Abstract:
A sub-array of slot-coupled microstrip antennas fed using microstrip lines on an opposing substrate. Also provided is an omni-directional antenna comprised of six of the sub-arrays arranged in a hexagonal fashion. The gain of the antenna is ˜6 dB with a 3 dB elevation beam width of ˜30 degrees. The design provides constant beam angle over frequency, which is important for frequency-hopping applications, and the potential to add beam control to mitigate jamming in different sectors.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to currently U.S. Provisional Patent Application No. 61/044,646 filed Apr. 14, 2008. 
    
    
     FIELD OF INVENTION 
     This invention relates to antennas and more specifically to series-fed aperture-coupled microstrip patch antenna arrays for use in wireless antenna communications. 
     BACKGROUND 
     Aperture-coupled microstrip patch antennas are desirable structures for use in wireless telecommunications. Their broad use is primarily due to ease of fabrication, low cost, and simplicity of design. These characteristics, combined with the straightforward integration with microstrip distribution networks, make them especially well suited for phased array applications. 
     High-gain omni-directional antennas find uses in several communications applications including those for small aerial vehicles. Several topologies for omni-directional radiators exist and include linear arrays using bifilar helical elements, periodic rod antennas, coaxial continuous transverse stub arrays (C-CTS), and patch arrays on a cylindrical body. These approaches typically suffer from beam-pointing variation over frequency, do not offer the capability for beam steering for attitude correction, and do not facilitate advanced beam-reconfiguration options such as eliminating coverage from certain sectors for jamming avoidance. 
     SUMMARY OF INVENTION 
     The present invention includes a dual series-fed, four microstrip patch array antenna that utilizes planar design for ease of fabrication and signal routing. The natural tendency of a series-fed array to have beam tilting over frequency is circumvented by using opposing, anti-symmetric balanced feed points. An embodiment uses 180-degree microstrip hybrid couplers to feed pairs of patch elements on each sub-array. This approach makes this element suitable for low-cost frequency-hopped phased array antennas. An approach for inter-element matching to evenly distribute power to each element is also described. 
     The present invention also includes an omni-directional antenna comprising multiple sub-arrays arranged in a cylindrical or hexagonal configuration. The three-dimensional antenna works over 600 MHz of bandwidth in the C-band with a maximum gain of ˜6 dB. 
     In accordance with the present invention, a microstrip patch antenna array is provided. The antenna comprises two aperture-coupled patch antenna elements positioned in a single row arrangement and a feed line coupled to the two antenna elements such that the two antenna elements are connected in series. The feed line has two open-circuit stubs—the first stub positioned between the two antenna elements and the second stub positioned on the second antenna element. The antenna may further comprise a third and a fourth aperture-coupled patch antenna elements, both positioned in a single row arrangement with the first two antenna elements, and a feed line coupled to the third and fourth antenna elements such that the third and fourth antenna elements are connected in series. The feed line connected to the third and fourth elements also has two open-circuit stubs—the first stub positioned between the two antenna elements and the second stub positioned on the third antenna element. Both feed lines are adapted to receive an input signal. The microstrip patch antenna array may further comprise circuitry for dividing an input signal into two component signals and phase shifting one of the component signals. The circuitry has two outputs, one coupled to each feed line. This circuitry may be a coupler. Alternatively, this circuitry may be a two-way power divider and a phase shifter. 
