TRI-POIAR ANTENNA ARRAY ELEMENT

FIELD OF THE INVENTION

The present invention relates to antenna elements and in particular to beamformed antenna elements.

BACKGROUND TO THE INVENTION

In beamformed or steerable antenna systems, such as may be used in base stations for cellular telephone networks, an antenna may be comprised of an array of identical antenna elements mutually spatially arranged in a grid of m by n elements in either a planar or surface conformal arrangement.

As transmission and user bandwidths and capacities increase in order to meet user demand, the number of signals that are radiated will also increase. Typically, such increase is achieved by installing more transmitters. With the increase in the number of transmitters, one solution is to add a correspondingly higher number of antenna elements.

However, with the advent of modern beamforming antenna arrays, the addition of another antenna is no longer a trivial task, as the array comprises a monolithic structure which calls for a suitable footprint. Unfortunately the demand for transmission and user bandwidth tends to concentrate in highly urban areas where physical space is often at a premium. As a result, many site managers now rent available footprint space according to the number of antenna elements.

An initial solution was to combine multiple transmitters in a common antenna element structure.

Unfortunately, it is generally accepted as a rule of thumb that combining a transmitter to an antenna element uses 3dB of the transmitted power for each combination. Thus, to combine 4 transmitters to a single antenna imposes a 6 dB power loss. To combine 8 transmitters to a single antenna imposes a staggering 9 dB loss.

Some efficiency can be obtained with a dual polarized antenna element, where the polarizations are orthogonally oriented at 90° to one another. The component antenna elements generally each have one port per polarization and a transmitter can be connected to each port without any combination losses.

However, even with the introduction of dual polarized antenna elements, capacity is still outstripping the number of antennas available, so further economy is desired in order to avoid the combination power loss problem.

Some antenna designers have introduced so- called quadrature-polarized antenna elements, but upon investigation, these prove to be simply packs of two dual-polarized antenna elements, which have been so packaged because lease rates for base station sites are generally by the antenna element so that an economy can sometimes be realized when dealing with less observant leasing managers.

Thus, while economies are therefore achievable in terms of cost, as a practical matter, such dual packs of dual-polarized elements occupy effectively the same physical footprint as a pair of discrete dual-polarized elements and do not resolve the power loss problem inherent to combining transmitters so that there is no real economy gained.

Accordingly, there is a need for an improved antenna element that effectively utilizes the space traditionally occupied by a dual polarized antenna element configuration to house a greater number of elements, and that maximizes use of available footprint and physical resources. In this regard, the present invention substantially fulfills this need.

SUMMARY OF THE INVENTION

Accordingly, it is desirable to provide a multiply-polarized antenna element for use in a beamforming antenna array that maximizes use of available footprint and improves the available power output.

It is further desirable to provide a multiply- polarized antenna element configuration that is compact so as to satisfy other antenna design constraints.

The present invention accomplishes these aims by providing a tri-polar antenna element that utilizes the space traditionally occupied by a dual polarized antenna element configuration.

In beamformed antenna arrays, the spacing between the centres of adjacent rows and/or columns imposes a performance constraint. For example, those skilled in the relevant art will understand that exceeding array spacing threshold maxima may introduce grating lobes in the radiated signal, which is generally undesirable. As an exemplary rule of thumb, array elements may be restricted to no more than 0.5 wavelength spacing.

In a typical two-dimensional antenna array, particularly one in which the signal is steered off

centre and a narrow beam is used, one could use the de facto 0.5 wavelength spacing in the azimuthal plane but increase the spacing in the elevation plane to 0.8 wavelength spacing. The greater wavelength spacing in the elevation plane is possible because the limited skew angle of the beam typically assists in preventing the formation of undesirable grating lobes, and is usually taken into account and used in laying out beamformed dual polarization antenna arrays.

Leaving aside the performance implications, it is generally desirable to optimize the array element spacing so as to produce an antenna array with the smallest physical footprint consistent with the required radiation pattern.

The present invention takes advantage of hitherto unused space in dual-polarized antenna elements, namely the additional 0.3 wavelength spacing between elements corresponding to the elevation plane.

A single polarization (nominally vertically oriented) antenna element occupies this spacing in the present with the result that a tri-polar antenna array may occupy the same physical footprint as a comparable doubly-polarized array.

