GB2405997A - An antenna and a method of receiving and transmitting signals via an antenna - Google Patents

Abstract

An antenna 2 for operating at mobile telecommunications frequencies. The antenna 2 comprises two electrically conductive plates 10,11 and a conducting feed element 13. The plates 10,11 are spaced apart and electrically insulated from each other. The surfaces of the plates 10,11 are substantially parallel to each other. The conducting feed element 13 comprises a plurality of elongate members 14,15 located between the plates 10,11. Each member 14,15 extends substantially parallel to the surfaces of the plates 10,11 to a free end. In use, one of the plates 10,11 forms a primary radiator and the series inductive reactance of the conducting feed element 13 is substantially compensated for by the series capacitance between the conducting feed element 13 and one of the plates 10,11.

Description

AN ANTENNA AND A METHOD OF RECEIVING AND TRANSMITTING

SIGNALS VIA AN ANTENNA

Backaround to the Invention

The present invention relates to an antenna and a method of receiving and transmitting signals via an antenna.

Currently available antenna technology is impedance bandwidth limited. A commonly used definition of impedance bandwidth is the frequency range over which the lo voltage standing wave ratio (VSWR) is less than 2:1.

Mobile communication operators place high importance on the VSWR pattern performance and gain and also the fact that these requirements may need to be achieved over a range of electrical tilts (typically 0 to 10 in the mobile communication sector). As a consequence of this, the VSWR of an individual radiating element should be less than about 1.3:1.

One way of overcoming the problem of limited bandwidth is described in Targonski. S. D., Waterhouse, R. B., Pozar, D. M., "Design of Wide-band Aperture Coupled Stacked Microstrip Antennas", IEEE Trans. Antennas and Propagation, Vol. AP-39, 1991, pp.1770-1776. In this document, the elements are described as stacked one on top of another. However, as the structure grows in size, so too do the cost and complexity. A simpler approach is to increase the thickness of the substrate, but then there is a high risk of introducing surface waves which result in low antenna efficiencies unless a dielectric of low permittivity is used.

A simple microstrip patch on a thick substrate fed with a coaxial probe introduces an inductance making the input impedance inductive and hence very difficult to match. One way of overcoming this problem is to add a 2 capacitance in series by etching a gap around the feed probe on the patch. Hence, at resonance, if the capacitance cancels the inductance, the end result is a feed point with a real impedance. Such a system is described in United States Patent US-A-6593887.

Another alternative is the L-feed design which is based on the same principle. That is, the structure presenting a series LC resonant circuit feeding a series resonant RLC circuit of the patch. The inductance of the lo vertical arm is compensated by the capacitance of the horizontal arm thus allowing for improved matching over a large bandwidth. Such systems are described in Jeon, J. "Design of Wideband Patch Antennas for PCS and IMT-2000 Service" Microwave Journal, http://www.swissatv. ch/documents/design_wideband_patch_ant enna.pdf, and also in US patent USB-6593887.

The L-feed design shows good electrical performance when there is no constraint to the design. The performance generally outperforms that of a Microstrip antenna of the same size. However, when the patch width is constrained to achieve the required azimuth beamwidth, the performance is generally equivalent to that of a Microstrip patch antenna using a combination of the thick substrate, parasitics and inset microstrip design. As to the parasitics, see Wood, C. "Improved Bandwidth of Microstrip Antennas Using Parasitic Elements", IKE Proc. H. Microwaves, Opt & Antennas, Vol.127, No. 4, pp 231 to 234, Aug 1980 and Kumar, G. and Gupta, K. C. "Broad-band Microstrip Antennas Using Additional Resonators Gap Coupled to the Radiating Edges", IEEE Trans. Antennas and Propagation, Vol. AP-32, No. 12, pp 1375 to 1379, Dec 1984.

