening poles 24-5 around the monopole 24-2 forms what might be termed a "bird cage" configuration. Accordingly, the configuration of the antenna in Figure 24 might be termed a birdcage configuration (or the antenna therein might be termed a birdcage antenna).

[00143] Importantly, whilst the birdcage antenna in Figure 24 is quite substantially modified/reconfigured, as compared to say a conventional half-wave dipole or quarter-wave monopole for example, nevertheless the birdcage antenna is still a species or kind of dipole (or monopole) antenna. As such, the birdcage antenna in Figure 24, like other conventional dipole (or monopole) antennas, is a resonant antenna. Accordingly, the sizes of the various parts of the birdcage antenna are inherently and necessarily dependent on the frequency of the radio signal with which the antenna is to operate. Or equivalently, it might be said that the sizes of the various parts of the birdcage antenna are inherently and necessarily dependent on the wavelength of the radio signal at the specified operating frequency. (Note: assuming the speed of light (c _SL ) is constant at c _SL = 299.79 x 10 ⁶ (m/s), then the wavelength (A) of a radio signal is related to the frequency (/) of the said radio signal by c _SL = fX or λ = ^C∑L /†-)

[00144] In the RFID applications described above for which the birdcage antenna in Figure 24 is thought to be highly suited, operating frequencies will generally be in the ultrahigh frequency (UHF) range. By way of example, typical operating frequencies for an antenna such as this might be in the range of, say, 860-960 MHz. In any case, because the size of the various parts of the birdcage antenna must necessarily change depending on the operating frequency with which the antenna is to be used, it is largely meaningless to describe specific sizes for the individual parts of the antenna (as this would, at best, describe an antenna suitable for use with one specific operating frequency only). Instead, it is more useful to describe the size of the various parts of the birdcage antenna with reference to (or as a function of) the wavelength of the radio signal. This is illustrated in Figure 30.

[00145] The following table outlines the various dimensions of the antenna, which are depicted in Figure 30, with reference to (or as a function of) the wavelength (λ) of the radio signal.

[00146] Note that, in Figure 30, there are a few dimensions depicted which are not given as a function of signal wavelength. This is because these dimensions often will not, or will not always, vary (like the other dimensions do) as the configuration of the antenna is varied to operate with different signal frequencies. These possibly non-varying dimensions depicted in Figure 30 include:

- the thickness of the antenna ground plane 24-3 (in Figure 30 this is actually the combined thickness of the antenna ground plane 24-3 and the dielectric layer 24-4) which in this example is -3 mm,

- the diameter of the shortening poles 24-5 which in this example is also -3 mm,

- the -10 mm space between the underside of the antenna (the underside of the antenna is either the underside of the ground plane, or the underside of the dielectric layer if the dielectric layer is present) and the bottom of the cavity into which the RFID reader (of which the antenna forms part) is inserted, and

- the small (typically -1 -2 mm) gap between the upper surface of the ground plane 24-3 and the bottom/first end of the monopole 24-2.

[00147] Note that the space mentioned in the last bullet point above, namely the -10 mm space between the underside of the antenna and the bottom of the cavity into which the RFID reader is inserted, is the space in which electronics such as the RFID reader controller, etc, will normally be located. Note also that, in Figure 30, there is a black solid line extending down from the base of the monopole into this space beneath the antenna. This solid line may be thought of as representing the feedline connecting the antenna to the controlling electronics. There are a number of other things that should be noted from Figure 30. For instance, as mentioned above, the lower end of the monopole does not actually connect/contact with the slotted ground plane (the slotted ground plane is represented by dashed lines in Figure 30; the dashes are intended to represent the slots). Thus, as depicted in Figure 30, there is a small horizontal gap between the upper surface of the ground plane and the lower end of the monopole. Also, there is a solid black line extending along the bottom/underside of the dielectric material. This is intended to represent the (typically non-slotted) conductive ground shield (which, like the dielectric material, is optional). There is also a box 30-1 illustrated in Figure 30. The vertical side edges of the box 30-1 extend along the side edges of the cavity in which the antenna is located, the lower horizontal side of the box extends along the base of the cavity, and the top of the box extends slightly above the level of the ground. The box 30-1 is intended to represent (the outline of) the housing or casing of a RFID reader. That is, the housing or casing within which the antenna (plus other reader electronics, power source or power connections, etc) are located.

