This invention relates to a dielectrically-loaded antenna for operation at frequencies in excess of 200MHz, and primarily but not exclusively to a quadrifilar helical antenna for operation with circularly polarised electromagnetic radiation.

Dielectrically-loaded quadrifilar helical antennas are disclosed in British Patents Nos. 2292638, 2310543 and 2367429. Antennas in accordance with these patents have been used mainly for receiving circularly polarised signals from satellites of the Global Positioning System (GPS) satellite constellation for position fixing and navigation purposes. GPS is a narrowband service. There are other satellite-based services requiring receiving or transmitting apparatus of greater fractional bandwidth than that available from the prior antennas.

In dielectrically-loaded loop antennas such as that disclosed in British Patent No. 2321785, bandwidth can be increased by replacing the individual helical radiating elements by a pair of mutually adjacent, substantially parallel, radiating elements connected at different positions to a linking conductor linking opposed radiating elements. In another variation, disclosed in British Patent No. 2351850, the single helical elements are replaced by laterally opposed groups of elements, each group having a pair of coextensive mutually adjacent radiating elements in the form of parallel tracks having different widths to yield differing electrical lengths. These variations on the theme of a dielectrically-loaded twisted loop antenna gain advantages in terms of bandwidth by virtue of their different coupled modes of resonance which occur at different frequencies within a required band of operation. A further extension in bandwidth can be obtained if at least one of the parallel tracks comprises a conductive strip having non-parallel edges or with opposing edges of different lengths, as disclosed in British Patent Application No. 2399948 A. In these latter variants, the edge of the strip which is furthest from the other track or tracks in its group is longer than the edge which is nearer the other track or tracks. Indeed, both first and second tracks of each group may have edges of different lengths, e.g., in that each such track which has an edge forming an

outermost edge of the group is configured such that the outermost edge is longer than the inner edge of the track.

Such differences in edge length are obtained by forming each affected track so that one of its edges follows a wavy or meandered path along substantially the whole of its radiating length. In a twisted loop antenna, each group of tracks executes a half turn around the central axis of a cylindrical dielectric core. Thus, the helical track has one edge which follows a strict helical path, whilst the other edge follows a path which deviates from the strict helical path in a sinusoidal pattern, for example.

It is an object of the invention to provide an antenna for circularly polarised signals which offers greater bandwidth than the prior quadrifilar helical antennas.

According to one aspect of the present invention, there is provided a quadrifilar helical antenna comprising first and second pairs of diametrically opposed antenna elements on a generally cylindrical dielectric substrate, at least one of the elements of each pair including a pair of mutually adjacent substantially parallel conductive tracks defining between them a channel, the track edges which define the channel being longer than the other edges of the respective tracks.

The preferred embodiment of the invention takes the form of a backfire antenna having an axial feeder structure located in a bore in a solid dielectric antenna core made of a material having a relative dielectric constant greater than 5. There are four radiating composite elements each extending from a feed connection where the feeder structure terminates at a distal end face of the core, to a linking conductor which interconnects the four composite elements at a location which is axially spaced from the feed connection. The linking conductor is connected to the feeder structure proximally with respect to the feed connection. The term "radiating" is to be construed broadly in the sense that, when applied to antenna elements, tracks or conductors it refers to elements, tracks or conductors which radiate energy when the antenna is used with a transmitter, or which absorb energy from the surroundings when the antenna is used with a receiver.

The composite elements forming the first pair of diametrically opposed antenna elements have a shorter average electrical length than those forming the second pair to yield substantial phase orthogonality between currents in the respective elements of the first and second pairs. As in conventional quadrifilar antennas, this phase orthogonality produces an operating band in which the antenna exhibits increased gain for circularly polarised signals.

