Field of the invention

The present inventive concept relates to the field of radiofrequency antennas, especially to patch antennas. In general, patch antennas have antenna elements disposed as a relatively thin layer of conductor material onto another material.

Background to the invention

Bancroft and Bateman's paper An Omnidirectional Planar Microstrip Antenna (IEEE Transactions on Antennas and Propagation, Vol. 52, No. 11, November 2004) describes a printed planar microstrip antenna for use in this technical field.

The described antenna has a substrate with a front face and a back face onto each of which are arranged alternating wide sections (ground planes) of antenna and narrow microstrip sections (patches) of antenna, with the ground planes on one face of the substrate being aligned with patches on the other face of the substrate. The Bancroft and Bateman arrangement is shown in Figure 1, which is an extract of the aforementioned paper, in respect of which all rights are acknowledged and respected.

The skilled reader will appreciate that the combination of patch and ground plane with a degree of separation and/or insulation therebetween gives rise to an antenna effect. Additionally where ground planes in close proximity are at different potentials due to fluctuations in the radiofrequency signal, that also gives rise to an antenna effect.

The radiating elements are a pairing between each of the patches and the ground planes. These patch and ground plane pairings in conjunction with one another in close proximity interact with one another at radio frequencies. In the cited prior arrangement patches and ground planes alternate on the same face of the substrate, so that electromagnetic energy flows from one patch to the adjacent ground plane (and so on) on the same face, and between the faces of the substrate by electrically conductive shorts or vias. The cumulative effect is seen at the driving or feed point.

Importantly, the Bancroft and Bateman arrangement provides for good transmission and reception of an electromagnetic signal in approximately all directions outwards from an axis of the antenna due to the arrangement of alternating patches and ground planes on the same face of the substrate. This is shown in Figure 2 in the present application, where the antenna is shown without the substrate. The arrows in Figure 2 indicate electromagnetic signal transmission and reception away from the substrate; the skilled reader will appreciate that in practice transmission and reception arises to varying degrees in approximately all directions from an axis as described above, so that the antenna is described as being omnidirectional.

The Bancroft and Bateman arrangement is also disclosed in US-A-2007/0052593.

Summary of invention

The present inventive concept provides a radiofrequency antenna adapted to be driven by an external source, the antenna comprising a substantially insulating substrate having at least two face portions in different geometric planes, the antenna comprising a plurality of radiating elements arranged end to end substantially co-linearly along an axis of the substrate, each radiating element having a ground plane and a patch substantially aligned with each other along an axis of the antenna and disposed on a different face portion of the substrate from each other, wherein a majority of the patches are connected electrically to a ground plane of a neighbouring radiating element and a majority of the ground planes are connected electrically to a patch of a neighbouring radiating element, and wherein a majority of the ground planes are arranged on the same face portion of the substrate as each other and a majority of the patches are arranged on the same other face portion of the substrate as each other, and wherein neighbouring ground planes are immediately electrically isolated from one another on the respective face portion of the substrate and neighbouring patches are immediately electrically isolated from one another on the respective face portion of the substrate.

In other words, for each radiating element, the ground plane and patch are not directly connected electrically to one another and on the respective face portions of the substrate the neighbouring ground planes are not directly connected to one another and neighbouring patches are not directly connected to one another. A patch may be connected electrically to a neighbouring ground plane by way of a via. A ground plane may be connected electrically to a neighbouring patch by way of a via.

This arrangement changes the shape/distance/material between any two neighbouring ground planes - they are now both on one side (in plane) of the substrate with a finite amount of air and/or substrate in the gap between them. This is in contrast to the previously known arrangement, in which any two neighbouring ground planes were on different faces of the substrate, out of plane, and with solely substrate between them.

