Abstract:
A circularly polarized patch antenna uses a square quarter-wavelength conductive plate, spaced away from a slightly larger backing conductor. Excitation uses a coaxial feed stem pair, whereof respective inner conductors join the patch at orthogonal locations on a reference circle, and outer conductors intrude past points of joining to the backing conductor to establish gaps that interact with patch and backing conductor size and spacing to jointly establish terminal impedance. A parasitic element in the propagation path broadens bandwidth, while a frame behind serves to define a cavity reflector. A power divider behind the frame converts a single applied broadcast signal into two equal signals with orthogonal phase, which signals are delivered to the feed stems with equal-length coaxial lines.

Description:
CLAIM OF PRIORITY 
     This application is a continuation of U.S. patent application Ser. No. 11/882,383 filed Aug. 1, 2007, now U.S. Pat. No. 8,373,597 issued Feb. 12, 2013, which application claims priority to U.S. Provisional Patent Application Ser. No. 60/836,398, titled “High-Power-Capable Circularly Polarized Patch Antenna Apparatus and Method,” filed Aug. 9, 2006, both of which are hereby incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to radio frequency (RF) electromagnetic signal broadcasting antennas. More particularly, the present invention relates to single-feed circularly polarized broadband patch antennas for broadcasting. 
     BACKGROUND OF THE INVENTION 
     Auction of the 700 MHz spectrum, specifically the lower S-Band, by the Federal Communications Commission (FCC), resulting in part from a conversion of television broadcast from analog to digital service, has created a need for new products specifically tailored for this band. Some of the new license holders have begun rollout of a Digital Video Broadcast to Handheld (DVB-H) mobile TV entertainment service, along with other services. Receivers for these services will likely be integral parts of cellular telephones, accessories for notebook computers, or similar devices in at least a significant proportion of embodiments. 
     Circular polarization of broadcast signals reduces dependence on receiving antenna orientation for received signal strength, so that a simple dipole in virtually any orientation, for example, can receive a usable signal. This can be a significant consideration, ensuring that low-cost mobile handheld devices can realize stable and clear entertainment video and audio reception, as well as high digital data rates. 
     As in other broadcasting, it can be desirable to achieve particular extents of signal reception range, and to employ a small number of minimally-powered transmitters in the course of realizing that propagation. To these ends, radiating devices are preferably capable of exhibiting high gain and are preferably configurable with any of a variety of directionality options. Along with gain and propagation pattern, light weight and relatively small size may ease strength and wind load requirements for tower construction, allowing extra height above average terrain (HAAT), more bays, more radiators per bay, and the like. 
     In addition to considerations of circular polarization and high gain in broadcast antennas, higher power levels than previously required in the lower S-band are allowed in DVB-H service. Effective radiated power (ERP, a function of a transmitter&#39;s emitted signal power and antenna design and height that corresponds broadly to reception range) is regulated by the FCC. Transmitter power up to 5 kW is permitted under new DVB-H regulations, so broadcast antennas capable of supporting this power level may be appropriate in pursuit of optimization in the lower S-band. The new DVB-H regulations also imply desirability of an economical antenna solution in a compact package, in view of expectations that a nationwide infrastructure will be implemented. 
     Many broadcast antenna configurations exist. One that is usable and of merit for many applications includes elements variously referred to as patch style or panel style radiators. Typical known patch antennas are strongly directional, producing a pronounced lobe of emission in a principal (zero degrees relative azimuth) direction, with little or no emission to the sides (+/−90 degrees azimuth) and to the rear (180 degrees azimuth). Examples of emission patterns, including those known as cardioid (wherein the lobe diminishes gradually so that there is substantial but generally less emission to the sides than forward), skull (wherein there is negligible emission to the sides but a vestigial lobe to the rear), and multi-lobe (wherein a strong and narrow central lobe is bracketed by nulls and lesser lobes), will be addressed in the discussion that follows. Patch antenna elevation signal strength patterns are likewise frequently broadly cardioid, skull, or multi-lobe in shape for typical patch antennas. 