In this preferred embodiment each composite radiating element has a first, radial portion on the distal end face of the core and a second, helical portion extending from the first portion to the linking conductor. Each of the four composite elements includes a pair of mutually adjacent parallel tracks defining the above- mentioned channel between them. Advantageously, each element is divided in this way only in its second, helical portion, the track edges defining the channel following, e.g., generally parallel meandered paths to increase the length of the channel within the available length of the radiating element. In this way, the electrical length of the channel can be increased to a half wavelength at an operating frequency of the antenna despite the fact that, in the preferred embodiment, the helical portion of each antenna element has an electrical length which is less than a half wavelength. Differences between the lengths of the conductive paths of which the tracks form parts, as well as dissociation of the currents in the tracks in each respective antenna element as a result of the half wavelength electrical length of the channel between them, promote a resonance for axially directed circularly-polarised radiation which has a greater bandwidth than that achievable with an equivalently-sized antenna having single track radiating elements. The bandwidth depends on, amongst other factors, the degree of dissociation between the currents in the respective composite elements each comprising parallel conductor tracks separated by a channel or slit. Current dissociation produces a phase dwell in the operating band in the sense that phase orthogonality between the average of the currents in each longer composite element is extended over a wider frequency band than in conventional quadrifilar antennas. Substantial phase orthogonality can typically be achieved over a fractional bandwidth of at least 0.4%. In some embodiments the fractional bandwidth may be 2% or more. Substantial phase orthogonality may be defined as exhibiting a phase difference of between 60° and 120°.

It is not necessary for the physical angular separation between the diametrically opposed pairs of shorter and longer composite elements to be exactly 90°, i.e. subtended at the axis of the helices. For example, the angular separation may be 70° or 80°. It is preferred that, in the operating band of the antenna, the phase difference between the average of currents in the shorter pair and the average of currents in the longer pair be within 30° of this angular separation or, better, within 20°. The operating band has the same fractional bandwidth limits referred to above. Thus, for instance, if the physical angular separation is 80°, the phase difference is preferably within the range of from 60° to 100° over the whole of a fractional bandwidth of at least 0.4%, or at least 2%.

By confining the channel to the helical portion of each antenna element, the radial portion on the distal end face of the core is made available for tuning by adjustments, i.e. removing conductive material from the antenna element, e.g., by forming cut-outs or apertures using laser etching, as disclosed in British Patent No. 2356086, the entire contents of which are incorporated in the present specification by reference.

In common with the antenna disclosed in the above-mentioned British Patents Nos. 2292638 and 2310543, the linking conductor of the preferred antenna comprises a conductive sleeve encircling the core and connected to the feeder structure. Each antenna element is joined to the rim of the sleeve with the channel preferably also extending to the level of the rim or very close to the level of the rim.

The mutually adjacent tracks of each antenna element may have different electrical lengths by, for example, having different average widths. Alternatively, differences in conductive path length may be achieved as a result of the inherent inclination of the tracks with respect to the rim and the consequent differences in the current patterns at their respective junctions with the rim.

According to another aspect of the invention, a helical antenna for circularly polarised electromagnetic radiation comprises: first and second pairs of helical conductive track groups on or adjacent the outer generally cylindrical surface of a

generally cylindrical insulative substrate, the track groups being distributed around the said outer surface and each comprising at least a pair of generally helical tracks which each have one edge longer than the other and each of which is nearer to the other track of the said pair that it is to the tracks of the other groups. The tracks of each group preferably define between themselves a substantially parallel-sided elongate channel of an average width which is less than half of the average spacing between neighbouring track groups.

According to a further aspect of the invention, there is provided a quadrifilar helical antenna for operation in a frequency band above 200MHz, wherein the antenna comprises four coextensive composite helical antenna elements each of which is formed as the combination of at least two coextensive elongate conductors separated by a slit, the width of the slit being less than half of the spacing between the respective composite element and either of the neighbouring composite elements, wherein the slit and the portions of the said coextensive conductors bounding the slit define elongate boundary regions which at a frequency within the operating band of the antenna have an associated electrical length which is greater than the electrical length of portions of the coextensive conductors which do not bound the slit.

The dependent claims accompanying this description include a non-exhaustive set of optional features not already set out above.