The said vias provide reactive elements distributed along the antenna. An antenna at its resonant frequency should be "matched" to the radio source driving it. The tuning of the antenna can swing either side of the optimum by being too capacitive or too inductive at the frequencies of interest. Inductive or capacitive features of an antenna are grouped together under the term "reactive" because unlike resistance the effect they have is dependent on frequency. By distributing vias across the antenna, reactive elements (mainly inductive) can be provided to change the tuning of the antenna. The effect seems to be attributable to the change (with respect to previous arrangements) in ground plane spatial relationships to each other, at their ends along the axis, and the distributed reactive features that the vias comprise. Furthermore, ground planes generally act as a barrier or shield to transmission and reception of an electromagnetic signal especially, so the arrangement provides a directionality or region in which the transmission and reception of such a signal is significantly less - especially in the region close to the ground planes. The shape etc. of this directionality or region will depend on the shape of the substrate, for example, as well as the sizes of ground planes, gaps therebetween and other factors. A key advantage is that transmission and reception losses can be reduced by this arrangement.

The inventive concept is advantageous because it provides for good transmission and reception in a more clearly defined directionality or region. Thus, for example, the antenna can provide more efficient transmission and reception in a situation whereby in one directionality or region an electromagnetically significant object, material or interference is present by directing the transmission and reception away from that object, material or interference.

For example, the antenna can be used to provide efficient transmission and reception when mounted on a person's body, or on part of a vehicle body, each of which is likely to comprise electromagnetically significant matter. If interference is likely to be expected from a particular direction then the antenna can be arranged and oriented so that it is able to transmit and receive efficiently in other directions such as away from that expected interference direction. For example if the antenna is mounted to a flying vehicle and interference is expected from below the vehicle in flight, the antenna can be mounted so as to provide efficient transmission and reception above the vehicle in flight - for example to a satellite. When the antenna is mounted close to a person's body, losses are reduced and it can provide a lower specific absorption rate (SAR) in respect of the body by mounting the antenna with the ground planes closer to the body than the patches are. Lower SAR is a further advantage due to less radiation being received by living tissue, with resulting health benefits.

The present inventive concept performs quite differently in practice than previously known arrangements. . The antenna propagates a high proportion of its electromagnetic energy away from the ground planes because of the arrangement of patches and ground planes on the same respective face portions of the substrate.

The substrate may be curved. In such a curved arrangement the propagation can be in several different directions (a degree of partial omnidirectionality) save for the directions/regions attenuated or shielded by the ground planes arrangement.

The antenna as described has been found to have good performance across a wide range of frequencies. Thus for a particular selection of dimensions for each ground plane and patch, materials etc. the antenna of the present inventive concept can provide high transmission and reception performance (i.e. low reflection) at a wide range of radio frequencies.

The antenna is thus especially useful in environments which were previously harder to effect good transmission and reception.

Furthermore, the antenna of the present inventive concept has been found to be useful in situations where, previously, circular polarisation would have been assumed to be desirable, such as when used for communication with satellites. For the avoidance of doubt, the present inventive concept is not expected to give rise to circularly polarised electromagnetic radiation. Thus, the present antenna provides a somewhat surprising result in those situations.

In an optional arrangement substantially all of the ground planes are arranged on the same face as one another, and substantially all of the patches are arranged on the same face as one another.

Providing a plurality of ground planes and patches provides for good performance. However it has been found that for different purposes and tuning frequencies the preferred number of ground planes and patches in an array as described can vary. By way of example only, the minimum number of ground planes and patches in an array is two of each. The applicant has successfully tested antennas with up to thirteen each of ground planes and patches, and believes that antennas with more than thirteen each are likely to provide good performance. In practice, most of the antennas of this type which have been trialled have seven of each of ground planes and patches. The antenna effect arises from electromagnetic interplay at an interface between the patches and ground planes and at an interface between radiating elements (a pairing between a patch and ground plane) in close proximity to one another. In general, a patch is adapted to give rise to less electromagnetic shielding/attenuation than a ground plane due to its size and shape.

One preferred patch is a thin strip - referred to herein as a microstrip. A microstrip patch has a conductor width significantly smaller than its length. A microstrip can be substantially straight. Alternatively a microstrip can form a meandering path. A meandering path microstrip can provide a longer section of conductor for the length of substrate on which it is arranged.