Various GSM900/GSM1800, GSM900/UMTS2100 and Cellular/PCS dual-band antennas are currently finding their way onto the market. Microstrip antennas have generally been popular for this purpose. - 3 -

Microstrip designs have been implemented because they are low profile, they can be mass produced and they can be manufactured relatively cheaply. They are deployed on mobile networks around the world. However, as the requirements for antennas are changing to steerable antenna arrays operating over a wider bandwidth, the advantages that a microstrip offers are eroded because of the relatively small bandwidth inherent in microstrip designs.

lo Adjustable electrical tilt (AET) capability allows mobile communication operators to optimise their networks more easily. However, the requirement for AET antennas capable of delivering dual-band performance that is cost effective with good radiation pattern stability and high gain, has forced antenna designers to seek alternative methods to microstrip antennas. Furthermore, environmental planning pressures have forced mobile communication operators to re-use existing sites for new antennas.

Dual-band base station antennas are gaining wide acceptance. In the majority of cases, the impedance bandwidth limitation of currently available technology has meant that designers generally design dual-band antennas only in the bands corresponding to GSM900 and GSM1800 (870MHz to 960MHz and 1710MHz to 1880MHz), GSM900 and UMTS2100 (870MHz to 960MHz and l900MHz to 2170MHz) and Cellular and PCS (806MHz to 896MHz and 1850MHz to l990MHz).

In an AET dual-band antenna, the performance of the conventional L-feed design is not adequate. The bandwidth performance can be enhanced by stacking the elements.

However, as each additional stacked patch introduces its own resonance, the combination results in wideband performance. Such a system is also expensive.

Furthermore, as the feed position is fixed to allow for the coaxial feed to excite the higher frequency component - 4 - of the dual-band element the performance is generally equivalent to that of a microstrip patch antenna using a combination of the thick substrate, parasitics and inset microstrip design.

Summarv of the Invention The invention in its various aspects is defined in the independent claims below, to which reference should now be made. Advantageous features are set forth in the appendant claims.

lo A preferred embodiment of the invention, described in more detail below, takes the form of an antenna for operating at mobile telecommunications frequencies. The antenna comprises two electrically conductive plates and a conducting feed element. The plates are spaced apart and electrically insulated from each other. The surfaces of the plates are substantially parallel to each other. The conducting feed element comprises a plurality of elongate members located between the plates. Each member extends substantially parallel to the surfaces of the plates to a free end.

In use, one of the plates forms a primary radiator, and the series inductive reactance of the conducting feed element is substantially compensated for by the series capacitance between the conducting feed element and one of the plates.

The present system is able to meet the requirements for VSWR, pattern performance and gain over the required range of electrical tilts. Furthermore, these benefits have been achieved over a much larger bandwidth than a microstrip antenna of the same size.

The present system provides a simple means of exciting a patch radiator over a much wider operating frequency bandwidth and with low VSWR than is obtained - 5 - with existing methods. In some embodiments, it also provides a means by which the basic feed can be extended to a dual-frequency device in which wide operating bandwidths are obtained at both frequencies.

The following advantages are obtained by embodiments described below. A low VSWR is achieved over a very large bandwidth even if the patch width is constrained to achieve the required azimuth beamwidth. While maintaining a low VSWR, the feed position may be constrained to allow lo coaxial access for high frequency elements in dual-band antennas. The feed structure makes the whole antenna element tolerant to dimensional errors, allowing for low cost construction on a mass scale. The radiating structure accommodates multi-frequency operation on a single plate and does not require stacked elements. This is cost effective without sacrificing performance. I Arrangements described below have a radiating element I for dual-band interleaved operation with the following characteristics. Radiation patterns (both elevation and azimuth) and gains are the same or better than a standard microstrip antenna. Close impedance matching is obtained over a wide bandwidth which covers both the worldwide mobile communication operator bands as summarised above.

This performance is achieved even with the patch width constrained to achieve a desired azimuth beamwidth and feed position fixed to allow for coaxial access to the higher frequency element. The arrangement provides lower cost construction than a traditional microstrip approach.

The element is compact in construction and very robust.