     Known patch antennas for low power applications may be relatively simple to implement. Within limits of materials, such antennas can be formed from sheet metal and insulating standoffs and can be fed using suitably sized connectors, coaxial lines, single conductors, and the like. Known radiative elements (radiators) may be square, shaped as incomplete rings, tee-shaped, formed as planar or bent bow-ties or bow-tie slots, or formed in numerous other configurations. At microwave frequencies (multiple gigahertz) and relatively low power per element, patch antennas can be made from dielectric layers (such as fiber-reinforced epoxy) and copper foil in much the same manner as circuit boards, trading off the dimensional and thermal limitations of the materials against high production rates and low costs. Limitations of many known designs generally focus on power handling per patch as a function of frequency; that is, element dimensions and interelectrode spacing decrease with wavelength, while voltage and current increase with power, so that a propensity for dielectric breakdown and arcing between components grows with power and frequency. 
     Circular polarization in known patch antennas can be realized using, for example, conductive, nearly-closed rings of about one wavelength circumference positioned above a planar reflector. Where several such rings are used to form an array, they can be connected with conductive rods to provide traveling wave feed. This particular design is severely limited in performance, however; see, for discussion,  Antenna Engineering Handbook, Third Edition , R. C. Johnson, ed., McGraw-Hill, New York, 1993, pp. 28.21-28.24, and FIG. 28.25 therein. 
     Deficiencies in existing antenna designs for the 700 MHz band include excessive cost, narrow bandwidth capability (i.e., low voltage standing wave ratio (VSWR) does not extend over the entire allotted band, or even a substantial fraction thereof), lack of support for high broadcast transmitter power, uncertain wind load, and limited ability to provide circular polarization, in a directional panel antenna. 
     Some existing high power (up to 1 kW) circularly polarized panel antennas include crossed dipoles or log periodic radiators fed with hybrids and power dividers. The complexity of these styles of antennas can result in high cost for the achieved performance. Simpler configuration could potentially achieve a much lower cost than available products without sacrifice of performance or reliability. 
     SUMMARY OF THE INVENTION 
     The foregoing disadvantages are overcome, to a great extent, by the invention, wherein in one aspect an antenna is provided that in some embodiments of the invention affords lower cost, broad bandwidth capability, support for high broadcast transmitter power, low wind loading, and strong circular polarization in a directional panel antenna. 
     In a first embodiment, a circularly polarized patch antenna is disclosed. The antenna includes a first patch radiator, further including a substantially planar, conductive surface having extents proportional to a wavelength of an electromagnetic signal within a specified frequency band, wherein a positive direction along a first-patch reference axis, passing through a centroid of the first patch radiator perpendicular to the surface thereof, is parallel to a sole principal direction of propagation of signals emitted from the antenna. The antenna further includes a first feed point and a second feed point on the first patch radiator, located at prescribed locations with reference to dimensions of the radiator, and a power divider, configured to accept an applied broadcast signal on an input port and to provide a first two divider output signals, having prescribed relative phase and amplitude, on a first two output ports. 
     The antenna further includes interconnecting signal lines between the first two divider output ports and the first patch radiator feed points, wherein the lines have prescribed relative lengths, a first backing conductor, substantially parallel to the first patch radiator, wherein a distance from the first patch radiator to the first backing conductor is negative with reference to the principal direction of propagation of signals emitted from the antenna, and a first parasitic radiator, substantially parallel to the first patch radiator, wherein a distance from the first patch radiator to the first parasitic radiator is positive with reference to the principal direction of propagation of signals emitted from the antenna. 
     In a second embodiment, a circularly polarized patch antenna is disclosed. The antenna includes a radiative patch element for radiating an electromagnetic signal with circular polarization with a principal axis of propagation, wherein the patch excites signal currents having orthogonal phase along axes that are physically orthogonal within the patch. The antenna further includes a power divider for dividing applied signal power from a single source into two parts having substantially equal power, wherein the parts are orthogonal in phase. The antenna further includes coaxial feed stems for coupling the orthogonal electromagnetic signals onto the patch, wherein spatial locations within the patch whereto the signals are coupled are orthogonal with reference to a circle associated with the patch, wherein the circle is centered on the principal axis of propagation. 
     The antenna further includes a backing conductor for reducing radiation in a negative primary axial direction along the principal axis of propagation, wherein the backing conductor further functions to establish impedance of the patch at least in part. The antenna further includes, between the backing conductor and the patch, an intrusion of each feed stem outer conductor, terminating in a gap between the maximum extent of each feed stem and the patch, wherein the intrusion into a spatial volume associated with the interrelationship of the patch and the backing conductor further functions to establish impedance of the patch at least in part. The antenna further includes a parasitic radiator for parasitically broadening bandwidth of the patch, wherein the parasitic radiator is interposed along the principal axis of propagation in a positive primary axial direction, and feed lines for connecting the power divider to the feed stems. 