Further envisaged patch configurations include a branched strip, a polygonal shape, a fractal shape. Differently sized and shaped patches can provide differing bandwidth performance depending on the situations in which the antenna might be deployed. In general the more conductor is present in a microstrip the wider the bandwidth performance, with a potential trade off with amplitude performance due to shielding/attenuation.

Preferably, apart from potentially at certain shorting points each ground plane and each patch is substantially the same length. This has been found to provide good transmission and reception efficiency.

Preferably, the ground planes and patches are arranged on the substrate with a relatively small gap therebetween so that the respective lengths can be matched.

The provision of a gap between respective ground planes and patches provides a good antenna effect. The ideal gap for a particular embodiment of the present antenna will vary according to the properties of the materials used, such as the dielectric constant of the substrate.

The patches can be arranged in a staggered fashion along the length of the substrate, for example each offset from a central axis of the substrate. Preferably, however, the patches are arranged substantially co-linearly with respect to one another. In other words, the patches are preferably arranged end to end along the substrate with a small gap therebetween. Preferably, the ground planes and patches are arranged substantially linearly along an axis of the substrate. If the substrate is curved, then the said axis will be correspondingly curved.

The predominant shape of each ground plane is preferably a rectangle. To enable a substantially linear arrangement of patch and ground planes and described, the ground planes can comprise one or more interlocking portion at an end thereof adapted to correspond to an interlocking portion of a neighbouring ground plane. The interlocking portions can form a kink or tab which fits into a corresponding void in the neighbouring ground plane. These interlocking portions, enabling a linear arrangement, provide for improved performance of the antenna.

Preferably, each patch connects to a corresponding ground plane in series so that if a direct current were to be fed to the antenna it would pass along substantially the whole length of the patch and along the corresponding ground plane before passing to the next patch, and so on. Thus, a direct current entering the antenna at a driving point would pass through alternating patches and ground planes until it reached a final patch or ground plane then would loop back through the remaining alternating ground planes and patch. In other words, preferably each patch is connected directly only to one or more neighbouring ground plane and each ground plane is connected directly only to one or more neighbouring patch.

For a radiofrequency signal, the skilled reader will appreciate that it is less relevant to describe an antenna in terms of a flow of direct current. However, the connections between the electrical elements of the antenna are described to make clear what elements are connected directly or only indirectly to one another.

The said sections of electrical conductor may be connected in series with one another by way of an intermediate conducting element. A preferred intermediate conducting element is a via. A via may be provided by creating a hole through the substrate (for example by drilling) and lining it with a conducting material (for example by plating). Alternatively a wire or flat conductor, such as a microstrip transmission line, may be provided as an intermediate conducting element.

The antenna may be driven by a direct electrical connection to one of the sections of electrical conductor, by a proximity-coupled feed or by an aperture-coupled feed. A direct electrical connection is preferred but the other arrangements are valid alternatives. For the avoidance of doubt, throughout this description the term "driven" has been used and the skilled reader will appreciate that because antennas are reciprocal the term includes both excitation of the antenna by an electrical source and the converse in which the antenna is excited by a received electromagnetic source to provide an electrical signal to the external source.

The antenna may further comprise one or more parasitic elements. Parasitic elements are conductive elements which are not electrically connected to other elements of the antenna, and are sometimes referred to as directors. One or more parasitic elements may be arranged nearby one or more of the patches.

Preferably the substrate is flexible or deformable. A flexible or deformable substrate enables an antenna to be arranged on an object which is not of precisely predictable dimensions. The substrate may comprise fibreglass. The substrate may comprise polytetrafluoroethylene (PTFE). The substrate may comprise a plurality of layers.