Antennas in accordance with the invention have particular use with the following bands of operation:

(a) 1559 - 1591 MHz (Galileo satellite positioning system)

(b) 1260 - 1300 MHz (Galileo satellite positioning system)

(c) 1164 - 1214 MHz (Galileo satellite positioning system) (d) 1563 - 1587 MHZ (GPS Ll)

(e) 1216 - 1240 MHz (GPS L2)

(f) 1164 - 1188 MHZ (GPS L5)

(g) 1602.56 - 1615.50 MHz (Glonass) (h) 1240 - 1260 MHZ (Glonass)

(i) 1610.0 - 1626.5 (Iridium satellite communications) 0) 2332.5 - 2345.0 MHz (XM satellite radio) (k) 2320.0 - 2332.5 MHz (Sirius satellite radio)

The services associated with these bands are indicated above in brackets.

The invention will now be described by way of example with reference to the drawings in which:-

Figure 1 is a perspective view of a dielectrically-loaded quadrifilar antenna in accordance with the invention, having four laterally opposed groups of elongate helical radiating conductors, viewed from the side;

Figure 2 is another perspective view of the antenna of Figure 1 , viewed mainly from the top;

Figure 3 is a third perspective view of the antenna of Figure 1, viewed from below and from the side; and

Figure 4 is a diagram showing variations in current phase in the conductor groups of the antenna of Figures 1 to 3.

Referring to the drawings, a quadrifilar antenna in accordance with the invention has an antenna element structure with four longitudinally extending groups of radiating conductive tracks 10AA, 10AB, lOBA, lOBB, lOCA, lOCB, 10DA, 10DB which are formed on the cylindrical outer surface 12C of a solid ceramic core 12.

The core has an axial passage and the passage houses a coaxial feeder structure having an outer conductor 16, an inner dielectric insulating layer 17 and an inner conductor 18. The outer conductor 16 of the feeder structure may be spaced from the wall of the axial passage through the core 12 in which it is housed by a dielectric layer (not shown in the drawings) having a relative dielectric constant less than the relative dielectric constant of the material of the core. In particular,

such a dielectric layer may consist of a plasties sheath as described and shown in the above-mentioned British Patent No. 2367429, the entire contents of which are incorporated in the present application by reference.

This coaxial feeder structure is for connecting radio communication apparatus (not shown) to the longitudinally extending track groups. The antenna element structure also includes four radial elements 10AR, lOBR, lOCR, 10DR formed as metallic tracks on a distal end face 12D of the core 12, connecting ends of the conductive tracks IOAA - 10DB of the respective four longitudinally extending track groups to the feeder structure. The other ends of the conductive tracks IOAA - 10DB are connected to a common virtual ground conductor 20 in the form of a plated sleeve surrounding a proximal end portion of the core 12. This sleeve 20 is, in turn, connected to the outer conductor 16 of the feeder structure in a manner described below. Two of the radial tracks lOCR, 10DR are connected at their inner ends to the inner conductor 18 of the feeder structure at the distal end of the core 12, and the other two radial tracks 10AR, IOBR are connected to the feeder screen formed by the outer conductor 16 of the feeder structure.

In this embodiment of the invention the four groups IOAA, IOAB - 10DA, 10DB of conductive tracks IOAA - 10DB are helical and have different lengths. Two of the groups lOBA, lOBB; 10DA, 10DB are longer than the other two IOAA, IOAB and 10CA ₃ IOCB by virtue of extending nearer to the proximal end of the core 12. The elements of each pair of conductive track groups IOAA, 10AB ₅ lOCA, IOCB; lOBA, lOBB, 10DA, 10DB are diametrically opposite each other on opposite sides of the core axis and each group of helical tracks IOAA - 10DB follows a helical path centred on the axis of the cylindrical core. The difference in length between the two pairs of track groups in this embodiment results from the upper rim or linking edge 2OU of the sleeve 20 being of varying height (i.e. varying distance from the proximal end face 12P of the core) to provide points of connection for the long and short track groups respectively. Thus, in this embodiment, the rim 20U follows a shallow zig-zag path around the core 12, the shorter track groups IOAA, IOAB; lOCA, IOCB meeting the rim 2OU at points on the rim which are further from the proximal end face 12P than the points where the longer track groups lOBA, lOBB; 10DA, 10DB meet the rim 2OU. The helical centre line of each of

the track groups 10AA, IOAB - 10DA, 1ODB subtends substantially the same angle of rotation at the core axis, here in the region of about 180°, i.e. or half turn.

