WO 2006/135956 PCT/AU2006/000834 -1 A RESONANT, DUAL-POLARIZED PATCH ANTENNA RELATED APPLICATION This patent application claims the benefit of, and is entitled to, the earlier filing date 5 of Australian Provisional Patent Application No. 2005903393 filed on 23 June 2005 in the name of Argus Technologies (Australia) Pty Ltd, which is incorporated herein by reference. TECHNICAL FIELD 10 The present invention relates generally to antennas and in particular to patch antennas. BACKGROUND Square or circular resonant patch antennas fed on two orthogonal axes are frequently used as elements of dual-polarised array antennas, in particular of base-station 15 antennas used in cellular telephone networks. If the required bandwidth of the antenna is more than a few percent, air dielectric is generally used and the height of the patch above the ground plane is selected to provide adequate bandwidth. The size of the patch is chosen to make the patch resonant. Feeding of the patch is generally achieved with slots, loops or probes containing resonant elements. The coupling of 20 these devices and the resonant elements is selected to achieve overall a double-tuned or higher order filter response. An example of such an implementation is shown in Fig. 1. Figs 1A, 1B and lC illustrate a resonant patch antenna 100 fed with a loop 130. The 25 patch 120 is positioned above the ground plane 110 to provide the desired bandwidth. For ease of illustration, the supports for the patch 120 are not shown, and the loop associated with one polarization only is shown. As best seen in Figs. lA and 1B, the loop 130 is "C" shaped with a signal source 140 disposed between the two ends of the "C". In practice, such a signal source might be implemented as a coaxial line 30 embedded in one side of the loop "C" with the centre conductor connected to the opposite side of the gap. A double-tuned impedance response is obtained by WO 2006/135956 PCT/AU2006/000834 -2 connecting a capacitor 620 in series at the feed point as shown in Fig. 6. A coaxial line 610 is coupled to the feedpoint in series with the capacitor 620. The outer conductor of the coaxial line is coupled to the loop 630 in the configuration 600 of Fig. 6. 5 A one-dimensional array of such patch elements 100 mounted above a ground plane typically results in a 3 dB beamwidth in the plane normal to the array of between 70 and 85 degrees. For cellular radio purposes, radiating slant polarization (linear polarization inclined at :450 to vertical) with a horizontal beamwidth between 60 and 10 65 degrees is frequently desirable. Some influence on the beamwidth can be exercised through the use of various metal fences or enclosures (not shown) around the patch elements 100. Another method of reducing beamwidth involves increasing the size of the patch. 15 However, this is accompanied by a reduction in the resonant frequency of the patch making impedance matching of the patch to the feed impossible. A need therefore exists for an improved method of controlling the beamwidth of a resonant, dual-polarized patch antenna 20 SUMMARY In accordance with an aspect of the invention, there is provided a patch antenna having a reduced beamwidth, comprising: a ground plane; a patch radiator suspended 25 above the ground plane, in which a central region of the patch radiator is shorted to ground; and a feed symmetrically disposed about the centre of the patch radiator and which excites opposite sides of the patch radiator in antiphase, the feed coupled to the patch radiator at locations outside the central region. 30 The patch antenna may further comprise a conductive member coupling the central region of the patch radiator and the ground plane to short circuit the central region to WO 2006/135956 PCT/AU2006/000834 -3 provide a desired resonant frequency of the patch radiator. The conductive member may be cylindrical in shape. The conductive member may be solid or tubular in form, or comprises a number of discrete connections between the patch and groundplane. 5 At least one microstrip or stripline board may implement the conductive member. At least one microstrip or stripline board may implement the feed. The patch antenna may further comprise two crossed microstrip or stripline boards. In accordance with another aspect of the invention, there is provided a patch antenna 10 having a reduced beamwidth, comprising: a ground plane; a patch radiator suspended above the ground plane; a conductive body coupled to one of the groundplane and the patch radiator disposed between the ground plane and the patch radiator to provide a stepped down central region of the patch radiator; and a feed symmetrically disposed about the centre of the patch radiator and which excites opposite sides of the patch 15 radiator in antiphase, the feed coupled to the patch radiator at locations outside the stepped-down central region. In accordance with either of the above aspects of the invention, the distance between the radiating edges of the patch radiator may be increased, so as to reduce the 20 beamwidth of the patch radiator while maintaining a resonant configuration. In accordance with either of the above aspects of the invention, the patch radiator may have a circular, square or other symmetrical shape. 