In an envisaged arrangement, the substrate is curved. Such a curved arrangement enables the antenna to be arranged on an object which is not flat. In one envisaged use case, the antenna may be shaped to fit over a user's shoulder, with portions of the substrate arranged approximately vertically to the front and back of a user's shoulder with a bent section to fit the antenna over the said shoulder. In this case the ground planes would generally be arranged closer to the user's body than the patches would generally be. This provides - as described - limited transmission and reception towards the user's body - which provides for more efficient transmission and reception in other directions. Arranging the substrate as described can provide a somewhat hemispherical transmission/reception directionality. As discussed above such an arrangement can provide a low specific absorption rate (SAR) in respect of the body with accompanying health benefits.

In a further envisaged use, the antenna may be shaped to attach to an exterior part of a vehicle.

Ideally, the conductor is selected from materials having high electrical conducting properties. A preferred material for the conductor is copper. The conductor may be provided on the face portions of the substrate by deposition or etching for example.

Antennas according to the present inventive concept can provide good performance at several different frequencies across a wide range of radio frequencies. For example, a single antenna tested by the applicant was able to transmit well at several frequencies between 200 MHz to 2.5 GHz.

A particular antenna according to the present inventive concept will tend to provide peak performance at several different frequencies and ranges of frequencies within a wide range. The specific frequencies of peak performance will depend on several factors, such as the physical dimensions of the ground planes and microstrips, the dielectric constant of the substrate, presence and nature of electromagnetically significant substances (including a user's body, or a vehicle) and the shape of the substrate (for example if curved). Thus, the skilled person will appreciate that the inventive concept includes a range of specific configurations within the scope of the inventive concept as described.

For example only, an antenna with seven patches may have seven microstrips having a width of approximately 1.16mm, five of which microstrips away from ends of the antenna have a length of approximately 39mm and two of which microstrips (one towards each end of the antenna) have a length of approximately 19.5mm, and the antenna could have seven ground planes with a width of approximately 12mm and a length of approximately 39mm.

In a generalised embodiment the present inventive concept can be said to provide a radiofrequency antenna comprising a substantially insulating substrate having at least two face portions in different geometric planes, the antenna comprising a plurality of ground planes and patches each being sections of electrical conductor arranged on a face portion of the substrate, the sections of electrical conductor being connected in series with one another, the antenna being driven by an external source, and wherein at least one ground plane is arranged on a different face portion from an electrically adjacent patch. Thus in general, ground planes and patches alternate in series electrically so that each ground plane is electrically adjacent one or more patch and thus each patch is electrically adjacent one or more ground plane. Detailed description of the invention

An exemplary embodiment will now be described in further detail with reference to the accompanying drawings, in which:

Figure 3 shows two plan views from opposite sides of an exemplary antenna;

Figure 4 shows a perspective representation of certain components of the same antenna of Figure 3; especially the antenna is shown without the substrate;

Figure 5 illustrates the electrical connections between sections of conductor in the antenna of Figure 3;

Figure 6 shows a plan view an alternative exemplary arrangement of an antenna;

Figure 7 shows a perspective view of an envisaged arrangement of an antenna of the present inventive concept, in a curved arrangement;

Figure 8 shows an electric field simulation of an embodiment similar to that shown and described in relation to Figures 3 to 5;

Figure 9 shows a result of a similar simulation of the electric field as shown in Figure 8 as a set of contours; Figure 10 shows a closer view of a similar simulation as shown in Figure 9;

Figure 11 shows a characterisation of electric field contours relating to a section of the prior art arrangement shown in Figure 1; and

Figure 12 shows a characterisation of electric field contours relating to a section of an embodiment of the arrangement of the present inventive concept. In Figure 3, an antenna 10 is shown with a front face portion F and a back face portion B. Antenna 10 has a substrate 12 which is substantially electrically insulating. On the front face portion F a series of five electrically conducting microstrip patches 14a, 14b, 14c, 14d, 14e are arranged co-linearly along axis A and with a small gap therebetween. Each of the patches 14 has a via 16 at each end (not all labelled so as to aid clarity in the drawing), each via 16 being an electrically conductive passage through the substrate 12 for electrical connection to the back face portion B of the antenna 10. On the back face portion B a series of five electrically conducting ground planes 18a, 18b, 18c, 18d, 18e are arranged co-linearly along axis A and with a small gap therebetween. Each of the ground planes 18 has at least one via 16 connected thereto (not all labelled so as to aid clarity in the drawing). The patches 14 and ground planes 18 are thus electrically connected to one another in a specific series. Each of the patches 14 is connected to one or more neighbouring ground planes 18 and not directly to the particular ground plane which is on the other side of the substrate 12 aligned along the axis A of the antenna.