     In a third embodiment, a method for broadcasting circularly polarized signals is presented. The method includes providing a single signal encompassing at least one transmission channel within a prescribed broadcast band, applying the single signal to a coaxial input port of a power divider configured to present, at a first coaxial output port, a first divider output signal having a first phase angle, and further configured to present, at a second coaxial output port, a second divider output signal having a second phase angle, orthogonal to the phase angle of the first divider output signal. The method further includes conducting the orthogonal divider output signals to respective first and second coaxial feed stems, wherein the divider output signals are applied to inner conductors of the respective feed stems, and wherein outer conductors of the respective feed stems have a common potential with the power divider input signal port outer conductor and power divider output port outer conductors. 
     The method further includes conducting the orthogonal divider outputs through a backing conductor via the respective first and second coaxial feed stems, wherein the feed stem outer conductors are electrically joined to the backing conductor at locations thereon where the outputs are conducted therethrough, and conducting the orthogonal divider outputs to orthogonal points of attachment on a patch radiator, wherein the patch radiator is a substantially planar, square, conductive surface, parallel to and smaller than the backing conductor, having extents proportional to a prescribed portion of a wavelength of a frequency within the band of the antenna, wherein the points of attachment are orthogonal with reference to a circle of prescribed diameter in the plane of the patch radiator, centered on the centroid of the patch radiator, whereon the points of attachment fall, and wherein the feed stem outer conductors terminate proximal to the patch radiator with a prescribed gap therebetween. 
     There have thus been outlined, rather broadly, features of the invention, in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments, and of being practiced and carried out in various ways. It is also to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description, and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be used as a basis for the designing of other structures, methods, and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a first perspective view of an antenna according to the invention disclosed herein. 
         FIG. 2  is a second perspective view of an antenna according to the invention disclosed herein. 
         FIG. 3  is a face view of one principal radiator component and a parasitic component according to one embodiment of the invention. 
         FIG. 4  is a side elevation in partial section illustrating features of the patch antenna of  FIGS. 1 and 2 . 
         FIGS. 5-12  are test charts representing gain and axial ratio versus azimuth and elevation at representative frequencies across a working band for a single patch antenna according to the invention disclosed herein. 
         FIG. 13  is a test chart representing voltage standing wave ratio (VSWR) versus frequency for a single patch antenna according to the invention disclosed herein. 
         FIG. 14  is a perspective view of another embodiment of an antenna according to the invention disclosed herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout. The invention provides an apparatus and method that in some embodiments provides a patch antenna for the lower 700 MHz band that emits a substantially single beam, circularly-polarized propagation pattern with high gain and relatively high power handling capability. 
     Typical patch antennas achieve directionality and impedance control in part by including a backing conductor. Without a backing conductor, a patch radiator exhibits an intrinsic property of emitting similar lobes before and behind (i.e., in the zero-azimuth and 180 degree-azimuth directions, with comparable elevation), known as a peanut pattern, and has an impedance that is a function of patch size and interaction with nearby conductors or free space. Square patches are commonly edge driven or center driven, as determined by the desired radiation pattern and by limitations of materials. 
     If a backing conductor is added in a plane parallel to that of the patch, with the backing conductor coextensive with the patch and larger than the patch to a greater or lesser extent, and if the backing conductor is connected to the outer conductor of a coaxial feed line whereof the patch is connected to the center conductor, the two parallel plate conductors exhibit a terminal impedance with respect to the coaxial line according to their dimensions and spacing, and the radiation pattern of the patch is substantially altered from that of a stand-alone equivalent. The interaction can cause the rear-directed lobe to be diminished and the forward-directed lobe to be increased. 