     In accordance with the present invention, a multi-directional antenna is provided. The antenna comprises at least two sub-arrays of microstrip patch antennas arranged such that each sub-array forms a single face of a multi-shaped three-dimensional geometric shape. Each of the at least two sub-arrays comprises two pairs of aperture-coupled patch antenna elements in which all four antenna elements are positioned in a single row arrangement. Each pair of antenna elements includes a feed line, which is coupled to each of the antenna elements such that the two elements of the pair are connected in series. Each feed line has two open-circuit stubs, the first stub positioned between the two antenna elements of the pair and the second stub positioned on one of the antenna elements of the pair. The at least two sub-arrays may further comprise splitting and offsetting circuitry for dividing an input signal into two component signals and phase shifting one of the component signals. The splitting and offsetting circuitry has two outputs, one coupled to each feed line. This circuitry may be a coupler. Alternatively, this circuitry may be a two-way power divider and a phase shifter. The antenna may further comprise a multi-way power divider, which is coupled to the coupler, the two-way power divider, or the splitting and offsetting circuitry of each of the sub-arrays. Each antenna element may be comprised of a feed substrate and a patch substrate. Each of the feed substrates faces toward the inside of the hexagonal three-dimensional geometric shape. Each antenna element may further comprise a ground layer positioned in between the feed substrate and the patch substrate, the ground layer being continuous between the sub-arrays. The ground layer may be formed from conductive silver epoxy and copper tape. The antenna may further comprise a reflector positioned within the multi-shaped three-dimensional geometric shape to preserve backside radiation. 
     In an additional embodiment, the multi-directional antenna comprises a first microstrip patch antenna element with a coupling slot and positioned such that the first antenna element forms a first face of a multi-shaped three-dimensional geometric shape, a first feed line forming an open-circuit stub on the first antenna element, a second microstrip patch antenna element having a coupling slot and positioned such that the second antenna element forms a second face of the multi-shaped three-dimensional geometric shape, and a second feed line forming an open-circuit stub on the second antenna element. The antenna may further comprise a multi-way power divider having a first output coupled to the first feed line and a second output coupled to the second feed line. Each antenna element may be comprised of a feed substrate and a patch substrate, wherein each of the feed substrates faces toward the inside of the hexagonal three-dimensional geometric shape. Each antenna element may further comprise a ground layer positioned in between the feed substrate and the patch substrate, the ground layer being continuous between the sub-arrays. The ground layer may be formed from conductive silver epoxy and copper tape. The antenna may further comprise a reflector positioned within the multi-shaped three-dimensional geometric shape to preserve backside radiation. 
     A method for providing symmetrical excitation of a microstrip patch array antenna about a central point in accordance with an embodiment of the present invention includes the step of providing a microstrip patch array antenna. The microstrip patch array antenna comprises two pairs of aperture-coupled patch antenna elements. Each pair of antenna elements includes a feed line, which is coupled to each of the antenna elements such that the two elements of the pair are connected in series. Each feed line has two open-circuit stubs—the first stub positioned between the two antenna elements of the pair and the second stub positioned on one of the antenna elements. The method further comprises applying a first signal to one of the feed lines and applying a second signal to the other feed line, wherein the first signal and the second signal are about 180 degrees out of phase. The provided microstrip patch antenna may further comprise circuitry for dividing an input signal into two component signals and phase shifting one of the component signals to create the two signals that are 180 degrees out of phase. The circuitry has two outputs, one coupled to each feed line for outputting the two component signals. This circuitry may be a coupler. Alternatively, this circuitry may be a two-way power divider and a phase shifter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a top view schematic of the feed network of a four-element patch antenna array having two feed points, one at each end of the array in accordance with an embodiment of the present invention. 
         FIG. 2  is a cross-sectional view of an aperture-coupled patch antenna element in accordance with an embodiment of the present invention. 
         FIG. 3  is a top view schematic of the feed network of a pair of aperture-couple patch antenna elements illustrating impedance matching in accordance with an embodiment of the present invention. 
         FIG. 4A  is a top view schematic of the feed network of a four-element patch antenna array having a coupler to split a inbound signal into anti-phase components in accordance with an embodiment of the present invention. 
         FIG. 4B  is a top view schematic of the feed network of a four-element patch antenna array having a splitter to split an inbound signal into two components and a phase shifter to create anti-phase components in accordance with an embodiment of the present invention. 
         FIG. 5A  is an isometric diagram of the omni-directional antenna in a hexagonal configuration as simulated in HFSS in accordance with an embodiment of the present invention. 
         FIG. 5B  is a diagram of the top view of the omni-directional antenna in a hexagonal configuration in accordance with an embodiment of the present invention. 
         FIG. 6  is a diagram of a four-element patch antenna array having a coupler fabricated on a planar PCB substrate receiving an input signal from a power divider in accordance with an embodiment of the present invention. 