In other words for a particular radiating element, say patch 14c and ground plane 18c, the patch and ground plane are not directly connected to one another; the patch 14c is connected to ground planes 18b and 18d; the ground plane 18c is connected to patches 14b and 14d.

Each ground plane 18 is generally rectangular. Adjacent ground planes 18 are provided with respective interlocking portions 20, 20' (not all labelled so as to aid clarity in the drawing) whereby an interlocking portion 20 in one ground plane 18 corresponds to an interlocking portion 20' of a neighbouring ground plane 18. The interlocking portions 20, 20' thus form a kink which fits into a corresponding void in the neighbouring ground plane 18. The antenna 10 can be driven externally by way of a driving point 22 which can be at one of the vias 16.

Each patch 14 has a conductor width significantly smaller than its length, and is substantially straight.

Substantially each ground plane 18 and patch 14 is substantially the same length as one another.

In Figure 4 the arrows indicate that the electromagnetic signal transmission and reception provided by the antenna 10 in use is predominantly not in a direction below the ground planes 18 in the orientation of the drawing. The reader will appreciate that transmission and reception will in practice be in most directions, with significantly less in the downward direction as depicted. Individual components are not labelled to aid clarity.

Figure 5 illustrates how the electrical connections are arranged between sections of conductor in the antenna. For clarity, some labels have been omitted but the skilled reader will appreciate that the conductor sections correspond to those described with respect to Figure 3. Each patch connects to a corresponding ground plane in series so that a direct current would pass along substantially the whole length of the patch and along the corresponding ground plane before passing to the next patch, and so on. Thus, a direct current entering the antenna at a driving point would pass through alternating patches and ground planes until it reached a final patch or ground plane then would loop back through the remaining alternating ground planes and patch.

Each of the patches 14 is connected to one or more neighbouring ground planes 18 and not directly to the particular ground plane which is on the other side of the substrate 12 aligned along the axis A of the antenna.

In other words for a particular radiating element, say patch 14c and ground plane 18c, the patch and ground plane are not directly connected to one another; the patch 14c is connected to ground planes 18b and 18d; the ground plane 18c is connected to patches 14b and 14d. These connection relationships are shown in Figure 5 as straight dashed arrows.

Figure 6 shows an alternative exemplary antenna 10'. Figure 6 is a plan view showing predominantly the front face of the antenna 10' but with features of the back face shown in dashed lines for context. This exemplary embodiment may further assist the reader to understand the electrical relationships between the patches 14' and ground planes 18'. The antenna 10' has a substrate 12' onto a front face portion of which are arranged a series of patches 14' (not all labelled for clarity). On a back face portion of the substrate 12', shown in dashed lines here, are arranged a series of ground planes 18' (not all labelled for clarity). Patches 14' and ground planes 18' are electrically connected through the substrate 12' by way of vias (not all labelled for clarity). In this exemplary embodiment, the patches 14' are arranged in a parallel staggered fashion along the length of the substrate 12'. This arrangement is an envisaged embodiment of the present inventive concept, and the drawing is also intended to help show how the patches 14' and ground planes 18' can be connected together in series. Thus, each patch 14' connects to a corresponding ground plane 18' in series so that in use a current passes along substantially the whole length of a patch 14' and along the corresponding ground plane 18' before passing to the next microstrip, and so on. Thus, a current entering the antenna at a driving point 22' would pass through alternating patches 14' and ground planes 18' until it reached a final patch 14' or ground plane 18' then would loop back through the remaining alternating patches 14' and ground planes 18'.