     The term “coextensive” as used herein refers to substantially similar geometric figures of comparable size, lying in parallel planes if planar, wherein lines perpendicular to the surfaces of the respective figures at respective centroids of the figures are approximately coincident. For nonplanar or complex coextensive figures, the approximate coincidence of lines perpendicular to and passing through centroids of the figures continues to apply, along with regular spacing and no contact between the figures. Nonplanar examples include concentric rotated parabolas, elliptical or cylindrical segments, or the like. Complex examples may include flat square bodies bounded by arcuate, dished perimeter surfaces, faceted surfaces of sufficiently similar shape to exhibit approximately uniform distributed electrical properties, and the like. For some such configurations, electrical characteristics may be well behaved, with impedance, electrical loading, emission, and the like well enough defined to permit their use for radiation of broadcast signals. For other configurations, transverse coupling may decrease suitability, at least for arrangements having a plurality of radiators. It may be observed that the antenna of  FIG. 1  includes flat, thin components with minimal edge thickness, affording low transverse coupling. 
       FIG. 1  shows a perspective view of a directional antenna  10  having two patch radiators  12 , in accordance with one embodiment of the invention. In order to overcome such limitations of typical patch antennas as low power and narrow band operation, the antenna  10  of  FIG. 1 , which may be sized for lower S-band operation, includes patch radiators  12  formed from a substantially flat and thin conductive material, having a square shape with dimensions perpendicular to the principal propagation axes  14  of the respective patches  12  approximating a half wavelength of a frequency within the intended passband of the antenna  10 . The patches  12  are spaced away from grounded backing conductors  16  by a distance  18  that is a function of the desired terminating impedance of the radiators  12 , in this instance roughly one-thirty-second of a wavelength, but generally requiring verification by test. The square shape of the patches  12  in the embodiment shown may be preferred for typical embodiments, although other proportions and shapes may be used. The relative dimensions of the patches  12  and backing conductors  16  similarly require verification for each embodiment: the backing conductors  16  in the embodiment shown are roughly 15% larger than the patches  12 , which can further reduce rearward emission in some embodiments, although various size ratios may be used. 
     Each patch  12  is further associated with a single parasitic element  20 , located on the propagation axis  14  in the direction of propagation, and electrically isolated from the patch  12  and the grounded backing conductor  16  by nonconductive fastenings. A single parasitic  20  can broaden bandwidth significantly, provided its size and spacing are suitable. In the embodiment shown, the parasitics  20  are round, and are equal in diameter to the respective edge lengths of the patches  12 , although parasitics  20  of different shapes and sizes may be used. As in the case of the backing conductors  16 , the distance  22  from each patch  12  to its parasitic  20  is a function of desired properties of the antenna  10 —about a sixteenth of a wavelength in the embodiment shown, although other spacings may be used. 
     Additional parasitics  20 , most often aligned with the other components of the respective radiators and located at selected distances from the patches  12 , can further enhance bandwidth, gain, and other attributes of radiators in some embodiments. Tradeoffs in the pluralization of parasitics  20  include cost, size, weight, stability of structure and function over time, and diminishing returns of increased performance with increased complexity. To cite a strictly hypothetical example, if a second parasitic were to add 10% to overall performance according to some criteria, then a third might add 5%, a fourth 2%, and the like, while antenna material cost increased by 8% per parasitic, wind loading by 3%, and so forth. Thus, in some embodiments, particularly those wherein an antenna&#39;s requirement for enhanced radiative performance outweighs some other considerations, two or more parasitics  20  may be preferred. The presentation of a single parasitic  20  in the present disclosure should be viewed as representative, and not construed as limiting. 
       FIG. 2  shows certain of the following elements with greater clarity; those also shown in  FIG. 1  may be identified there as well. Behind (i.e., opposite to the principal propagation direction of) each assembly of a patch  12 , a backing conductor  16 , and a parasitic  20  is a frame  24 . This frame  24  is another generally planar, grounded, conductive surface, spaced away from the backing conductor  16  by a distance  26  approximating a quarter wavelength in the example shown. 
     It is to be understood that a signal propagating from the patch  12  toward the frame  24  has opposite handedness of circular polarization to a signal propagating in the desired (positive) direction. As a consequence of reflecting the negative-going signal, the frame  24  reverses the signal&#39;s polarization, so that the reflected signal has common polarization with and is propagating in the same direction as the signal originating from the patch  12  in the positive direction. The reflected signal returning to the patch  12  is retarded by one half wave, but the patch  12  has reversed phase by one half cycle in the interval, so that the signal reflected from the frame  24  reinforces the forward-directed signal. 