[00148] A further important point to note, and one which is also signified by the ( ^* ) in the headings of the table above, is that the dimensions listed in the table and in Figure 30, even when given as a function of the signal wavelength, are approximate only. In other words, these approximate dimensions form a starting point from which an appropriate antenna configuration may be obtained for a specific intended operating signal frequency. Therefore, after the specific signal frequency with which an antenna is to operate has been determined, the particular dimensions given above (and illustrated in Figure 30) may be used as a starting point, and thereafter further fine adjustment of the antenna configuration (e.g. by slightly altering one or more of the said dimensions whilst keeping others fixed, amongst other things) may be used to "fine tune" the antenna for maximum performance (return loss, resonance, bandwidth, etc). (This fine tuning may also involve altering the particular configuration of the slots in the ground plane, altering dimensions or design attributes of the antenna that are not related to signal wavelength, etc.) Such "fine tuning" within size, design and other physical constraints is common to most, if not all, antenna design, as those skilled in the art will readily understand. By way of example (and it is to be stressed that this is a purely hypothetical and non-limiting example), for an antenna having a configuration like the one depicted in Figure 24 and Figure 30 and which is to operate with a signal of frequency /=936.85 MHz and wavelength of λ =320 mm (i.e. as per the example in the table above), the actual dimensions of such an antenna after fine tuning might be: a=28 mm; b=45 mm; c=1 10 mm; d=128 mm; e≥170 mm; and f=20 mm. Again, this is a purely hypothetical and non-limiting example.

[00149] Even though the antenna dimensions which are given with reference to signal wavelength in the table above are approximate only, as has just been explained, it can nevertheless be appreciated from the discussion of the general antenna configuration in Figure 24 and Figure 30 that for signal frequencies between 860-960 MHz, the signal wavelength will be between 348 mm and 312 mm, and even with a wavelength towards the upper end of this range (348 mm), the overall antenna should still fit within a structure, such as the housing of a RFID reader, having a diameter of, say, 180 mm or less and a height of, say, 40 mm or less. In fact, the antenna should fit within a housing of this size and still leave room for the frame and supporting parts of the housing, etc. furthermore, the housing itself (even if fully recessed/buried in the surface of the road, with no part left above the road surface) should not penetrate the road by much (if any) more than 40 mm. This should therefore allow such an RFID reader housing to be placed in the road without compromising the integrity of the road. The gaps between the antenna (or the RFID reader housing) and the road structure generally should not cause the radiation properties of the antenna to change, however, this may influence the antenna return loss. In any case, this minor effect might be compensated for by adjustment to the input power level to help guarantee sufficient power emitted from antenna body.

[00150] In comparison with the antenna discussed above with reference to Figure 24 and Figure 30, an "inverted F antenna" (IFA) as depicted in Figure 25 may be a simpler alternative construction to achieve an antenna that has a low profile physical structure and which is able to provide a low, flat radiation pattern. As shown in Figure 25, the IFA therein has an upstanding antenna element (the resonant part of the antenna) which is shaped like a sideways capital letter "F". The single long edge of the "F" is oriented parallel to the antenna's ground plane. The "F"-shaped antenna element also has two "prongs". The prong which would be the lower of the two if the "F" were in a normal upright orientation will be referred to here as the second prong. The other prong, namely the one which would be the upper of the two if the "F" were in a normal orientation, will be referred to here is the first prong. As illustrated in Figure 25, the second prong extends vertically downwards from partway along the long horizontal portion of the "F" and inserts through a small hole in the centre of the antenna's ground plane (note that the second prong inserts through the hole in the ground plane but does not contact with the ground plane). The first prong extends vertically down from one end of the long horizontal portion and connects to the antenna's ground plane slightly to one side of the second prong.

[00151] Unlike the ground plane of the antenna in Figure 24 (which is round), the ground plane of the IFA in Figure 25 is rectangular. In Figure 25, even though no dielectric layer or underlying conductive shield layer is depicted, these could optionally be provided. Also, the configuration of the periodic slots in the ground plane differs. In the particular IFA depicted in Figure 25, there are two sets of slots. In each set, there is a number (1 1 ) of arcuate slots. Each slot is shaped like an arc, although it will be noted that the slots in this IFA ground plane are shaped like longer arcs than the slots in the ground plane in Figure 24. Having said this, the slots in the ground plane of the IFA in Figure 25 are similar to the slots in the ground plane in Figure 24 in that they are oriented to form concentric arcs centred on the centre of the antenna. And in each set of slots in Figure 25, the respective slots are radially spaced equally from one another, and each set extends radially outwards in a direction perpendicular to the axis of the horizontal portion of the "F". In Figure 25, the arcuate length of the individual slots becomes greater as the radial distance from the centre of the antenna increases, and one set of arcuate slots is separated from the other set of slots, on both sides, by a solid, un-slotted portion of the ground plane. The slots in the IFA ground plane are formed through the thickness of the ground plane. Importantly, as for the antenna depicted in Figure 24 above, the number, the relative shape, the relative size, the relative depth (into/through the ground plane and/or into any underlying dielectric layer), the relative position, etc, of the slots may be varied in order to alter the performance of the IFA (i.e. these things may be varied in order to "tune" the antenna).