As described in, for instance, the above-mentioned British Patent No. 2310543, the differing lengths of the conductive paths constituted by the combinations of the radial tracks IOAR - 10DR and helical track groups 10AA, IOAB - 10DA, 10DB produce different transmission delays at a central operating frequency located between the resonant frequencies associated with the longer and shorter conductive paths, such that the antenna has a resonant mode for receiving or transmitting circularly polarised signals.

With the left-handed sense of the helical paths of the conductive track groups 10AA, IOAB - 10DA, 10DB, the antenna has its highest gain for right-hand circularly polarised signals incident the antenna on the core axis and from above the distal end face 12D. If the antenna is to be used for left-hand circularly polarised signals, the direction of the helices is reversed and the pattern of connection of the radial elements is rotated through 90°. In the case of an antenna suitable for receiving both left-hand and right-hand circularly polarised signals, the conductive track groups can be arranged to follow paths which are generally parallel to the axis of the core.

The conductive sleeve 20 covers a proximal portion of the antenna core 12 and is proximally connected to the outer conductor 16 of the feeder structure by conductive plating 22 on the proximal end face 12P of the core 12. As described in the above-mentioned British Patent No. 2310543, the combination of the sleeve 20 and the plating 22 forms a balun so that signals on the transmission line formed by the feeder structure 16, 17, 18 are converted between an unbalanced state and the proximal end of the feeder structure and at at least approximately balanced state at the distal end. The disclosure of Patent No. 2310543 is incorporated in the present application by reference.

The combination of the sleeve 20 and proximal end face plating 22 also has the effect of isolating the rim 2OU from the outer conductor 16 of the feeder structure at the operating frequencies of the antenna so that currents in the conductive track

groups 10AA, IOAB - 10DA, 1ODB circulate between the inner and outer conductors 18, 16 of the feeder structure at its distal termination via conductive loops formed by respective pairs of the conductive track groups and portions of the sleeve rim 2OU.

In practice, the conductive tracks in each group 10AA, IOAB - 10DA, 10DB provide respective alternative conducting paths between the feed connection and the sleeve rim 2OU. The tracks of each pair 10AA, IOAB - 10DA ₅ 10DB are separated by a respective channel or slit 26A ₃ 26B, 26C, 26D extending from, firstly, a point at or very close to the connection of the track group to its respective radial element 10AR, lOBR, 10CR ₃ 1 ODR to, secondly, the region where the tracks 10AA, IOAB - 10DA, 10DB are connected to the sleeve rim 2OU. More precisely, each channel 26A, 26B, 26C, 26D extends to the level of the rim 2OU where it is connected to the respective tracks on each side of the channel. Preferably, however, the channel ends at a level just short of the level of the rim 2OU at the respective track connections.

In this embodiment, the edges of the tracks IOAA - 10DB which bound the respective channel 26A, 26B, 26C, 26D deviate from respective helical lines, e.g., by following respective meandering paths so that the channel 26A, 26B, 26C, 26D has a wave-like sinusoidal configuration with substantially parallel sides. In this way, the electrical length associated with each channel 26A ₃ 26B, 26C, 26D is increased so as to be greater than the electrical lengths of the outwardly facing edges of the tracks of the corresponding track group 10AA ₃ IOAB - 10DA ₃ 10DB. Each track takes the form of an elongate conductive strip which has first and second ends and two opposing edges extending from the first end to the second end. The edge which bounds the channel is longer than the outwardly facing edge because if follows a less direct path between the ends of the strip. In practice, the electrical length associated with each channel approaches equivalence to a half wavelength at an operating frequency within the operating band of the antenna. The currents in the conductive paths on either side of the channel then develop a phase independence one from the other. This effect can be visualised by the analogy of two half-wave simple-harmonic resonances independently carried on respective tracks. According to the required electrical characteristics, including