     In the embodiment shown, the frame  24  is formed from flat sheet metal by cutting and by bending up fins  28  to establish a shallow box shape, variously known in the art as having a basket shape or as establishing a cavity-backed antenna. In other embodiments, the material and configuration of the frame  24 , or indeed its presence, may differ, such as by using perforated or expanded metal, mesh, or another material reflective in the frequency range of interest. 
     When the antenna  10  is excited, the region between the backing conductors  16  and the frame  24  is hot—that is, contains relatively high field gradients—despite the backing conductors  16  being at roughly the same potential as the frame  24 . As a result, the configuration of any conductors in that space tends to affect the overall emission pattern of the antenna  10 . Therefore, any conductors in this region are preferably highly stable and uniform in configuration, and any signals coupled through this region shielded, in order to assure predictable performance. Each dimension of the frame  24 , as well as the spacing to the radiative parts, is subject to verification for a specific embodiment. 
     The space behind the frame  24  is relatively shielded from radiation. Into this space in the embodiment shown are placed a power divider  30  having an input connector  32  and sufficient output connectors (concealed by mating cable-end connectors  34  or obscured by the divider  30  in  FIG. 2 ) to provide feed signals to the patches  12 . Split-off signal portions are carried by interconnecting signal lines to the patches  12 , with the interconnecting signal lines made up of respective coaxial feed lines  36 ,  38 ,  40 , and  42  and coaxial feed stems  44 ,  46 ,  48  and  50 . An overall enclosure  52 , shown in phantom and mounted to the frame  24 , covers the divider  30  and the feed arrangement, with the input connector  32  protruding through the enclosure  52  in the embodiment shown in  FIG. 2 . The enclosure  52  may be conductive in some embodiments, thereby affording additional radiation uniformity, protection, and like benefits. A radome  54  provides overall mechanical protection of the radiating parts against wind force, wind-blown matter, rain, icing, and like hazards, and establishes in part a uniform and quantifiable wind drag characteristic. The mailbox-shaped radome  54 , shown in phantom and mounted to the frame  24 , is preferably fairly light in weight, strong, and resistant to sunlight and pollutant degradation, while substantially transparent to radio emissions in the frequency band of the antenna  10  to a desirable extent. 
     The divider  30  provides four outputs in the embodiment shown. These outputs may be equal in phase, magnitude, and spectral content in some embodiments. In other embodiments, while otherwise equal, each two outputs may differ in phase by 90 degrees or another amount, as discussed below. Similarly, the coaxial feed lines  34 ,  36 ,  38 , and  40  may differ by a quarter wavelength, may be equal in length, or may differ by another amount, as also discussed below. All conductive parts other than the inner parts of the divider  30 , the inner conductors of the feed lines  34 ,  36 ,  38 , and  40  and stems  44 ,  46 ,  48  and  50 , the patch radiators  12 , and the parasitics  20 , are connected electrically, and thus are approximately at a common ground potential presented to the antenna on the outer conductor of the input connector  32  to the divider  30 . 
       FIG. 3  is a schematic diagram  60  showing a surface of a representative patch  12  having equal height  62  and width  64 , with the direction of propagation toward the viewer. For convenience, an approximate value for a speed of propagation of electromagnetic signals in the vicinity of the antenna of 0.88 times the speed of light is used herein. It is to be understood that this approximation is a function of the physical properties of the components and materials of the antenna, and that this velocity differs, for example, within coaxial cables filled with a dielectric material, along conductive surfaces spaced apart from other conductive surfaces and separated by air, and the like. The dimensions in  FIG. 3 , in inches, are approximately those used in the prototype antenna discussed below. The patch  12  is about a quarter-wavelength on each edge at 722 MHz at the assumed propagation velocity. 
     The patch radiator  12  achieves circular polarization by receiving the applied signal at two feed points  66  and  68 , each placed midway along one of two orthogonal edges  70  and  72  of the patch  12  and inward from the respective edges  70  and  72 , effectively placed on a feed point reference circle  74 , centered on the patch radiator  12  and having a specified diameter. If the signals applied to the feed points  66  and  68  are orthogonal in phase, that is, are two samples of a single signal, substantially identical but differing in phase by one-quarter wave (90 degrees), they establish currents in the patch  12  with separate and orthogonal phase in space and time, which couple out of the patch  12  as a single signal propagating with circular polarization. To the extent that stations at which the feed points  66 ,  68  are placed have nonorthogonal angular and/or radial separation with respect to the reference circle  74 , or that the phase and/or strength of the applied signals are not orthogonal/identical as indicated above, polarization may be elliptical, i.e., ellipticity will vary from a value of one. 
