Abstract:
A circularly polarized array antenna ( 30 ) is disclosed. A single layer dielectric substrate ( 36 ) has a ground plane ( 32 ) located on its upper surface of the substrate and covering only part of the upper surface. A plurality of antenna elements ( 40 - 54 ) are also located on said upper surface of the substrate. Each antenna element has a slot element ( 60 - 74 ) formed in the ground plane and a respective loading element ( 80 - 94 ) located within each slot element. The antenna elements being arranged in a regular array where each respective slot element is sequentially rotated in space with respect to adjacent slot elements, and the loading elements generate a perturbation under excitation. A microstrip feed network ( 100 ) is located on the underside of the substrate to provide excitation to each slot element, and including feeds of different lengths to be electrically sequentially rotated in common with spatial rotation of the slot elements. A single microstrip feed point ( 108 ) extends to the edge of the substrate for connection purposes. A reflecting plane is located parallel to and spaced apart from the underside of the substrate. The ground plane extends to cover the entire microstrip feed array.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/AU2008/000121 filed Feb. 2, 2009, and which claims the benefit of Australian Patent Application No. 2008900495, filed Feb. 4, 2008, the disclosures of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to circularly polarized array antennas. 
     BACKGROUND 
     There is a commercial demand for antennas that operate in the millimeter wave region, equating to frequencies in the range 30-300 GHz. Such antennas find application in Wireless Personal Area Networks (WPANs) used in the wireless transmission of high definition television data and for high-speed internet access, and also in video on demand and short-distance high data-rate transmission used to replace fixed cabling. 
     A similar demand also exists for antennas that operate below millimeter wavelengths, down to 1 GHz, for use in Wireless Local Area Networks (WLANs). 
     Circularly polarised antennas are of interest because they do not need to be aligned/oriented in the way that do linearly polarised antennas to send or receive radio waves. A circular polarised antenna need only be directed towards another circularly (or linearly) polarised antenna. 
     Known circularly polarised antennas operating at millimeter wave frequencies typically rely upon Low-Temperature Cofired-Ceramic (LTCC) materials, and use arrays of apertures fed by waveguide feed networks, such as that described in Uchimura, H., Shino, N., and Miyazato, K., “Novel circular polarized antenna array substrates for 60 GHz-band,” 2005  IEEE MTT - S International Microwave Symposium Digest , pp. 1875-1878, 12-17 Jun. 2005. 
     Another example of a circularly polarized antenna is taught by K.-L. Wong, J.-Y. Wu and C.-K. Wu, “A circularly polarized patch-loaded square-slot antenna”,  Microwave and Optical Technology Letters , vol 23, no. 6, pp. 363-365, Dec. 20, 1999. Wong et al teaches a patch-loaded square-slot antenna that uses a rectangular patch as the perturbation element for the excitation by a slot of two orthogonal, phase shifted resonant modes of circularly polarized radiation. 
     It is also of interest to achieve high-gain and wide bandwidth in circularly polarized antennas, which can not be achieved by the two exemplary known antennas referred to immediately above. 
     U.S. Pat. No. 4,843,400, Tsao et al, issued on Jun. 27, 1989, teaches an array of radiating patch elements mounted on a single waveguide that enables the synthesis of a larger aperture than would be the case for a single antenna element. 
     A paper by P. S. Hall, “Application of sequential feeding to wide bandwidth, circularly polarised microstrip patch arrays”,  IEE Proc ., Vol. 136, Pt. H, No. 5, October 1989, pp. 390-398, describes the sequential rotation of the feeding of circularly polarised microstrip patch antennas and arrays coupled with appropriate offset of the feeding phase leads to significant improvements both in bandwidth and purity. 
     SUMMARY 
     It is an object of the invention to substantially achieve and improve upon one or more high gain and wide bandwidth, to be susceptible of cost-effective mass production, or to provide a useful alternative. 
     Accordingly, there is provided an antenna comprising:
         a single layer dielectric substrate;   a ground plane located on the upper surface of the substrate and covering only part of said upper surface;   a plurality of antenna elements also located on said upper surface of the substrate, each antenna element having a slot element formed in the ground plane and a respective loading element located within each slot element, said antenna elements being arranged in a regular array where each respective slot element is sequentially rotated in space with respect to adjacent slot elements, and said loading elements generate a perturbation under excitation;   a microstrip feed network located on the underside of the substrate to provide excitation to each slot element, and including feeds of different lengths to be electrically sequentially rotated in common with spatial rotation of said slot elements, and a single microstrip feed point extending to an edge of said substrate for connection purposes; and   a reflecting plane located parallel to and spaced apart from the underside of the substrate; and   wherein said ground plane extends to cover the entire microstrip feed array.       