required bandwidth, the level of dissociation of the current flowing in the tracks of each pair 10AA, IOAB - 10DA ₅ 10DB produces a region of phase dwell in an otherwise linear phase-versus-frequency characteristic of the composite line formed by the pair of tracks. In effect, depending on the level of current dissociation, the band over which substantial phase orthogonality is achieved between currents in the shorter composite lines lOAA, IOAB; lOCA, IOCB and those in the longer composite lines lOBA, lOBB; 10DA, 10DB can be altered. This, in turn, affects the band over which the antenna is resonant in a mode associated with circularly- polarised radiation. Such phase dwell can be observed using a test arrangement such as that described and shown in the above-mentioned Patent No. 2356086. By bringing capacitive probes into juxtaposition with the lower ends of the composite lines, i.e. adjacent the sleeve rim 2OU, the phases of the currents in the lines can be monitored as the antenna is supplied with a swept frequency signal from a generator coupled to the feed structure 16, 17, 18. The graph of Figure 4 represents the phase-versus-frequency characteristics obtained from four probes in juxtaposition with the four composite lines. The antenna gain for circularly- polarised radiation incident along the axis of the antenna core 12 from the end having the feed connection towards the proximal end reaches a maximum when the two characteristics 30A, 30C for the shorter lines 10AA, IOAB; lOCA, IOCB exhibit about a 90° difference compared to the characteristics for the longer lines lOBA, lOBB; 10DA, 10DB respectively. As shown in Figure 4, approximate phase orthogonality is achieved over a bandwidth B. In this embodiment B is approximately 16MHz about a centre frequency of 1618MHz.

The tracks of each track group 10AA, IOAB - 10DA, 10DB may have differing average widths to yield different average electrical lengths within each group. However, it will be noted that a first track 10AA, lOBA, lOCA, 10DA of each track group meets the sleeve rim 2OU with an acute included angle between them whereas, conversely, a second track IOAB, lOBB, IOCB, 10DB of each group meets the sleeve rim 20U with an obtuse included angle. These differences in the way in which the respective tracks meet the sleeve rim also cause small differences in the average electrical length of the tracks of each track pair. Consequently, even if the tracks of each pair have the same average width, there are two coupled resonances at slightly different frequencies associated with the longer track groups

lOBA, lOBB; 1ODA ₅ 1ODB and, similarly, there are two coupled resonances of different frequencies associated with the shorter track groups 10AA, 10AB; lOCA, lOCB.

The width of the channel is typically less than the average width of the tracks on each side. In general terms, the width of the channel is less than the spacing between adjacent groups of tracks 10AA, IOAB - 10DA, 10DB, and preferably less than half of the spacing between the adjacent track groups. Each mutually adjacent pair of conductive tracks 10AA ₅ IOAB - 10DA, 10DB may be considered as forming part of a composite radiating helical antenna element.

By restricting the longitudinal extent of the channels 26A - 26D so that at least part of each radial portion IOAR - 10DR on the distal end face 12D of the core remains undivided, the radial portions can be reserved for forming cut-outs in the form of apertures 28 for trimming the antenna using, e.g., laser etching as described in the above-mentioned Patent No. 2356086.

In the above-described embodiment, the electrical lengths associated with the channels 26A - 26D are increased compared with the average electrical lengths of the respective track groups 10AA, IOAB - 10DA, 10DB by arranging for the edges of the tracks bounding the channels to be longer than the other edges of the tracks. In other words, each track inner edge deviates from a mean helical path whereas the outer edge of the respective track follows a simply helical path or deviates from such a path to a lesser degree than the deviation of the inner edge. In effect, the helical wave velocity associated with the channels 26A - 26D is reduced by compared with the wave velocity associated with the outer edges of the tracks.

Such slowing of the wave velocity may be achieved in other ways. For instance, the physical lengths of the channels may be approximately be the same as the average helical length along the corresponding portion of the group of tracks but each channel may have, above, below, or within itself, an elongate dielectric element made of a material having a relative dielectric constant which is higher than the relative dielectric constant of the core 12. Thus, the core may be formed with integral higher-dielectric helical strips in registry with each channel or strips

of a higher dielectric constant may be applied over or in each channel after the conductive tracks have been plated on the core 12 so that the average relative dielectric constant associated with the channel is higher than that associated with the outer edges of each track group.