Brief Description of the Drawinas

The invention will now be described in more detail, by way of example, with reference to the drawings, in which: - 6 Figure 1 shows an antenna embodying the present invention; Figure 2 shows an exploded view of the conducting feed element of the antenna and the means for mechanically connecting the conducting feed element of Figure 1; Figure 3 shows the conducting feed element of Figure 2; Figures 4 to 6 show alternative conducting feed elements to those shown in Figure 2; Figure 7 shows top view of the conducting feed element of Figure 4 mechanically connected to a printed circuit lo board; Figure 8 shows a bottom view of the conducting feed element of Figure 7; Figure 9 shows the element of Figure 4 connected to a coaxial cable; Figure 10 shows a graph of VSWR against frequency for a conventional microstrip element and an antenna embodying the present invention in the GSM900 band; Figure 11 shows a graph of VSWR against frequency over a wide band for a conventional microstrip element and an so antenna embodying the present invention; - 7 Figure 12 shows a graph of VSWR against frequency in the cellular frequency band for an L- feed probe design and an antenna embodying the present invention; Figure 13 shows a graph of VSWR against frequency over a wide band for an L-feed probe design and an antenna embodying the present invention; Figure 14 shows a graph of VSWR against frequency over the GSM900 band for an L- feed probe design with two stacked elements and an antenna embodying the present invention lo with one suspended patch; Figure 15 shows another embodiment of the present invention viewed from below having dual conducting feed elements for operation on two separate bands; Figure 16 shows a magnified view from the bottom of the s high frequency radiating element of Figure 15; Figure 17 shows a magnified view from the top of the high frequency conducting feed elements of Figure 15; Figure 18 shows the dual conducting feed elements of Figure 15 viewed from the side; and Figure 19 shows a graph of VSWR against frequency for the high frequency component of a dual band antenna (GSM1800 and UMTS2100) of Figures 15 to 18. - 8

Detailed Description of the Preferred Embodiments

Referring to Figure 1, an antenna 2 is shown with a multi- layer structure comprising a conductive ground plane 10 and a suspended patch 11. The suspended patch 11 is rectangular and spaced from the conductive ground plane 10 by four electrically insulated pillars 12, one at each corner of the suspended patch 11. The pillars 12 extend through through holes 19 in the suspended patch 11 and the conductive ground plane 10 and they are held in place by snap-fit fastenings lo 28. The pillars 12 are typically O.1A to 0.15A high where A is the required wavelength. The suspended patch 11 is approximately 0.5A long. Its width must be selected to provide the required H-plane beamwidth and is typically 0.2A to 0.3A. A conducting feed element 13 is coupled to the ground plane 10 and is electromagnetically coupled to the conductive suspended patch 11.

In the example of Figures 1 and 3, the conducting feed element 13 is shown extending upwardly from the conductive ground plate underneath the suspended patch 11.

At one end, the conducting feed element 13 has a fork shape comprising a planar base 30 extending vertically and three members 14,15, forming probes, extending outwardly from the end of the base 30 and perpendicular to the base 30. The members 14,15 are spaced apart and extend horizontally parallel to each other. As shown in Figure 2, there are two outer members 14 and one inner member 15. The inner element 15 is displaced vertically from the outer members 14 such that the outer members 14 are closer to the suspended patch 11 than the inner member 15. The electro magnetic couplings between the inner and outer members 14,15 interact to create a much wider impedance bandwidth than can be obtained from a single member.

At the other end to the elongate members 14,15, means are provided for mechanically connecting the conducting feed element 13 to a surface such as the conductive ground plane 10. The conducting feed element 13 has a lower flange 16 which extends outwardly, perpendicularly from the base 30 in the opposite direction to the members 14,15. The lower flange 16 is used to connect the conducting feed element 13 mechanically to the conductive ground plane 10. It is supported against an insulating pad 18 and secured by insulating fasteners 17. The lower flange 16 is fed by a coaxial cable 20 having an outer conductor 22 and an inner conductor 24. The outer conductor 22 is electrically lo connected to the conductive ground plane 10 and the inner conductor 24 is electrically connected to the conductive feed element 13.

In use, the suspended patch 11 forms a primary radiator and the series inductive reactance of the conducting feed element 13 is substantially compensated for by the series capacitance between the conducting feed element 13 and the suspended patch 11.

Alternative forms of conducting feed element 13 are shown in Figures 4 to 6.

Figure 4 shows an alternative conducting feed element 13 in which the two outer members 14 are conjoined at their outer ends 26. This configuration is easily cut or pressed from a single sheet and provides greater mechanical stability than is obtained from the arrangement of Figures 1 to 3.