     Preferably, the ground plane covers the substrate to the extent that at least ½ wavelength at an operational frequency between the edges of the ground plane and the edges of the substrate is not covered, except where said ground plane covers said feed point. The reflector typically is at least as large in surface area as said substrate. The regular array typically is at least of dimensions 2×1. A housing that supports said substrate at the substrate edges and supports or incorporates said reflector can be provided. The substrate typically is formed of a liquid crystal polymer material. 
     Other aspects are disclosed. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are plan and elevation views respectively of a known patch-loaded square slot antenna element. 
         FIG. 2  is a partial view of a 4×2 array antenna assembly embodiment. 
         FIG. 3  is a plan view of the 4×2 array antenna assembly showing the microstrip feed network. 
         FIG. 4  is a computed reflection coefficient at the input of the 4×2 array antenna assembly. 
         FIG. 5  is a computed realised gain of the 4×2 array antenna assembly. 
         FIG. 6  is a computed axial ratio of the 4×2 array antenna assembly. 
         FIG. 7  shows computed RHCP radiation patterns of the 4×2 array antenna assembly at φ=0°. 
         FIG. 8  shows computed RHCP radiation patterns of the 4×2 array antenna assembly at φ=90°. 
         FIG. 9  is a plan view of the 4×2 array antenna assembly with an extended feed line and ground plane. 
         FIG. 10  is another view of the assembly of  FIG. 9 . 
         FIG. 11  is a plan view of a 2×2 array of patch-loaded square slot antenna assembly. 
         FIG. 12  is a plan view of a 4×4 array of patch-loaded square slot antenna assembly. 
         FIG. 13  is a plan view of an 8×2 array of patch-loaded square slot antenna assembly. 
         FIG. 14  is a plan view of another 2×2 array of patch-loaded square-slot antenna assembly. 
         FIG. 15  shows various other antenna element embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Introduction 
       FIGS. 1A and 1B  show the known antenna element taught by Wong et al, referred to above. The antenna  10  consists of a square slot  12 , of length L, formed in a ground plane  14 . The ground plane  14  is formed by metalisation contacted to the surface of a liquid crystal polymer (LCP) substrate  16 . The substrate  16  is of thickness h. The slot&#39;s major axes are rotated by 45 degrees with respect to the edge of the ground plane  14 . The slot  12  is loaded with a conducting rectangular patch  18  of dimensions w by L 1 . The slot  12  is fed by a microstrip line  20  with a width of W f , which is contacted on the opposite side of the substrate  16  to the slot  12 . The length d p  of the probe portion of the feed line  20  allows tuning of the impedance of the antenna  10 . 
     A conductive reflector  22  is located at a distance h 2  from the lower face of the substrate  16 . The reflector  22  limits the radiation of the slot antenna to the positive z direction. Without the reflector  22  being present, the antenna  10  will radiate almost equally in both the positive and negative z directions. The distance h 2  is typically a quarter of a wavelength long at the centre frequency of the design bandwidth. 
     By adjusting the ratio of length to width (L 1 /w) of the patch  18 , a perturbation of the symmetry of the slot  12  is achieved, such that it is then possible to excite two orthogonal modes in the rectangular slot  12  that couple together with the correct phase shift to generate circularly polarized radiation. A typical value for L 1 /w is 2.6. L 1  is typically 0.7 L. 
     4×2 Array Embodiment 
       FIG. 2  is a plan view of a constituent assembly  30  of a 4×2 array of patch-loaded square slot antenna. This assembly  30  has been designed to operate from 57 to 66 GHz for Wireless Personal Area Network (WPAN) applications. The dimensions of the ground plane  32  are length=16.34 mm and width=8.17 mm. The single layer dielectric substrate  36  has the dimensions of length=24 mm and width=15.83 mm, and thickness of 100 μm. The substrate  36  is formed of a LCP material, having a dielectric constant=3.2 and tan δ=0.004. A suitable substrate is the Rogers ULTRALAM 3850, or Nippon Steel Chemical Co. Ltd, Espanex L Series. 
     As is apparent, the ground plane  32  extends only over a portion of the total surface area of the substrate  36 . This is important in terms of packaging the antenna in a housing, as will be described below. The distance between the edge of the ground plane  32  and the edge of the substrate  36  should be at least a ½ wavelength to avoid the housing unduly influencing the radiation characteristics of the assembly  30 . 
     The area occupied by the ground plane generally is optimised to give best antenna performance by numerical simulation software. In general, the size is proportional to the array spacing, the number of array elements and the type of slot and substrate material. 
     The antenna assembly  30  has eight antenna elements  40 - 54  (each equivalent to the antenna  10  of  FIG. 1 ), each consisting of a slot  60 - 74  and a loading element in the form of a patch  80 - 94 . The antenna elements  40 - 54  are sequentially rotated in space about a common slot axis. 