25 In accordance with either of the above aspects of the invention, the patch antenna may further comprise a series capacitor and impedance transformer to provide a wide-band double-tuned configuration. In accordance with a further aspect of the invention, there is provided a method of 30 controlling the beamwidth of a resonant, dual-polarized patch antenna. The method comprises the steps of: short circuiting a central region of a patch radiator suspended WO 2006/135956 PCT/AU2006/000834 -4 above a ground plane to the ground plane of the antenna; and feeding a signal symmetrically using a feed disposed about the centre of the patch radiator and which excites opposite sides of the patch radiator in anti-phase, the feed coupled to the patch radiator at locations outside the central region. 5 In accordance with yet another aspect of the invention, there is provided a method of controlling the beamwidth of a resonant, dual-polarized patch antenna. The method comprises the steps of: providing a stepped-down central region of a patch radiator using a conductive body coupled to one of a groundplane and the patch radiator, the 10 patch radiator suspended above the ground plane and the conductive body disposed between the ground plane and the patch radiator; and feeding a signal symmetrically using a feed disposed about the centre of the patch radiator and which excites opposite sides of the patch radiator in antiphase, the feed coupled to the patch radiator at locations outside the stepped-down central region. 15 Other aspects of the foregoing methods may be implemented in accordance with the aspects of the invention set forth herein with respect to the patch antenna. BRIEF DESCRIPTION OF THE DRAWINGS 20 A small number of embodiments of the invention are described hereinafter with reference to the drawings, in which: Figs lA, lB and IC are perspective, side elevational and plan views, respectively, of a resonant patch antenna fed with a loop; Figs 2A, 2B and 2C are perspective, side elevational and plan views, 25 respectively, of a patch antenna with a shorted section in accordance with an embodiment of the invention; Fig. 3 is a perspective view of a circular patch fed with dual printed circuit loops in accordance with another embodiment of the invention; Figs. 4A, 4B and 4C are perspective views of a structure for feeding the patch 30 antenna of Fig. 3 in two polarizations with two printed circuit boards; WO 2006/135956 PCT/AU2006/000834 -5 Figs. 5A and 5B are side elevation views illustrating details of the two sides of the printed feed of Figs. 4A to 4C; Fig. 6 is a side elevation views of a resonant patch antenna fed with a loop having a capacitor in series; 5 Fig. 7 is a simplified side-elevation view of a resonant patch antennas having a stepped down gap region between a patch radiator and a groundplane in accordance with another embodiment of the invention; and Fig. 8 is a simplified side-elevation view of another resonant patch antennas having a stepped down gap region between a patch radiator and a groundplane in 10 accordance with still another embodiment of the invention. DETAILED DESCRIPTION Patch antennas and a method for controlling the beamwidth of a resonant, 15 dual-polarized patch antenna are described hereinafter. In the following description, numerous specific details, including particular conductive materials, frequency ranges, materials, and the like are set forth. However, from this disclosure, it will be apparent to those skilled in the art that modifications and/or substitutions may be made without departing from the scope and spirit of the invention. In other 20 circumstances, specific details may be omitted so as not to obscure the invention. In accordance with an embodiment of the invention, a method of reducing beamwidth is described in which the size of the patch is increased. Increasing the patch size normally is accompanied by a reduction in the resonant frequency of the patch, 25 making impedance matching of the patch to the feed impossible. In the embodiment of the invention, the resonant frequency of the patch antenna is returned to the desired value by introducing a region in the centre of the patch where the patch is connected to the ground plane. 30 Figs 2A, 2B and 2C illustrate a patch antenna 200 with a short-circuited region 225 in accordance with an embodiment of the invention. The configuration shown in Fig. 2 WO 2006/135956 PCT/AU2006/000834 -6 effectively increases the spacing of the radiating edges of a patch radiator 220 (simply, the patch hereinafter), so as to reduce the beamwidth of the patch 220 but at the same time maintaining a resonant configuration. In this embodiment, the patch 220 has a square shape. However, in other embodiments, differently shaped patches 5 may be used, such as a circularly shaped patch. The size of the resonant patch 220 required for resonance is increased. The shorted section 225 (short circuit region) is connected between a central region of the patch 220 and the ground plane 210. In this embodiment, the shorted section 225 is cylindrical in shape and may be solid or tubular in form and of conductive material, e.g. copper. The central region of the 10 patch 220 is shorted to the ground plane 210. Alternatively, the spacing between the patch 220 and the ground plane 210 is stepped down in the central region only. This has a similar effect. The shape of the shorted region is not critical, but should retain symmetry about both the orthogonal feed planes. 15 To maintain symmetry in the presence of this shorted region 225, an anti-symmetrical feed 236, 230 such as that shown Fig. 2 is used. The feed probes or loops 236 are coupled to the resonant patch 220 on opposite sides of the patch 220 and fed by signal sources 240. Other feed probes or loops 230 may be used to excite the orthogonal polarization. 20 A larger shorted region 225 requires a larger patch 220 to maintain resonance. As the size of the short-circuited region 225 is increased and the size of the patch 220 is increased to maintain resonance, the radiated beamwidth of the configuration 200 decreases smoothly. This is the desired effect. A limit to this process occurs when 25 higher order modes become excited, making the field distribution at the patch edges deviate significantly from that of a simple resonant patch. In Fig. 2, a circular grounded region 225 in the centre of the patch 220 is shown. Fig. 3 shows an alternative implementation 300 using two crossed microstrip or 30 stripline boards 330 in accordance with another embodiment of the invention. In this embodiment, the patch 320 is circular in shape. However, other shapes may be used.
WO 2006/135956 PCT/AU2006/000834 -7 The two crossed boards 330 are used both to connect the central region of the patch 320 to the ground plane 310 and to feed opposite sides of the patch in anti-phase. The two crossed boards 330 combine a magnetic-loop feed function and an adequate central grounding provided by the four inner tabs 340 and 344 on the printed boards. 5 A series capacitor 360 and impedance transformer 365 can be used to provide a wide band double-tuned frequency response. In the arrangement 300 shown, the two orthogonal boards provide feeds for two orthogonal linear polarizations. A signal is provided to each feed board through a 50-ohm coaxial cable 370. 10 Figs. 4A, 4B and 4C show in detail the two crossed microstrip printed circuit boards 330. In particular, Figs. 4A to 4C illustrate the method of feeding the patch 320 in two polarizations with two printed circuit boards 330. On one side of the board, two balanced loops 380 are etched. On the other side of the board are tracks 365 that feed the two loops 380 in anti-phase. The capacitive stubs at the ends of the track 360 15 resonate with the loops 380. These resonant circuits coupled to the resonant patch 320 form a wideband double-tuned impedance characteristic. Fig. 5 illustrates details of the two sides of the printed feed. As Figs. 5A and 5B show, the crossed microstrip printed circuit boards 330 may be implemented as two 20 separate boards, each adapted with a notch in a central region of the boards so that the boards can be assembled together to make the crossed boards 330 without interrupting the tracks required on each board. The tracks 365 provide impedance transformation to match the patch to the 50 ohm feed cable 370. This board 330 also provides the short circuit connection for the centre region of the patch while providing a 25 symmetrical anti-phase feed with a double-tuned, wideband impedance characteristic. This may be implemented on a low-loss microwave substrate. Fig. 7 illustrates a cross-section of a patch antenna 700 with a stepped down gap region 740 in accordance with another embodiment of the invention. In this 30 embodiment, the patch radiator 720 may have a square or circular shape. However, in other embodiments, differently shaped patches may be used. The patch radiator 720 is disposed at a position above the groundplane 710 (support not shown). The ground WO 2006/135956 PCT/AU2006/000834 -8 plane 710 is formed to have a central region 740 that is much more closely spaced to the patch radiator 720 than the rest of the groundplane 710. While this region 740 is shown in such a manner to indicate an internal cavity in the central region 740, this region may in fact be solid conductive material. In this embodiment, the central region 5 740 of the groundplane 710 is cylindrical in shape and may be solid or tubular in form and of conductive material, e.g. copper. This has a similar effect to the short-circuited region 225 of Fig. 2. To simplify the drawing, an anti-symmetrical feed is not shown, however one such as that shown Fig. 2 may be used used. The feed probes or loops are coupled to the resonant patch on opposite sides of the patch and fed by signal 10 sources. Other feed probes or loops may be used to excite the orthogonal polarization. Fig. 8 illustrates a cross-section of a patch antenna 800 with a stepped-down gap region 840 in accordance with still another embodiment of the invention. In this 15 embodiment, the patch radiator 820 may have a square or circular shape. However, in other embodiments, differently shaped patches may be used. The patch radiator 820 is disposed at a position above the groundplane 810 (support not shown). The patch radiator 820 is formed to have a central region 840 in the lower surface of the patch radiator 820 that is much more closely spaced to the groundplane 810 than the rest of 20 the patch radiator 820. This region 840 is preferably solid conductive material. In this embodiment, the central region 840 of the patch radiator 840 is cylindrical in shape and may be made of conductive material, e.g. copper. This has a similar effect to the short-circuited region 225 of Fig. 2. To simplify the drawing, an anti-symmetrical feed is not shown. However one such as that shown Fig. 2 may be used. The feed 25 probes or loops are coupled to the resonant patch on opposite sides of the patch and fed by signal sources. Other feed probes or loops may be used to excite the orthogonal polarization. Patch antennas and a method for controlling the beamwidth of a resonant, 30 dual-polarized patch antenna have been described. In view of this disclosure, it will WO 2006/135956 PCT/AU2006/000834 -9 be apparent to one skilled in the art that modifications and/or substitutions may be made without departing from the scope and spirit of the invention.