According to a first broad aspect of an embodiment of the present invention, there is disclosed a tri-polar antenna element for use in a beamforming antenna array comprising a plurality of mutually spaced- apart antenna transmitters in azimuthal and elevation directions, the antenna element comprising: a pair of slots oriented mutually orthogonally to one another and defining a first plane thereby incorporating the azimuthal and elevation directions; a dual feed network

extending along a second plane parallel to the first plane and subimposed thereunder; a dual polarization patch radiator extending along a third plane parallel to the first plane and superimposed thereover; a linear slot extending in a direction bisecting the azimuthal and elevation directions along a fourth plane coplanar with the first plane and adjacent thereto along the elevation direction; and a linear feed network extending along a fifth plane coplanar with the second plane and adjacent thereto along the elevation direction; wherein adjacent elements in the aray are sufficiently proximate so as to avoid the formation of grating lobes in a radiated signal .

According to a second broad aspect of an embodiment of the present invention, there is disclosed a tri-polar antenna element for use in a beamforming antenna array comprising a plurality of mutually spaced apart antenna transmitters in azimuthal and elevation directions, the antenna element comprising: a pair of slots oriented mutually orthogonally to one another and defining a first plane thereby incorporating the azimuthal and elevation directions, wherein one of the pair of slots extends in a direction oriented at +45° relative to the elevation direction and a second one of the pair of slots extends in a direction oriented at -45° relative to the elevation direction; a dual feed network extending along a second plane parallel to the first plane and subimposed thereunder; a dual polarization patch radiator extending along a third plane parallel to the first plane and superimposed thereover; a linear slot extending in a direction bisecting the azimuthal and elevation directions along a fourth plane coplanar with the first plane and adjacent thereto along the elevation

direction; and a linear feed network extending along a fifth plane coplanar with the second plane and adjacent thereto along the elevation direction; wherein adjacent elements in the array are sufficiently proximate so as to avoid the formation of grating lobes in a radiated signal .

According to a third broad aspect of an embodiment of the present invention, there is disclosed a tri-polar antenna element for use in a beamforming antenna array comprising a plurality of mutually spaced apart antenna transmitters in azimuthal and elevation directions, the antenna element comprising: a pair of slots oriented mutually orthogonally to one another and defining a first plane thereby incorporating the azimuthal and elevation directions; a dual feed network extending along a second plane parallel to the first plane and subimposed thereunder; a dual polarization patch radiator extending along a third plane parallel to the first plane and superimposed thereover; a linear slot extending in a direction bisecting the azimuthal and elevation directions along a fourth plane coplanar with the first plane and adjacent thereto along the elevation direction; and a linear feed network extending along a fifth plane coplanar with the second plane and adjacent thereto along the elevation direction; wherein adjacent elements in the array are sufficiently proximate so as to avoid the formation of grating lobes in a radiated signal, and wherein the spacing between antenna elements extending in the azimuthal direction is less than substantially 0.5 of the operating wavelength of the antenna array, and the spacing between antenna elements extending in the elevation direction is less than

substantially 0.8 of the operating wavelength of the antenna array.

The advantage of the present invention is that it provides an improved tri-polar antenna element for use in a beamforming antenna array that maximizes use of available footprint and physical resources, and which is compact so as to facilitate other antenna design constraints .

A further advantage of the present invention is that it provides an improved tri-polar antenna element that effectively utilizes the space traditionally occupied by a dual polarized antenna element configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention is described below with reference to the accompanying drawings, in which:

Figure 1 shows a passive array antenna system in accordance with a preferred embodiment of the present invention;

Figure 2 is a cross section of a composite

polarization antenna element of the embodiment of Figure 1;

Figure 3 is a partially exploded view of the

element of Figure 2;

Figures 4A-4E show various embodiments of a patch radiator for use in the embodiment of Figure 1;

Figure 5 shows a linear slot for use in the

embodiment of Figure 1;

Figure 6 shows a crossed slot structure for use

in the embodiment of Figure 1;

Figure 7 shows a linear slot feed network

structure for use in the embodiment of Figure 1;

Figure 8 shows a crossed slot feed network

structure for use in the embodiment of Figure 1; and

Figure 9 shows an air bridge structure for use in the embodiment of Figure 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 shows the primary elements of a passive array antenna system comprising a plurality of multiply-polarized antenna elements 100. Each port corresponding to a polarization of an antenna element is connected to a plurality of first beamforming networks 110 used to shape the beam in a first radiating plane, which may be the azimuthal plane or the elevation plane. In one embodiment of the present invention, a narrow beam is generated by the antenna array and is steered off centre .

In accordance with the above-described generally accepted rule of thumb, the spacing between

array elements extending in the azimuthal direction is less than substantially 0.5 of the operating wavelength of the antenna array, while the spacing between array elements extending in the elevation direction is less than substantially 0.8 of the operating wavelength of the antenna array.

The combined ports 120 of the first beamforming networks 110 are connected to input ports 130 of a second beamforming network 140 used to shape the beam in a second radiating plane orthogonal to the first radiating plane. Thus, the two sets of beamforming networks 110, 140 are capable of forming a composite three dimensional radiating pattern. The output 150 of the second beamforming network 140 passes to the output port 160 of the antenna array, which is connected to the input of a base station system (not shown) .

Figure 2 shows a composite polarization antenna element in cross-section. In the exemplary design, the antenna element comprises a cavity backed, slot fed tri- polarized patched element. Such an element comprises the components that one of ordinary skill in the relevant art might use to implement a cavity-backed slot fed dual polarized patched element as is known in the art, namely, in order from the back of the radiating element to the front, a cavity structure 210, a dual feed network 230, a double sided printed circuit board 220, a pair of slots 240, a linear slot 250, a plurality of field suppression fingers 290, and a patch radiator 270 on a substrate 260. With reference again to Figure 2, the feed network 230 is connected to the external beamforming elements by coaxial tube assemblies 280. The dual feed network 230 is largely to provide fields to drive the patch radiator 270 by

exciting the right field structure on the patch radiator 270.

The cavity structure 210 is preferably a sheet metal structure that ensures that substantially all of the radiated energy emerges out of the front of the antenna element. The general arrangement of the cavity structure 210 is shown in exploded view in Figure 3. It comprises a shallow 5-sided brass box 210 for each antenna element which may be, for exemplary purposes only, approximately 10 mm deep and may be manufactured using a variety of different materials such as would appeal to one having ordinary skill in this art, taking into account the choice and relative merits of each. The cavities may be individual or grouped together as shown in Figure 3.

The exemplary depth of 10 mm ensures that the array element may achieve an adequate bandwidth for the application, in this case, the PCS frequency band of approximately 1900 MHz. However, those having ordinary skill in this art will readily recognize that the depth of the cavity structure 210 may be adjusted to accommodate other applications and/or operating frequency bands. In an embodiment of the present invention, the patch radiator 270 is supported by the cavity structure 210 proximate to the surface of the substrate facing away from the cavity structure.

The slot pairs 240 are used in dual polarization elements in order to minimize any mutual coupling between adjacent antenna elements. In one embodiment of the present invention, the pair of slots 240 intersect one another. Preferably, the slots 240 are set out in a crossed slot configuration to minimize its

footprint. In a preferred embodiment of the present invention, one of the slots extends in a direction oriented at +45° relative to the elevation direction and the other slot extends in a direction oriented at -45° relative to the elevation direction. The general arrangement of this crossed slot structure is shown in Figure 6. It comprises a crossed slot pair 610 printed on a low loss substrate 600 and is used to provide radiation in two orthogonal planes. The choice of substrate is well known to those having ordinary skill in this art. In the case of an exemplary cellular base station antenna system, these are the +45° and -45° planes.

As is known to those having ordinary skill in this art, and as can be seen in Figure 2, the feed network 230 and the slots 240 are mounted on opposite sides of a double sided printed circuit board 220 supported by the cavity structure 210, with the dual feed network 230 disposed on the surface of the double sided printed circuit board 220 and facing the inside the cavity structure 210, and with the slots 240 facing toward the patch radiator 270.

The general arrangement of the crossed slot feed network structure is shown in Figure 8, and comprises a pair of substantially identical (but reversed in orientation) tuning fork feed networks 810 with matching sections 820. The fork feed networks 810 and matching sections 820 may be approximately mirrored. In an embodiment of the present invention, each of the feed networks 810 are substantially a mirror image of the other. This mirroring would be approximate as only the large features would be mirrored; small features could be adjusted to suit the matching between the two feed networks .