     A typical range for the dimension of the square slots  60 - 74  is 1.69 mm to 1.86 min. A typical range for the dimensions of the patches  80 - 94  is 1.22 mm to 1.45 mm×0.43 mm to 0.48 mm. The antenna element separation of the array is typically 3.86 mm (0.79λ, at 61.5 GHz) in the x-direction, and 3.41 mm (0.702 at 61.5 GHz) in the y-direction. 
     A metallization thickness of 9 μm is used for the ground plane  32 , the patches  80 - 86  and the feed network  100 . The conductivity of the metallization is 3×10 7 S/m. 
     The reflector (not shown) located below the substrate  36  should have equal or larger dimensions than the substrate  36 , and be separated by a typical air gap of 1.25 mm. 
       FIG. 3  shows the microstrip feed network  100  on the underside of the substrate  36  with the ground plane  32  and the 4×2 array of patch-loaded square slot antenna elements  40 - 54  shown in phantom, and superimposed onto the feed network  100  to show their relative positions. The relative (electrical) phase shifts provided by the feed network  100  are given for each antenna element  40 - 54 . These phase shifts coincide with the spatial sequential rotation of the rectangular patches  80 - 94 . The angle between the respective probe and slot  60 - 74  is at substantially 45° to the major axes of the slot. Variations of between +/−1° to +/−5° can be tolerated. 
     The feed network  100  is formed as two (2×2) sub-arrays  102 ,  104 , constituted by a series of power dividing T-junctions beginning with the principal junction  106  from the input feed line  108 . The characteristic impedance of the microstrip feed network  100  is approximately 71Ω (excluding T-junctions), corresponding to a line width of 123 μm on an LCP substrate with a height of 100 μm. The lengths of the individual feeds to each antenna element  40 - 54  vary to achieve an electrical delay, leading to a relative phase difference, as indicated. 
     The antenna assembly  30  can be fabricated using known photolithography techniques, where the substrate  36  initially has full metallisation on both surfaces, and the metallisation is appropriately removed to create the ground plane  32 , patches  80 - 94 , and feed network  100 . 
     Each of the 2×2 sub-arrays  102 ,  104  uses sequential rotation of the antenna elements to increase the axial ratio bandwidth. The feed network delivers equal amounts of energy to the antenna elements  40 - 54 . The phase delay of each element in the 2×2 sub-array is sequentially increased by 90° (ie 0°, 90°, 180°, 270°) as the elements are rotated in space about a common square slot axis. This sequential rotation increases the overall axial ratio bandwidth for the individual sub-arrays  102 ,  104 . By using two arrays, the overall gain of the antenna is increased compared to one, and the beamwidth of the radiation pattern is narrowed (in the φ=0° plane in this case). 
     The designed performance of the array antenna assembly  30  is as follows:
         Minimum realised gain (57-66 GHz): 14.7 dBic   Maximum axial ratio (57-66 GHz): 2.84 dB   Maximum reflection coefficient, S 11  (57-66 GHz) −14.9 dB   Impedance bandwidth (where the reflection coefficient is less than −10 dB) extends from 49.16 GHz to 77.16 GHz (44%).       

     The antenna assembly  30  is believed to have good insensitivity to tolerance errors in manufacturing, and particularly in shifts of the metallisation patterns in the top and bottom surfaces of the LCP substrate of up to ±100 μm. This is particularly advantageous where low-cost manufacture is desired where tolerances may not be closely controlled. 
       FIG. 4  is a plot of computed reflection coefficient at the input (i.e. the end of the feed line  108 ) for the antenna assembly  30 . The reflection coefficient is less than −14.9 dB over the specified bandwidth of operation, thus providing a well-matched connection/interface to a silicon integrated circuit. 
       FIG. 5  is a computed realised gain for the antenna  30  assembly. The realised gain is greater than 14.7 dBic over the specified operating bandwidth to provide the necessary signal level for typical WPAN applications, such as transmission of HDTV signals. 
       FIG. 6  is a computed axial ratio of the antenna assembly  30 . The axial ratio is less than 2.84 dB over the specified bandwidth, thus ensuring the purity of the circularly polarized radiation, and reduces antenna orientation errors associated with linearly polarized antennas. 
       FIG. 7  is a computed right hand circularly polarised radiation pattern for the antenna assembly  30  at φ=0° (being the x-z plane in  FIG. 3 ). Sidelobe levels are below −10 dB across the specified bandwidth, and the beamwidth of the radiation patterns is narrower than that of the φ=90° plane (y-z plane), deemed suitable for WPAN applications. 
       FIG. 8  is a computed right hand circularly polarised radiation pattern for the antenna assembly  30  at φ=90° (being the y-z plane in  FIG. 3 ). Sidelobe levels are below −10 dB across the specified bandwidth, and the beamwidth of the radiation patterns is relatively wide ensuring that alignment of antennas in a WPAN application is relatively easy. 