[00152] One difficulty associated with an IFA is that the asymmetrical configuration of an IFA can result in a non-perfect (in particular non-symmetrical) toroid radiation pattern. In the example in Figure 25, the particular configuration of the periodic slotted ground plane is used to for several reasons including: to help correct the said asymmetry of the radiation pattern, to reduce the size of the ground plane, to manipulate the surface impedance to help ensure a uniform radiation pattern, and to limit static and changing ground effects. It is thought that, in this way, the periodic slotted ground plane may be matched (at least to a suitable extent) to direct the beam of the IFA up and down the road. In Figure 25 the IFA is matched with a rectangular periodic slotted ground plane, etc, as discussed above, and this particular configuration results in a radiation pattern up and down the road as illustrated in Figure 26.

[00153] The periodic slotted ground plane of the IFA could possibly also be used/changed to correct/adapt the IFA's radiation pattern to be nearer to the preferred "dropped doughnut" or "toroid on the ground" shape shown in Figure 23, but with such an alternative configuration the IFA would likely still generate substantially more vertically upward energy than the particular top loaded monopole (i.e. "birdcage") antenna depicted in Figure 24. A cap or top load may be used to reduce upwards radiation (as is indeed the case in the birdcage antenna in Figure 24). Such a cap or top load might therefore also be used with an IFA. However, in the case of an IFA, due to the antenna's inherent asymmetry, the cap or top load placement must generally also be asymmetrical, and the antenna's radiation pattern, it has been found, is extremely sensitive to the cap placement. The overall height of the antenna is also increased with the cap. Nevertheless, it is thought to be possible that through appropriate tuning and use of a top load, etc, an IFA configuration may be used to provide a height-restricted antenna which also has an overall low, flat shaped radiation pattern, as desired.

[00154] Following on from the above IFA discussion, and as a further possible enhancement thereto, an additional (typically higher frequency) antenna may be integrated into the top load or cap of an IFA. Such an additional antenna may be used to provide data communications from the device (where the device incorporates the main antenna with the "dropped doughnut" radiation pattern used for RFID) to another device such as a controlling device. For example, the additional antenna may be used for Wi-Fi or WAVE, as described in the IEEE 802.1 1 standard set. Figure 27 illustrates a possible example of a capped IFA with an elongated slot formed in the cap which functions as the additional antenna. In this particular example the slot forms a 2.4 GHz Wi-Fi antenna. The periodic slotted circular ground plane in this example is used to correct for or accommodate the resulting imbalanced radiation pattern. Figure 28 illustrates the Wi-Fi radiation pattern, which may be ideal for an in/on road device to communicate with another device located on the roadside, or on a vehicle or on a pole, etc. The combined use of the RFID antenna and the additional data communications capability that may be provided by the additional antenna may help to reduce deployment and maintenance costs.

[00155] As has been discussed at length above, the antenna proposed herein will often be (and should be suitable to be) placed on the ground, or in the ground just below the surface, or at any position in between, as conditions or the application requirements dictate. This is illustrated in Figure 29.

[00156] Another possible application for antennas in accordance with the present invention is to place the antenna upside down on the underside of a bridge or a gantry. A conducting reflector (preferably one having a diameter λ) may be mounted on the underside of the overhead structure, such that the reflector becomes mounted between the structure and the antenna on the underside of the structure. A birdcage antenna like the one in Figure 24 for example may thus be mounted beneath and in the centre of this reflector with a standoff between the reflector and the antenna ground plane of ~ λ/16. Such a configuration may thus use the same birdcage antenna as described above but the toroid radiation pattern may be pushed downwards, preferably with the angle of maximum gain at ~45°. This change in the radiation pattern (i.e. with the angle of maximum gain pushed down preferably by -45°) may (it is thought) be caused largely by the use of the conducting reflector mentioned above. The conducting reflector may also (it is thought) help to isolate the unpredictable near-field effect caused by the bridge/gantry structure and also help to maintain antenna efficiency. This radiation pattern (i.e. with the angle of maximum gain pushed down preferably by -45°), which may be suitable for use with windscreen tags that are mounted so as to be effectively vertically polarised, may create a RFID beam which is effective for reading windscreen tags, possibly nearly as well as plate tags, in 2 lanes in any direction. It is believed that the vertical polarisation may (even in this "upside down" configuration) alleviate some of the multi-path problems described above. The toroid radiation pattern may also alleviate blinding reflections of the roof of, for example, buses. The small size of the antenna may also be very useful where space is restricted, as under bridges and in tunnels.

[00157] In the present specification and claims (if any), the word 'comprising' and its derivatives including 'comprises' and 'comprise' include each of the stated integers but does not exclude the inclusion of one or more further integers.

[00158] Reference throughout this specification to One embodiment' or 'an embodiment' means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearance of the phrases 'in one embodiment' or 'in an embodiment' in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more combinations.

[00159] In compliance with the statute, the invention has been described in language more or less specific to structural or methodological features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims (if any) appropriately interpreted by those skilled in the art.