Because the two feed networks will cross over, as can be seen in Figure 8 where the feed networks overlap at an overlap location 860, cross coupling at the points of intersection is minimized by tapering the lines in this region, as shown in detail 830, so that a width of the feed networks can be tapered proximate to the overlap location. A fine printed line 840 is used for one feed and a ceramic chip zero ohm resistor 850 is used at the overlap location to bridge the line for the other feed over it while ensuring conductive isolation between them. Those having ordinary skill in the art will readily recognize that according to the designer' s preference, a conventional air bridge structure 910, such as is shown in Figure 9 could also be used at the overlap location in place of the zero ohm resistor 850. In another embodiment of the present invention, one of the feed networks may be spaced apart from the other feed network at the overlap location. In the illustrated exemplary embodiment of Figure 8, a consistent structure is used for each of the crossovers, although it will be recognized by those having ordinary skill in this art that other arrangements may be used generally or in specific instances as warranted. However, the nature of the implementation of the crossovers will affect the precise detail of the design of the corresponding matching network 870. In any event, coaxial transitions are provided for both polarizations.

The patch radiator 270 is the active or radiating part of the antenna element. Preferably, the patch radiator is annular and is silkscreened onto a substrate such as polycarbonate using a highly conductive ink, such as a silver-loaded ink, etched copper on a microwave quality printed circuit board or solid metal

suspended by plastic spacers. Figures 4A through 4E illustrate various possible examples of patch radiators or patch elements that could be utilized m the present invention, though it will readily understood by one skilled in the art that numerous other variations are possible from those illustrated. As shown m Figures 4A through 4E, the patch radiator 270 comprises patch element 410, printed on a supporting board structure 400 mounted over the remainder of the antenna elements via mounting holes 420, and may be manufactured using a variety of materials such as foam, sheet or composite dielectric materials. Optionally, a central region of the patch element 410 may be devoid of material 430, as seen m Figures 4B,4D and 4E.

Frequently, because of the layout constraints of such devices, the additional 0.3 wavelength spacing in the elevation plane is used up in the layout of the patch radiator 270 on the substrate.

A preferably electrically clear radome 295 (shown in Figure 2) is provided over the element for environmental protection.

To minimize mutual coupling between the various elements and between element assemblies when mounted into an array, a plurality of field suppression fingers 290 (shown in Figure 3) are built into the cavity structure 210 and are also used to support the double sided printed circuit board 220 and patch radiator 270 thereon. The fingers 290 are provided on four of the sides of the cavity structure 210 to control and limit any mutual coupling between elements. Those having ordinary skill m this art will readily recognize that other manufacturing methods may be feasible and well known and

may be implemented without departing from the spirit and scope of the present invention.

To implement the third vertically polarized antenna element, that portion of the printed circuit board 220 that corresponds to the additional 0.3 wavelength spacing in the elevation plane (corresponding to an additional length of 47 mm at the exemplary PCS operating wavelength of 1900 MHz is available for use and is occupied by a linear slot feed network structure 750 (shown in Figure 7) adjacent to the dual feed network 230 and a corresponding linear dumbbell slot structure 550 on low loss substrate 500 (shown in Figure 5) adjacent to the crossed slot structure 610 on low loss substrate 600

(shown in Figure 6) .

The general arrangement of the linear slot feed network structure is shown in Figure 7. It comprises a 50 ohm line (not shown) exciting the slot together with a single section matching network 720 and a coaxial transition 730, all printed on the low loss substrate 700, which is common with the dual sided printed circuit board 220.

Because it is not dual polarized, no patch radiator is superimposed thereover, with the result that no accommodation need be made in the layout of the patch radiator 270 for the dual polarized antenna element across the additional 0.3 wavelength spacing in the elevation plane, which the patch radiator 270 in any event occupies in part.

Because the polarization on the dual polarized antenna element is typically at +45° and -45° and the third polarization of the present invention is vertically polarized, there is in fact some cross coupling between

the signal for the third polarization and those for the dual polarities of the dual polarized antenna element. However, because any single user is typically concerned with only one polarization, the cross-coupling problem can be easily dealt with using sector design considerations known to those having ordinary skill in the relevant art.

Because there are now 3 independent radiators, one would expect to achieve an overall 1.77 dB improvement in the available power output using the present inventive tri-polar antenna element structure.

Other embodiments consistent with the present invention will become apparent from consideration of the specification and the practice of the invention disclosed therein.

Accordingly, the specification and the embodiments are to be considered exemplary only, with a true scope and spirit of the invention being disclosed by the following claims.