In Figure 7 an antenna 10" of the present inventive concept is shown with a curved substrate 12”. The substrate 12” is curved so as to fit around a user's shoulder S, for example under or as part of clothing. The antenna 10” is connected via an electrical cable 24” to a co-axial connector 26” which can in turn interface with a radio frequency transceiver to drive the antenna 10”. The back face portion B" of the substrate 12” is arranged to face inwards towards the user's shoulder S, so that the ground planes (not shown) are arranged closest to the user's shoulder S and the patches (not shown) face outwards away from the user's shoulder S.

Figure 8 shows a result of a simulation of the electric field gradient when an antenna similar to the arrangement of Figures 3 to 5 and described in relation thereto. A strong electric field gradient is shown formed between the ground planes. This shows that the ends of the ground planes are a critical location of such an antenna - and important in the generation of good radio transmission and reception relative to certain other locations such as the sides of the ground planes, the sides of patches or the gap between the microstrip and the ground plane on the opposing face of the substrate. This further contrasts the present inventive concept from previously known arrangements.

Figure 9 shows a result of a similar simulation of the electric field as shown in Figure 8, with lines 30 (not all labelled) connecting points of equal electric field intensities at certain electric field intensities in the form of an isoline or contour. The contour lines 30 are iso-volts-per-metre maps of the electric field indicative of the antenna 10. They are mapped onto a cross sectional plane of the antenna 10 which is slightly offset from the main axis centre, and represent the boundary between two adjacent ground planes 18. The view shown is somewhat orthographic towards the ground plane 18 side of the antenna 10 - i.e. is not quite a plan view of the antenna 10 from above the face portion B as described with respect to Figure 3. This view is aimed at aiding understanding. Figure 9 shows that the electric field gradient (i.e. rate of change of electric field strength) is much stronger between and in the regions 32 (only one of these is labelled to aid clarity) of close proximity of the ground planes than in the regions 34 (only one of these is labelled to aid clarity) further away from the regions 32. The lines 30 which are shown are spaced in a logarithmic way, with the rate of change of electric field increasing very strongly towards the regions 32. The result of the configuration of the antenna 10 can be seen in a relatively uniform electric field at a distance away therefrom.

Figure 10 shows a closer view of a similar simulation as shown in Figure 9, where the view shows in more detail the electric field contour lines 30 close to the region where ground planes 18 have respective edges 34 close to one another on the common face of the substrate of the antenna. In a region 32 between edges 34 of ground planes 18, the electric field gradient is strongest.

The skilled reader will note that there are some discontinuities in certain contour lines 30. These discontinuities are inside the substrate and are believed to result from different dielectric values of air and the substrate material, as well as the mesh resolution of the simulation.

Figure 11 shows characterisation of a section of the prior art arrangement shown in Figure 1, the cross section being along the line N-N in Figure 1. Figure 11 shows a characterisation of electric field contours 30 which the skilled reader will appreciate are shown in a simplified form. The key point is that the point of peak electric field strength 40 is approximately located centrally to the antenna and substrate 12 thereof, due to the arrangement of ground planes 18 in the prior art arrangement.

Figure 12 shows a characterisation of a section of the embodiment of the arrangement of the present inventive concept, for example that shown in Figure 3, the cross section being along the line N'-N' in Figure 3. Figure 12 shows a characterisation of electric field contours 30 which the skilled reader will appreciate are shown in a simplified form. A key difference from the arrangement shown in Figure 11 is that the point of peak electric field strength 40 is approximately located at an edge of the substrate 12, and with the region of high electric field strength being approximately half within the substrate 12 and half outside the substrate so that it is formed in surrounding material (not shown). This is due to the arrangement of ground planes 18 of the inventive concept. The location of the point 40 of peak field strength and region of high field strength is different to that which arises from the aforementioned prior art arrangement and is advantageous to the performance of the antenna 10.