         FIG. 7  is a three-dimensional polar plot of the omni-directional antenna in a hexagonal configuration simulated in HFSS in accordance with an embodiment of the present invention. 
         FIG. 8  is a graph showing the measured S 11  (the amount of power reflected from the antenna) for each of six sub-arrays of the omni-directional antenna in a hexagonal configuration in accordance with an embodiment of the present invention. 
         FIG. 9  is a plot of the azimuth radiation pattern at theta=90 degrees (broadside) for the omni-directional antenna in a hexagonal configuration in accordance with an embodiment of the present invention. 
         FIG. 10  is a plot of the elevation radiation pattern for the omni-directional antenna in a hexagonal configuration in accordance with an embodiment of the present invention. 
         FIG. 11  is a graph showing the return loss for different sizes of ground planes for the single sub-array in accordance with an embodiment of the present invention. 
         FIG. 12  is a plot of the elevation radiation pattern at phi=0 degrees (broadside) for different sizes of ground planes for a single sub-array in accordance with an embodiment of the present invention. 
         FIG. 13A  is an isometric diagram of the omni-directional antenna in a hexagonal configuration with a single patch antenna positioned on each face of the hexagonal prism as simulated in HFSS in accordance with an embodiment of the present invention. 
         FIG. 13B  is a diagram of the top view of the omni-directional antenna in a hexagonal configuration in accordance with an embodiment of the present invention. 
         FIG. 14A  is a plot of the simulated radiation pattern for the omni-directional antenna of  FIG. 13  with a single element excited in accordance with an embodiment of the present invention. 
         FIG. 14B  is a plot of the simulated radiation pattern for the omni-directional antenna of  FIG. 13  with a three neighboring elements excited in accordance with an embodiment of the present invention. 
         FIG. 14C  is a plot of the simulated radiation pattern for the omni-directional antenna of  FIG. 13  with a all six elements excited in accordance with an embodiment of the present invention. 
         FIG. 15  is a diagram of a four-element patch antenna array fabricated on a planar PCB substrate receiving an input signal from a power divider illustrating pair spacing and element spacing in accordance with an embodiment of the present invention. 
         FIG. 16A  is a graph showing the top view and side view of the patch elements with equal element spacing and pair spacing in accordance with an embodiment of the present invention. 
         FIG. 16B  is a diagram illustrating the pattern variation for the element spacing and pair spacing as shown in  FIG. 16A  in accordance with an embodiment of the present invention. 
         FIG. 17A  is a graph showing the top view and side view of the patch elements with a variation in pair spacing in accordance with an embodiment of the present invention. 
         FIG. 17B  is a diagram illustrating the pattern variation for varying pair spacing as shown in  FIG. 17A  in accordance with an embodiment of the present invention. 
         FIG. 18A  is a graph showing the top view and side view of the patch elements with equal element spacing and pair spacing and showing the feed phase, φ in accordance with an embodiment of the present invention. 
         FIG. 18B  is a diagram illustrating the pattern variation for the relative feed phase as shown in  FIG. 18A  in accordance with an embodiment of the present invention. 
         FIG. 19  is a graph illustrating the power rating characteristics of substrate materials used in fabricating the antenna in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention. 
     The design of a low-cost microstrip patch antenna suitable for frequency-hopped communications is presented. Two of the main considerations were to achieve an instantaneous bandwidth greater than 10% and to minimize the elevation beam-angle variation over frequency. A suitable solution to these requirements is an N×1 microstrip patch array. As shown herein, the use of an aperture-coupled feed along with the proper choice of substrate materials provides sufficient bandwidth and also avoids the need for live vias or their equivalent. A series-fed approach, combined with an anti-symmetric dual excitation from both ends of the array, helps to address the elevation beam-pointing specification and reduces the distribution network complexity. The dual-feed forces excitation symmetry about the center of the array, thereby keeping the elevation beam fixed at broadside independent of frequency. 
     With reference to  FIG. 1 , dual series-fed microstrip patch antenna array  10  in accordance with an embodiment of the present invention is illustrated, comprising two pairs of antenna patch elements (first pair  20  and second pair  40 ), and two microstrip lines (first microstrip line  23  and second microstrip line  43 )—one used to excite each pair of patch elements. The four elements are positioned in a single line (or row) as shown in  FIG. 1 . 
     First pair  20  comprises two aperture-coupled patch antenna elements—first patch element  21  and second patch element  22 . First pair  20  also comprises first microstrip line  23  having first feed  24  positioned at a first end of antenna array  10 . First feed  24  is used to excite first patch element  21  and second patch element  22 . 
     The design of second pair  40  mirrors the design of first pair  20 . Second pair  40  includes two aperture-coupled patch antenna elements—third patch element  41  and fourth patch element  42 . Second pair  40  also includes second microstrip line  43  having second feed  44  positioned at a second end of antenna array  10 . Second feed  44  is used to excite third patch element  41  and fourth patch element  42 . 
     Second patch element  22  has microstrip stub  25 , which is a short, open-circuit stub of microstrip line  23  that extends just beyond coupling slot  26 . Third patch element  41  has microstrip stub  45 , which is a short, open-circuit stub of microstrip line  43  that extends just beyond coupling slot  46 . Microstrip stub  25  and microstrip stub  45  facilitate impedance matching in antenna array  10 . 
     In addition, first pair  20  includes stub  27  located between first patch element  21  and second patch element  22 . Stub  27  is used to achieve equal power distribution between coupling slot  28  of first patch element  21  and coupling slot  26  of second patch element  22 . Similarly, second pair  40  includes stub  47  between third patch element  41  and fourth patch element  42 . Stub  47  is used to achieve equal power distribution between coupling slot  46  of third patch element  41  and coupling slot  48  of fourth patch element  42 . In order to account for the difference in the microstrip feed-line directions, the signals applied to each end of array  10  (at first feed  24  and second feed  44 ) are 180 degrees out of phase. 
     A cross-sectional view of aperture-coupled patch  50  is shown in  FIG. 2 . The basic design of aperture-coupled patch  50  was first presented by D. M. Pozar and typically consists of two substrates, patch substrate  51  and feed substrate  52 , separated by ground plane  53  that is perturbed by coupling slot  54 . “A Microstrip Antenna Aperture Coupled to a Microstrip Line,” Electronics Letters, Vol. 21, pp. 49-50 (Jan. 17, 1985). Microstrip patch antenna  55  is positioned on patch substrate  51  and microstrip feed line  56  is positioned on feed substrate  52 . The thickness and dielectric constant of the two substrates can be independently selected to optimize radiation characteristics (patch substrate  51 ) and feed network loss or size (feed substrate  52 ). The power from feed line  56  is coupled through ground plane slot  54  to patch antenna  55 . In an exemplary embodiment of the present invention, a 20 mil Rogers 4003 (∈ r ˜3.6) feed substrate and a 125 mil Rogers 5880 (∈ r ˜2.1) patch substrate were used and the overall size of antenna array  10  was 45×115 mm 2 . 
     The antenna arrays may be fabricated using standard lithography and copper etching methods. In an embodiment, copper tape may be used to bond the ground planes of adjacent sub-arrays together in order to provide continuity of the ground plane around the three-dimensional structure. Silver epoxy was utilized to ensure proper connection between ground planes and the copper tape. Proper alignment of the feed network to the patch antenna layer was achieved through the use of Teflon screws as alignment marks. 
     Equivalent circuit models were used to optimize the feed network for return loss performance and equal power distribution. Numerical electromagnetic simulations were performed using Agilent&#39;s  Momentum , and from these results, equivalent circuit models for individual aperture-coupled patch designs were extracted and validated. The topology used in the model, illustrated in  FIG. 3 , consisted of an inductor to represent the coupling slot  62 , in parallel with a series RLC to emulate each patch element—first patch element  64  and second patch element  65 . The circuit models were used in a network representation for each pair of elements. Along with the patch models, the network included open-circuit stub  61  terminating the inner-element feed-line, inter-element matching network  63 , and input feed line  66 . Matching network  63  was used to transform the input impedance of second patch  65  to Z′ 2 , such that
 
 Z′   2 =conj( Z   1 )  (1)
 
where Z 1  is the impedance of first patch  64 . Note that the input impedance of second patch  65  includes the effect of open-circuit stub  61 , which, in part, controls the resonance of second patch  65 . As a result of the impedance transformation, the input impedance at the feed point becomes
 
 Z   in =2 ·Re ( Z   1 )  (2)
 
in accordance with the series configuration. Neglecting transmission line loss, Equation (1) ensures equal power delivery to the antenna elements. The real-valued Z in  could be further transformed in order to maximize return loss although in this design the value was sufficiently close to 50 Ohms. Furthermore, the impedance matching approach does not, in itself, ensure equal phase excitation at the two elements; this is a requirement for broadside radiation from this pair. As shown in  FIG. 1 , however, a phase imbalance can be tolerated because second pair  40  restores phase symmetry about the center of four-element array  10 . The phase symmetry is frequency independent and ensures a fixed broadside pattern.
 
     With reference to  FIG. 4A , microstrip patch antenna array  70  in accordance with an embodiment of the present invention is illustrated, comprising four aperture-coupled microstrip patch antenna elements  81 ,  82 ,  91 ,  92  positioned in a single line (or row). Each pair of patches  80 ,  90  employs a series-fed approach with an open-circuited stub for matching purposes as described in above and illustrated in  FIG. 1 . 
     The feeding approach uses coupler  93 , which operates as a splitter/combiner. In an exemplary embodiment a hybrid rat race coupler is used, which provides a 180-degree phase offset between the two outputs of the coupler. The input signal for coupler  93 , which is carried along input line  94 , is equally split into anti-phase components by coupler  93 , and these components are subsequently used to feed array  70  from each end. Assuming proper phase balance from coupler  93  over the desired frequency band, this configuration ensures symmetric excitation of array  70  about the central point of antenna array  70  and thus a fixed beam angle. 
     The coupler was designed and simulated using Agilent&#39;s  Momentum  and optimized for performance at 5 GHz. In an exemplary embodiment, the microstrip lines leading into the coupler were meandered to reduce size and to avoid adverse effects of fringing fields near the coupling slots. A comparison of the simulation results between the array fed by coupler  93 , and the same array fed at each side by anti-phase signals ( FIG. 1 ) showed negligible coupling effects from the close proximity of coupler  93  to the coupling slots for the patches. 
     With reference to  FIG. 4B , microstrip patch antenna array  100  in accordance with an embodiment of the present invention is illustrated, comprising four aperture-coupled microstrip patch antenna elements  111 ,  112 ,  121 ,  122 . Each pair of patches  110 ,  120  employs a series-fed approach with an open-circuited stub for matching purposes as described in above and illustrated in  FIG. 1 . Antenna array  100  is similar to antenna array  70  shown in  FIG. 4B  except that antenna array  100  employs a splitter  101  and phase shifter  102  instead of a coupler. 
     The feeding approach of antenna array  100  uses splitter  101  to divide the incoming signal carried on input line  103  into two equal components. One signal component carried on line  104  is used to feed pair  120 . The other signal component is carried on line  105  to phase shifter  102  where it is phase-shifted 180 degrees. This phase-shifted signal is then carried on line  106  and used to feed pair  110 . The splitter and phase shifter combination thereby creates equally split anti-phase components of the incoming signal, which are then used to feed array  100  from each end. 
     With reference to  FIG. 5 , omni-directional antenna  150  in accordance with an embodiment of the present invention is illustrated, comprising a plurality of sub-arrays  160 ,  161 ,  162 ,  163 ,  164 ,  165 ,  166  arranged in a manner such that each sub-array  160 ,  161 ,  162 ,  163 ,  164 ,  165 ,  166  is positioned on the face of a hexagonal prism.  FIG. 5A  is a diagram of an isometric view of omni-directional antenna  150  as rendered by Ansoft&#39;s HFSS. A diagram illustrating the top view of omni-directional antenna  150  is shown in  FIG. 5B . Each sub-arrays  160 ,  161 ,  162 ,  163 ,  164 ,  165 ,  166  comprises an array of antenna elements and may include the array embodiments described above and illustrated in  FIGS. 1 and 4 . A hexagonal shape is used for illustrative purposes. Any three-dimensional geometrical shape conducive to creating a multi-directional coverage is anticipated by the present invention including, but not limited to, cylinders, cones, and multi-sided prisms, such as hexagonal prisms, triangular prisms, rectangular prism, pentagonal prisms, octagonal prism, and a decagonal prism. 
     In the exemplary embodiment shown in  FIG. 5 , the array illustrated in  FIG. 4A  was used for each of six sub-arrays placed in a hexagonal prism shape. 
       FIG. 6  shows a schematic of antenna array  70  (also illustrated in  FIG. 4A ) fabricated on planar PCB substrate  71 . A radio signal is received by power divider  97 . Power divider  97  is illustrated in  FIG. 6  as a six-way power divider; however, any number of power divisions is contemplated according to the number of array used to create the omni-directional antenna. Each divided signal is routed via a transmission line to one of the sub-arrays in the omni-direction antenna. For illustrative purposes,  FIG. 6  only shows a single transmission line coupled to one of the sub-arrays. However, the remaining power divider outputs would each be connected to each remaining sub-array. 
     The three-dimensional structure illustrated in  FIG. 5  was simulated in Ansoft&#39;s  HFSS . Hexagonal prism  155  located around the array structure is the radiation boundary used in the simulations. 
     The three-dimensional plot of the simulated radiation pattern, given in  FIG. 7 , illustrates superior omni-directional coverage. Although the antenna is not perfectly cylindrical (rather, it is hexagonal) the variation in gain over azimuth was only approximately +/−0.5 dB. The simulated maximum gain was ˜6 dB at 5 GHz. 
     Six sub-arrays were fabricated and the return loss of each was measured to verify reasonable uniformity in the prototype fabrication process. As shown in  FIG. 8 , the measured S 11  for all six arrays are in relatively close agreement. The variation that is observed may be due to using an inconsistent amount of non-conductive epoxy in bonding each antenna layer and feed layer together. 
     To assemble the three-dimensional structure, six fabricated sub-array feed layers were first mounted on a hexagonal Teflon apparatus. The Teflon holder was designed to provide structural support for the sub-arrays while minimizing electromagnetic interaction with the feed layers that face toward the center of the holder. As described above, copper tape and conductive silver epoxy were used to form a continuous ground plane between the sub-arrays. Continuity of the ground layer was essential in preventing the occurrence of nulls in the azimuth radiation pattern. The six sub-arrays were fed using an 8-way 0-degree coaxial coupler (Mini Circuits P/N ZB8PD-6.4) with two ports terminated in a matched load. 
     Comparisons between measured and simulated radiation patterns are given in  FIGS. 9 and 10 . The measured azimuth radiation pattern at 5 GHz agrees closely the simulated results from HFSS ( FIG. 9 ), and demonstrates a variation of ˜+/−1.5 dB over the 360-degree span. Although the antenna measurement system that was used did not readily enable a full elevation cut to be measured, the data taken between +/−45 degrees is in good agreement with the HFSS simulation results ( FIG. 10 ). The simulated and measured 3 dB beam widths are 35 and 30 degrees, respectively. 
     Additional HFSS simulations were performed in order to investigate the impact of increasing the size of the ground plane, and thus the substrate surrounding the patch elements. The fabricated sub-arrays had 10 mm of ground/substrate extending beyond the edges of patches, partly to accommodate the Teflon alignment screws. S 11  results for different ground plane extensions for the single sub-arrays showed that minimal performance variation was introduced for ground extensions ranging form 4.8 to 12.8 mm ( FIG. 11 ). The impact of ground plane size on the sub-array radiation pattern was likewise relatively small, with the most noticeable differences occurring at the back-lobe direction ( FIG. 12 ). The sub-array widths can be reduced in order to shrink the diameter of the hexagonal structure, and further improve the uniformity of the omni-directional coverage. 
     With reference to  FIG. 13A , omni-directional antenna  200  in accordance with an embodiment of the present invention is illustrated, comprising a plurality of patch antennas elements  210 ,  220 ,  230 ,  240 ,  250 ,  260  arranged in a manner such that each patch antenna element  210 ,  220 ,  230 ,  240 ,  250 ,  260  is positioned on the face of a hexagonal prism.  FIG. 13A  is diagram of a isometric view of omni-directional antenna  200  as rendered by Ansoft&#39;s HFSS. A diagram illustrating the top view of omni-directional antenna  200  is shown in  FIG. 13B . Each patch antenna element  210 ,  220 ,  230 ,  240 ,  250 ,  260  having an aperture-coupled patch antenna. 
     Radiation patterns from simulations of this designed performed using HFSS are shown in  FIGS. 14A-14C .  FIG. 14A  shows a radiation pattern of omni-directional antenna  200  with excitations of a single element, such as element  230 .  FIG. 14B  shows the radiation pattern when three neighboring elements, such as element  230 , element  240 , and element  250  are excited.  FIG. 14C  shows the radiation pattern when all six elements  210 , 220 , 230 , 240 , 250 , 260  are excited. The results indicate that, when all six elements are excited, the variation in directivity versus azimuth angle is ˜+/−0.35 dB. 
     Analyses using ideal point radiators were performed in order to study the impact of design parameters including pair spacing and element spacing. The parameters involved in pair spacing and element spacing are illustrated in  FIG. 15 . For analysis purposes, the design assumes six sub-arrays  300  spaced around a hexagonal body (as was illustrated in  FIGS. 5A and 5B ). Each sub-array  300  is comprised of two pairs of two patch antenna elements (first pair  310  and second pair  320 ). First pair  310  comprises first patch antenna element  311  and second patch antenna element  312 . Second pair  320  comprises third patch antenna element  321  and fourth patch antenna element  322 . 
     Pair spacing  350  is the distance between first pair  310  and second pair  320 . Element spacing  360  is the distance between two patch antenna elements of a pair, such as the distance between first element  311  and second element  312 . The simulated patterns assuming uniform spacing (pair spacing  350  equal to element spacing  360 ), perfect phase balance, and an inner element amplitude that is 70% of the outer element amplitude, are shown in  FIGS. 16A and 16B . In this example, a side lobe begins to form at ˜λ/4 spacing. The difference in the amplitudes between the inner element (second element  312  and third element  321 ) and the outer elements (first element  311  and fourth element  322 ) was included due to the series nature of the feed. The choice of 70% is arbitrary and only used for demonstration purposes. 
     A second example, looking at the variation of pair spacing d 1 , with a constant sub-element spacing of λ/4, is shown in  FIGS. 17A and 17B . Once again, the inner sub-element amplitude is assumed to be 70% of the outer sub-element amplitude. These results show that pair spacing  350  can be reduced to minimize side lobes, thereby providing some flexibility after pair design is optimized for impedance match, etc. 
     As a final example,  FIGS. 18A and 18B  show the pattern with variations in the relative phase on the right side (or top) feed. As before, the inner sub-element amplitude is 70% of the outer sub-element amplitude and the sub-element and pair spacings are λ/4. The results show that a phase difference of 15 degrees results in a pattern shift of ˜5 degrees, thus indicating that the performance is relatively insensitive to feed phase error. 
     Power handling capacity is usually associated with temperature rise and maintaining the operating temperature below the rated value for the given material. The main concern is that the traces will delaminate. For the materials selected for an exemplary embodiment of this invention (Rogers 4003 and 4350) maintaining the continuous operating temperature below 125° C. is recommended, which means that the temperature rise should be less than 100° C. (assuming 25° C. ambient temperature). The minimum substrate thickness planned is 20 mils, and according to  FIG. 19 , at 2 GHz˜180 W is required to yield 100 degrees C. temperature rise. A conservative estimate is that the 200 W total power will be split between 6-8 elements, such that the power handling capability will be considerably more than adequate for the antenna elements and feed network traces. 
     It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall there between.