     All of the indicated physical dimensions, in addition to signal phase, strength, and spectral equivalence, affect antenna performance. Spacing between and dimensions of the backing conductor  16 , parasitic  20 , frame  24 , and fins  28 , shown in  FIGS. 1 and 2 , and feed point placement along the respective edges  70  and  72  (described above as midway, although other orientations may be used), as well as feed point reference circle diameter  74 , affect emission. 
       FIG. 4  is a schematic side view  80  of an antenna  10  according to the invention, shown in partial section. In this view, it may be seen that the outer conductors of the coaxial feed stems  44 ,  46 ,  48  and  50  are electrically and mechanically joined by a suitable method to the frame  24  and the backing conductors  16 , and end with gaps  84  between respective termination loci  86  and the patches  12 . The inner conductors  82  of the coaxial feed stems  44 ,  46 ,  48  and  50  are electrically joined by a suitable method to the respective patches  12 . The joining methods illustrated in  FIG. 2  are nuts over threaded tubes or rods;  FIG. 4  suggests soldering, brazing, welding, or a combination of such methods. Methods appropriate to an embodiment may be determined in part by the selection of materials for the radiative elements, power levels, tradeoffs between cost and reparability, and the like. 
     The gap distances  84  between the respective outer conductors of the coaxial feed stems  44 ,  46 ,  48  and  50  and the patches  12  represent factors affecting the impedance of the signal paths over frequency. The divider  30 , the associated feed lines  36 ,  38 ,  40 , and  42 , and the coaxial feed stems  44 ,  46 ,  48  and  50  may be configured to provide relatively uniform impedance, such as fifty ohms, through choice of dimensions, dielectrics, and like factors. Similarly, size and spacing between the patches  12  and the backing conductors  16  and placement of the feeds (inner conductors  82 ) on the patches  12  may be defined to control signal emission and polarization, as well as impedance, over a selected frequency range. The gaps  84  function as transformers whereby the feed components (divider, coaxial lines, feed stems) and the radiative components (patches, backing conductors, parasitics, and the frame) can be integrated to provide low voltage standing wave ratio (VSWR) over a broad bandwidth, while permitting high power to be applied and emitted. 
     The enclosure  88  shown in  FIG. 4  houses a power divider  90  differing in shape from the divider  30  of  FIG. 2 , with an additional feed line  92 . It is to be understood that any arrangement of components that meets the operational description herein is included. 
     Mounting standoffs  94  are incorporated in order to position the conductive components relative to one another. The configuration shown is one of many practical styles. Multiple slender, non-conductive posts having opposite-sex screw threads on respective ends, as shown in some parts of the standoff  94  arrangement, allow conductive elements to be assembled with relatively low complexity, using a single small-diameter hole in each conductive component at each post location, stacking the posts to the extent practical, and completing assembly with screws as required. Suitable materials for such posts include at least polymers and ceramics. The materials may be reinforced with fibers or other filler materials or unfilled, and resilient or rigid, depending on considerations relevant to specific applications, such as vibration, temperature, electromagnetic radiation level, and the like. Dielectric constants and dissipation factors of selected materials may affect signal distortion, signal power loss through conversion to heat, and other effects of the mounting provisions. Conductive or semiconductive materials may be suited to some applications at least in part. Configurations other than the standoffs  94  shown in the figures, including clip-retained (non-threaded) fittings otherwise generally similar to the threaded posts shown, a single central post stack per patch, slotted or relieved frameworks external to the conductive parts, retention fittings molded or bonded into the radome, and other types may prove practical in some embodiments. The feed stems may contribute a portion of overall structural strength in some embodiments. 
       FIGS. 5-12  are charts showing measured test results for a prototype antenna in a standard test range.  FIGS. 5 ,  7 ,  9 , and  11  show azimuth performance for a single antenna  10  (two patches  12 , one divider  30 , and associated parts) as a function of polarization, using the customary procedure of transmitting a series of single-channel signals from the antenna  10  under test while slowly rotating it. A linearly polarized receiving antenna located at a single azimuth in far field is oriented to detect horizontal polarization, then subsequently vertical polarization, and finally is rotated rapidly (in comparison to the transmitting antenna rotation rate) to detect the axial ratio of the antenna under test. 