Figure 5 shows an alternative arrangement to that in Figure 4. It differs from the arrangement of Figure 4, in that the lower flange 16 of the feed element 13 has been bent in the opposite direction, so that it extends away from the base 30 in the same direction as the inner and outer members 14,15.

Figure 6 shows an alternative and equivalent arrangement to that shown in Figure 4. In this example, the inner member 15 is elevated above the outer members 14. - 10

In an alternative arrangement shown in Figures 7 and 8, rather than using a coaxial cable to connect to the conducting feed element 13, a microstrip is used. The conducting feed element 13 is mechanically attached to the top surface of a printed circuit board 40 in the same way as shown in Figure 2. A microstrip line 42 extends along the underside of the printed circuit board 40. It is connected to the conducting feed element 13 by a conducting pin 44, and the conducting feed element 13 is placed in a cleared lo area 46 on the upper surface 48 of the conductive ground plane 10.

The most critical dimensions of the elongate members 14,15 and base 30 are illustrated in Figure 9. They are the vertical length Vlengthl, the horizontal length Hlengthl and the width W1 of the inner member 15. Vlengthl is the length between the ends of the base 30. Hlengthl is the distance the inner member 15 extends from the base 30. Wl is the width of the inner member 15.

The width W2 between the outside edges of the outer members 14, the distance Vleng,h2 between the upper surfaces of the inner member 15 and the outer members 14, and the length Hlength2 of the outer members 14 can also be adjusted in order to vary the VSWR.

The dimensions Wl, W2, Hlengthl, Hlength2, Vlengthl and Vlength2 can be optimised experimentally by trimming and adding features to provide the required low VSWR over the desired operating frequency and bandwidth. Alternatively, the dimensions W1, W2, Hi, Hlength2, Vlengthl and Vlength2 can be defined as variables on a computer simulator and optimised using an iterative process.

In a preferred embodiment of the present system, Wlis 0.015A, W2is 0.04A, Hleng,hl is O.1A, Hlength2 is 0.2A, Vlengthl is 0.07A, and Vlength2 is 0.015A. In other embodiments of the present system, W1 can be between 0.01A and 0.03A, W2 between 0.01A and 0.08A, H1engthl between 0.07A and 0.2A, Hlength2 between - 11 - 0.15A and 0.3A, Vlengthl between 0.05A and 0.15A, and Vlenqth2 between 0. 01A and 0.03A.

The inductive reactance of the base 30 is cancelled by the capacitive reactance of the inner member 15. This cancellation creates a real impedance match to the suspended patch 11 thus increasing the impedance bandwidth of the antenna 2. This impedance bandwidth is larger than a conventional microstrip approach.

By providing inner and outer members 14,15 and putting lo the inner member 15 at a different height to the outer members 14, a multi-step impedance matching transformer between the input and the suspended patch 11 has effectively been created.

In the embodiment of Figure 9, Vlength2 can be adjusted to improve the match. Furthermore, a better match can be obtained over a larger bandwidth than a conventional L-Feed design. It also outperforms the traditional microstrip antenna element of the same size. With the microstrip and L-Feed design, a lmm to 2mm change in the height of the suspended patch 11 will cause the match to degrade significantly at cellular frequencies. The antenna 2 described above allows the distance of the suspended patch 11 above the coupling feed element 13 to change by more than lOmm before the VSWR degrades to the same level as that caused by changing the distance by lmm to 2mm using the microstrip and L-feed approaches. With the change in length of the suspended patch 11, the L-feed design is more robust than the microstrip approach but not as robust as the antenna 2. The other benefit of splitting and stacking the feed is that all parameters (Hlengthl, Hlength2, Vlength1' V1ength2' W1' W2) are less sensitive to errors, which is ideal for mass production. When there is no constraint imposed on the antenna 2, the performance far outperforms the results shown in this application. A further benefit of the antenna 2 is that the whole structure remains relatively compact. - 12

A comparison of the performance has been made between the arrangement described above and a conventional Microstrip antenna element used in a production antenna made by CSA Wireless.

The conventional element included the following modifications to increase the bandwidth: a thick substrate, an inset microstrip, parasitics (as described in Wood, C., "Improved Bandwidth of Microstrip Antennas Using Parasitic Elements",IEE Proc. H. Microwaves, Opt. & Antennas, Voll27, lo No. 4, pp231-234, Aug 1980 and Kumar, G., Gupta, K., "Broad band Microstrip Antennas Using Additional Resonators Gap Coupled To The Radiating Edge", IEEE Trans. Antennas Propagation, vol AP-32, pp. 1375-1379, Dec 1984.) and stacked patches (as described in Chen, C. H., Tulintseff, A., Sorbello, R. M., "Broad-band Two-Layer Microstrip Antenna", IEEE Antennas and Propagation Symp. Digest, 1984, pp. 251-254 and Targonski. S. D., Waterhouse, R. B., Pozar, D. M., "Design of Wide-band Aperture-Stacked Patch Microstrip Antennas", IEEE Trans. Antennas and Propagation, Vol. AP-39, 1991, pp. 1770-1776).

With these modifications, the conventional Microstrip antenna element achieved low VSWR across the required operating band as shown in the graph in Figure 10. Figure shows a graph of VSWR against frequency in the GSM9OO frequency band. The grey line 100 shows the VSWR for a conventional antenna element. This is very good performance considering the width of the suspended patch was fixed at 0.12A to achieve the required azimuth beamwidth of 90 .

This also gave a very narrow bandwidth performance and a lot of effort and research went into achieving such a low VSWR of the Microstrip element. Figure 10 also shows the performance of the antenna 2 described above using the conducting feed element 13 shown in Figures 4 and 9 across the same frequency band and using the same suspended patch (i.e. with the width fixed at 0.12A). The position of the conducting feed element 13 was also fixed, in this example, - 13 to allow for space for a coaxial cable to access a second conducting feed element for higher frequency components for dual-band operation. The black line 102 in Figure 10 shows the VSWR for such an arrangement.

Referring to Figure 11, when the frequency band is increased from 806MHz to 970MHz, the conducting feed element 13 (as shown in Figures 4 and 9) indicated by the black line 104, far outperforms the conventional microstrip antenna element approach indicated by the grey line 106.

lo Figure 12 shows a graph of VSWR against frequency for a conventional Lfeed probe design compared with the fork shape conducting feed element 13 across the cellular frequency band (806MHz to 896MHz). The results are optimised for both feeds with the following constraints.

Firstly, feed position is in a fixed position near the edge of the suspended patch to allow access for the coaxial cable to feed the high frequency component in a dual-band design.

Secondly, the suspended patch width is fixed at 0.12A to achieve an azimuth beamwidth of 90 . The grey line 108 shows the performance of a conventional L-feed probe design and the black line 110 shows the performance of the antenna 2.

The results indicate that both designs are good across the cellular frequency band. In the graph of VSWR against frequency in Figure 13, the system using the conventional L feed probe design is shown by the line 112 and the system using the conducting feed element 13 (in the example of Figures 4 and 9) is shown by the line 114. Figure 13 shows that, when the frequency band is widened, the conducting feed element 13 gives low VSWR over a much wider band than a conventional L-feed probe design.

Figure 14 shows a graph of VSWR against frequency for an L-feed design using two L-feed radiating elements stacked one on top of the other. This is shown by the grey line 116. Such an arrangement gives a low VSWR over a wide - 14 bandwidth which is equivalent to using only one conducting feed element 13 in the antenna 2 as shown by the black line 118.

A further embodiment of the present invention is shown in Figures 15 to 18. It is a dual-band system 200 which comprises two coupling feed elements 202 and 204.

The conducting feed elements 202 and 204 will be described as of the Figure 4 and Figure 6 type, but could alternatively be as in Figures 3 or 5. They do not have to lo be the same type.

The dual-band system has a conductive ground plane 206 and a suspended patch 208 forming a radiating patch element.

The suspended patch 208 is rectangular and spaced from the conductive ground plane 206 by four insulating pillars 210, one at each corner of the suspended patch 208. The pillars 210 extend through through holes 211 in the suspended patch 208 and the conductive ground plane 206 and they are held in place by snap-fit fastenings 209.

The first coupling feed element 204 provides the high frequency component of the dual-band system. As shown best in Figures 16 and 17, in this system, the radiating patch element 208 is penetrated by a transverse substantially rectangular aperture 212 which is offset to one end of the suspended patch 208. The long edges of the aperture 212 are parallel to the short edges of the rectangular suspended patch 208. The aperture 212 has a rectangular extension 214 extending from the centre of the inner long edge of the aperture 212 towards the centre of the radiating patch element 208.

The first coupling feed element 204 is of the same shape as the feed element shown in Figure 6. That is, it has a fork shape comprising a planar base 30 extending vertically and three members 14,15 extending outwardly from the end of the base 30 and perpendicular to the base 30.

The members 14,15 are spaced apart and extend horizontally - 15 parallel to each other. There are two outer members 14 and one inner member 15. The free ends of the outer members 14 are joined together. The inner member 15 is displaced vertically from the outer members 14 such that the outer members 14 are closer to the suspended patch 208 than the inner member 15.

The planar base 30 of the first coupling feed element 204 passes through the rectangular extension 214 such that the inner and outer members 14,15 are located between the lo suspended patch 208 and the conductive ground plane 206.

The inner and outer members 14,15 extend parallel to the surface of the suspended patch 208 and the conductive ground plane 206, towards the aperture 212 such that they can be viewed through the aperture 212.

The first coupling element 204 is mechanically connected to the suspended patch 208 in the same way as the conducting feed element 13 in the first embodiment described above. That is, the flange 16 of the first coupling element 204 is retained on an insulating pad 18 by means of insulating fasteners 17. It is excited by a coaxial cable 213 whose outer conductor 215 is connected to the side of the suspended patch 208 facing the conductive ground plane 206. The coaxial cable 213 extends towards the conductive ground plane 206 and passes through a through hole in it to a point of connection in a feed network which may be of coaxial, microstrip or other RF transmission line construction.

A second conducting feed element 202 is coupled to the ground plane 206 and is electromagnetically coupled to the conductive suspended patch 208. The second conducting feed element 202 is coupled to the ground plane 206 in the same way as the feed element 13 of the first embodiment described above. It is offset to the opposite end of the conducting ground plane 206 to the aperture 212. The second conducting feed element 202 is of the shape shown in Figure 4. The inner and outer members 14,15 of the second conducting feed element 202 are located between and extend parallel to the surface of the suspended patch 208 and the conductive ground plane 206 towards the aperture 212. The inner member 15 is located below the outer members 14, between the ground plane s 206 and the outer member 14. The inner conductor 221 of the coaxial cable 217 is electrically connected to the flange 16 of the second conducting feed element 202. The coaxial cable 217 extends through a through hole 219 in the conducting ground plane 206. It is spaced from and extends lo parallel to the coaxial cable 213 connected to the first conducting feed element 204.

The second conducting feed element 202 is located towards the edge of the patch 208 to allow a coaxial feed 213 to pass through the antenna 200 to excite the higher frequency component provided by the first conducting feed element 204. The position of the second conducting feed element 202 does not matter, the match and bandwidth of the antenna 200 remain the same.

This dual-band system 200 provides operation on two separate wide frequency bands, having a wide frequency separation between the two bands.

The structure of the conducting feed elements 202,204 is again formed by a plurality of spaced apart members 14,15 to provide impedance compensation of the antenna 200. The 2s conducting feed elements' 202,204 elongate members 14,15 are dimensioned and extend across the aperture 212 in such a manner as to provide the required degree of coupling in the upper operating frequency band.

The upper frequency radiator is formed by the aperture 212 forming a radiating slot in the suspended patch 208 of the conducting feed element 202,204. The suspended patch 208 forms the patch element for the lower frequency band.

No separate radiating element is required for the upper frequency band. As the frequency band extends beyond mobile telecommunication bands, the tolerances need to be much - 17 tighter and the benefit of exciting an aperture 212 is more pronounced as performance stability can be hampered by using the stacked approach in a traditional design.

The inner and outer members 14,15 are dimensioned and extend across theaperture 212 to provide the required degree of coupling in the upper operating frequency band.

The parameters illustrated in Figure 9 again should be optimised for each feed element. The second vertical length Vlength2 can be adjusted to cancel the combined effects of lo Vlength1, Vlength2 and Hlength1. This allows improved VSWR performance over a much larger bandwidth. It does not matter whether the feed to the element 202 is located at the edge of the suspended patch 208 or in the middle, provided the parameters are appropriately optimised.

Figure 19 shows a graph of the measured performance of VSWR against frequency for the high frequency component (i.e. GSM1800 to UMTS2100) of the antenna 200 of the dual band system described above. The VSWR is shown to be well within the required level throughout this frequency range.

The arrangment of Figures 15 to 18 provides several useful advantages. An important one is that both the elevation and azimuth radiation patterns and gains are substantially the same or better than a standard microstrip antenna. Close impedance matching can be obtained over a 2 wide bandwidth, which covers the various worldwide mobile communication band as described above.

A plurality of the antennas 2 described above may be arranged in a linear array. The conducting feed elements 13,202,204 of the antennas 2 may be fed from a feed network comprising microstrip or coaxial lines.

It will be seen from the foregoing that the antenna structures illustrated achieve a low VSWR over a relatively large bandwidth, even if the patch width is constrained to achieve the required azimuth bandwidth. There is no 3s degradation to radiation patterns or gains. Also, the feed position can be constrained to allow coaxial access for the high frequency elements in dual-band antennas, while still - 18 maintaining a low VSWR. The radiating structure accommodates multi-frequency operation on a single plate, and does not require stacked multiple elements, thus providing cost effective designs without sacrificing performance. Finally, the feed structure makes the whole antenna element tolerant to dimensional errors, again allowing for a low-cost method of construction on a large scale.

Embodiments of the present invention have been lo described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention. In particular, features of the different structures described can be used in combinations other than those shown. Also, it has been assumed in the foregoing that the patch radiating element is oriented for vertical polarization, i.e. the H-plane beamwidth lies in the azimuth plane.

However, the patch radiating element may alternatively be so oriented for horizontal polarization, or indeed for polarization at any intermediate angle should this be desired. - 19

Claims (33)

1. An antenna for operating at mobile telecommunications frequencies, the antenna comprising: two electrically conductive plates, the plates being spaced apart and electrically insulated from each other, the surfaces of the plates being substantially parallel to each other; and a conducting feed element, the conducting feed element comprising a plurality of elongate members located lo between the plates, each member extending substantially parallel to the surfaces of the plates to a free end; such that, in use, one of the plates forms a primary radiator and the series inductive reactance of the conducting feed element is substantially compensated for by the series capacitance between the conducting feed element and one of the plates.

2. An antenna according to claim 1, wherein the plates are substantially rectangular.

3. An antenna according to claim 2, wherein one plate is a patch which is narrower and shorter than the other plate which is a conductive ground plane.

4. An antenna according to any of claim 1 to 3, wherein one of the elongate members is a protruding member which is closer to one of the plates than the other elongate member or members.

5. An antenna according to claim 4, wherein the elongate member which is the protruding member is closer to the plate that forms a patch than the other elongate member or members. -

6. An antenna according to any preceding claim, wherein there are three elongate members, an inner member and two outer members.

7. An antenna according to claim 6, wherein the inner member is closer to one of the plates than the outer members.

8. An antenna according to any of claims 1 to 3, wherein the elongate members are co-planar.

9. An antenna according to any preceding claim, lo wherein the free ends of the elongate members are connected together.

10. An antenna according to claim 6 or 7, wherein the free ends of the outer members are connected together.

11. An antenna according to any preceding claim, wherein the plurality of elongate members extend from an electrically conductive base which is substantially perpendicular to the elongate members.

12. An antenna according to claim 11, wherein the base and the plurality of elongate members each provide a substantially linearly polarised signal which are orthogonal to each other.

13. An antenna according to claim 12, wherein the base and the plurality of elongate members are fed in phase quadrature to provide substantially circularly polarised signals.

14. An antenna according to any of claims 11 to 13, wherein the base is substantially rectangular. - 21

15. An antenna according to claim 14, wherein a leg extends from one edge of the base towards one of the plates.

16. An antenna according to claim 15, wherein the leg extends from a short edge of the base.

17. An antenna according to claim 15 or 16, wherein the leg has a flange which extends from and substantially perpendicular to the base and mechanically connects the conducting feed element to one of the plates.

lo

18. An antenna according to any preceding claim, comprising a second conducting feed element, wherein one conducting feed element is mechanically connected to one electrically conductive plate and the other conducting feed element is mechanically connected to the other electrically conductive plate.

19. An antenna according to claim 18, wherein one of the electrically conductive plates comprises a through hole through which a portion of the second conducting feed element passes such that the through hole forms a secondary radiator.

20. An antenna according to claim 19, wherein the through hole is substantially rectangular.

21. An antenna according to claim 19, wherein the through hole is substantially rectangular and has a substantially rectangular extension extending from the centre of one of its long sides and the portion of the second conducting feed element which passes through the through hole passes through the substantially rectangular extension. - 22

22. An antenna according to any of claims 19 to 21, wherein the secondary radiator has dimensions to operate at a higher frequency than the primary radiator.

23. An antenna according to any preceding claim, wherein at least one conducting feed element is electrically connected to a radio frequency transmission line.

24. An antenna according to claim 23, wherein the radio frequency transmission line is a microstrip lo transmission line.

25. An antenna according to claim 23, wherein the radio frequency transmission line is a coaxial transmission line.

26. Antennas according to any preceding claim, wherein the antennas form a linear array and the conducting feed elements of the antennas are fed from a feed network comprising microstrip or coaxial lines.

27. An antenna for operating at mobile telecommunications frequencies, the antenna comprising: two electrically conductive plates, the plates being spaced apart by at least one pillar between the plates, the surfaces of the plates being substantially parallel to each other, and the plates being electrically insulated from each other; and a conducting feed element, the conducting feed element comprising: an elongate base portion extending substantially perpendicular to the surfaces of the plates and a plurality of elongate members extending from one end of the elongate portion, the elongate members being located between the plates and each member extending - 23 substantially parallel to the surfaces of the plates to a free end.

28. An antenna for operating at mobile telecommunications frequencies, the antenna comprising: two electrically conductive plates, the plates being spaced apart and electrically insulated from each other, the surfaces of the plates being substantially parallel to each other; and lo at least two conducting feed elements, each conducting feed element comprising a plurality of elongate members located between the plates, each member extending substantially parallel to the surfaces of the plates to a free end; IS such that, in use, one of the coupling feed elements excites a high frequency component and another coupling element excites a low frequency component of the mobile telecommunications frequencies at which the antenna operates.

29. A method of receiving and transmitting signals via an antenna, the method comprising the steps of: receiving or transmitting a signal via: two electrically conductive plates, the plates being spaced apart and electrically insulated from each other, the surfaces of the plates being substantially parallel to each other; and a conducting feed element, the conducting feed element comprising a plurality of elongate members located between the plates, each member extending substantially parallel to the surfaces of the plates to a free end; such that the series inductive reactance of the conducting feed element is substantially compensated for by the series capacitance created by the conducting feed element to one of the plates and one of the plates forms a primary radiator. - 24

30. A method of receiving and transmitting signals via an antenna, the method comprising the steps of: receiving or transmitting a signal via: two electrically conductive plates, the plates being spaced apart by at least one pillar between the plates, the surfaces of the plates being substantially parallel to each other, and the plates being electrically insulated from each other; and a conducting feed element, the conducting feed JO element comprising: an elongate base portion extending substantially perpendicular to the surfaces of the plates and a plurality of elongate members extending from one end of the elongate portion, the elongate members being located between the plates and each member extending substantially parallel to the surfaces of the plates to a free end.

31. A method of receiving and transmitting signals via an antenna, the method comprising the steps of: receiving or transmitting a signal via: two electrically conductive plates, the plates being spaced apart and electrically insulated from each other, the surfaces of the plates being substantially parallel to each other; and at least two conducting feed elements, each conducting feed element comprising a plurality of elongate members located between the plates, each member extending substantially parallel to the surfaces of the plates to a free end; such that one of the coupling feed elements excites a high frequency component and another coupling element excites a low frequency component of the mobile telecommunications frequencies at which the antenna operates. -

32. An antenna as substantially described above with reference to and as illustrated by the accompanying drawings.

33. A method of receiving and transmitting signals via an antenna as substantially described above with reference to and as illustrated by the accompanying drawings.