     Referring now to  FIG. 9 , a further antenna  30 ′ is shown. The ground plane  32 ′ is “T-shaped” to extend to the edge of the substrate  36  to accommodate an extended microstrip feed line  108 ′. A supporting housing  120  also is shown. The housing provides structural integrity for the substrate  36 , and can be of metal or plastics material.  FIG. 10  is a view of the antenna  30 ′ showing the feed network  100 . The elements are shown as wireframe outlines so as to appear transparent. The optimal width Wgnd of the ‘leg  33  is determined by a numerical simulation optimisation, and for the present embodiment a width of 5 mm is chosen. By this arrangement, a feed port  110  and ground return path are provided at the edge of the substrate which makes for easy external connection, most usually to an integrated circuit, which needs to be in close proximity to the antenna. Additionally, the leg  33  of the ground plane prevents the feed line  108 ′ from radiating. The base of the housing (omitted in FIG.  10 ) forms the reflector, and therefore needs to be fabricated from a conductive material. 
     The array size may also be varied to suit other applications, depending upon the gain required by the antenna. In the present embodiment of 4×2 array elements, the required gain is 14 dBic. However, other applications may need less directive radiation performance and would use less array elements. For increased gain and narrower beamwidth of the antenna more elements can be used (e.g. 4×4, 8×8, 16×16, 8×2, 16×2, etc.). For best axial ratio bandwidth performance a minimum of 2×2 array elements are required to enable complete sequential rotation of the element in 90 degree intervals. A 2×1 array with sequential rotation is also possible but the axial ratio bandwidth is less than the 2×2 array, but better than the single element. 
     2×2 Array Assembly Embodiment 
     A 2×2 array antenna assembly  130  is shown in  FIG. 11 , where the elements are shown as wireframe outlines so as to appear transparent. The ground plane  132  extends over a portion of the substrate  134 . The antenna elements  136 - 142  are shown in phantom with reference to the feed network  144  and feed port  146 . 
     4×4 Array Assembly Embodiment 
     A 4×4 array antenna assembly  150  is shown in  FIG. 12 , where the elements are shown as wireframe outlines so as to appear transparent. The ground plane  152  extends over a portion of the substrate  154 . The antenna elements  156 - 186  are shown in phantom with reference to the feed network  188  and feed port  189 . 
     8×2 Array Assembly Embodiment 
     A 8×2 array antenna assembly  190  is shown in  FIG. 13 , where the elements are shown as wireframe outlines so as to appear transparent. The ground plane  192  extends over a portion of the substrate  194 . The antenna elements  196 - 226  are shown in phantom with reference to the feed network  228  and feed port  230 . 
     Alternative 2×2 Array Assembly Embodiment 
     The array layout used may also be varied. Referring again to  FIG. 11 , note that the edges of the square slots are at 45 degrees compared to the x and y axes, and the microstrip feed lines are parallel to these axes. It is also possible to have the edges of the slots parallel to the x and y axes, and the microstrip feed line at 45 degrees. This variation is illustrated for a 2×2 array antenna assembly shown in  FIG. 14 . This orientation of the slots allows a closer spacing of the array elements  136 ′- 142 ′, and uses a more compact feed network  144 ′. The feed port  146 ′ is shown. Closer element spacing is advantageous to reduce sidelobe levels in the radiation pattern, and to avoid grating lobes when steering the beam in phased-array applications. 
     OTHER EMBODIMENTS 
     A diagram of some of the possible variations on the basic array element is shown in  FIG. 15 , in which: (a) patch-loaded square-slot ( FIGS. 3 and 4 ), (b) patch-loaded circular-slot, (c) ellipse-loaded circular-slot, (d) patch-loaded rectangular-slot, (e) circle-loaded rectangular-slot (f) ellipse-loaded rectangular-slot, (g) ellipse-loaded elliptical-slot, (h) circle-loaded elliptical-slot, (i) patch-loaded elliptical-slot, (j) patch-loaded pentagonal-slot, (k) ellipse-loaded pentagonal-slot, (l) patch-loaded hexagonal-slot, (m) ellipse-loaded hexagonal-slot, (n) patch-loaded heptagonal-slot, (o) ellipse-loaded heptagonal-slot, (p) patch-loaded octagonal-slot, and (q) ellipse-loaded octagonal-slot. 
     In general, the slot element of the antenna element may be any polygon with n sides, where n is greater than three. This polygon may be loaded by either a planar metallic ellipse or a planar metallic patch, where the ratio between the major and minor axes of the ellipse or patch determines the circular polarization and hence the axial ratio of the element. The loading element may also be a polygon with n sides (n is greater than three) that contains a perturbation to its shape such that it also has a major axis and a minor axis to control the axial ratio of the antenna.