     The respective horizontal polarization envelopes  102 ,  112 ,  122 , and  132  were detected at low, intermediate, and high frequencies within the 700 MHz to 750 MHz band. The directivity and uniformity of directivity over frequency are evident. Gain is normalized in the plots. 
     The respective vertical polarization envelopes  104 ,  114 ,  124 , and  134  at the same frequencies are also shown to be highly uniform, and comparable to the horizontal envelopes. Measured axial ratio at zero degrees off axis remains above 0.6 at the lowest frequency and exceeds 0.8 over most frequencies, decreasing to roughly 0.5 at 30 degrees off axis at the low end The remaining curves  106 ,  116 ,  126 , and  136  demonstrate that there is substantially continuous and uniform circular polarization, rather than isolated horizontally and vertically polarized elements alone. 
       FIGS. 6 ,  8 ,  10 , and  12  chart performance of the prototype versus elevation, with testing performed by mounting the transmitting antenna prototype on its side and using substantially the test setup of  FIGS. 5 ,  7 ,  9 , and  11  otherwise. Chart measurements  140 ,  142 ,  144 , and  146  are clearly similar to corresponding azimuth measurements, with the two patch radiators reinforcing to provide increased vertical directivity—narrower relative beam width due to the presence of two wavelength-spaced radiators—at some cost in developing side lobes with nulls around 25 to 35 degrees off axis and peaks in the vicinity of 60 degrees off axis for the entire band. Measured axial ratio at zero degrees elevation exceeds 0.8 at all frequencies, and generally improves off-axis. 
       FIG. 13  graphs VSWR versus frequency, with the plot line  150  showing that markers 1 (698 MHz, VSWR=1.1050), 2 (713 MHz, VSWR=1.0246), 3 (722 MHz, VSWR=1.0391), and 4 (746 MHz, VSWR=1.1029) demonstrate an ability of an antenna according to the invention to accept and radiate power that is exceptionally broadband (near 1.1 VSWR for 6.65% bandwidth) for a patch design in general or for a broadcast antenna for use in the lower 700 MHz band. 
     The provision of four-way power division within the patch antenna  10  assembly, the addition of four rigid coaxial feed stems delivering signal energy to the patches  12 , the distance from the patches  12  to the backing conductor  16  and other grounded surfaces, and the absence of masses of dielectric material between the backing conductor  16  and the patch  12  all permit increased power handling compared to previous patch antenna designs, while providing uniform broad-band performance. 
     A single antenna assembly according to the indicated embodiment of the invention includes a doublet of patches  12  scaled specifically for the lower 700 MHz band and enclosed in a mailbox shaped radome. Such a configuration affords comparatively low wind load while managing complexity. Single patches within radomes, as opposed to the doublet configuration shown, use twice the external feed complexity (power dividers, cables) of the doublets, and have increased housing surface area and thus wind load. Placing three or more patches within each radome is likewise feasible, further reducing wind loading. Placing four patches in a two-dimensional planar array within a single radome, for example, may be preferred for so-called sector type service, but may be incompatible with some omnidirectional applications where transmitter power output is modest. The same four patches  12 , placed at angles to one another, as shown in  FIG. 14 , may provide wider azimuthal coverage while reducing configuration complexity by incorporating coaxial lines into the assembly, again at a cost of providing an eight-way divider, two four-ways preceded by a two-way, or an equivalent power distribution arrangement. 
     Note that 0 degree and −90 degree feed lines are provided to feed the patches  12  as shown in  FIGS. 1 and 2 , an arrangement that produces circular polarization. If the 0, −90 degree phasing is provided within the power divider  30  and the feed lines are equal in length, then, for at least some configurations of divider, impedance cancellation at the divider may be realized. To the extent to which the divider appears nonreactive to its input over the band of interest, this impedance cancellation can improve divider, and thus antenna, bandwidth. In the alternative, the 0, −90 phase relationship may be realized using differential lengths of the feed lines. The latter arrangement renders impedance cancellation within the divider  30  more difficult. In addition, phasing that is realized using feed line length tends to vary more greatly over the working band. Thus, reliance on differential feed line length for setting phase tends both to lower uniformity of phase circularity over frequency and to narrow antenna bandwidth. 
     The many features and advantages of the invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and, accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention.