Abstract:
A circularly polarized, omnidirectional, corporate-feed pylon antenna uses multiple helically-oriented dipoles in each bay, and includes a vertical and diagonal support arrangement of simple structural shapes configured to provide a frame strong enough to sustain mechanical top loads applied externally. The radiators in each bay fit within the vertical supports. The radiators are integrally formed with cross-braces, and are fed with manifold feed straps incorporating tuning paddles. A single cylindrical radome surrounds the radiative parts and the vertical supports. The antenna admits of application to the upper L-band at the full FCC-allowed ERP. Beam tilt, null fill, and vertical null can be readily accommodated.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Patent Application titled, “Circularly Polarized Omnidirectional Low Wind Load Antenna Apparatus and Method”, filed Aug. 9, 2006, having Ser. No. 60/836,397, which is hereby incorporated by reference in its entirety. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to radiating systems. More particularly, the present invention relates to single-feed circularly polarized omnidirectional broadcast antennas. 
       BACKGROUND OF THE INVENTION 
       [0003]    The auction of the 700 MHz spectrum by the Federal Communications Commission (FCC) resulted in part from the shift of television broadcasting from analog to digital service. Some of the new license holders have begun rollout of a Digital Video Broadcast to Handheld (DVB-H) mobile television (TV) entertainment service. Since receivers for this service may be expected to be integrated into cell phones and similar devices, circularly polarized broadcast signals will likely be preferred. 
         [0004]    By providing a signal with horizontal and vertical components of comparable strength, circular polarization offers independence between receiving antenna orientation and reception, at least within a plane perpendicular to a line of propagation between the transmitting and receiving antennas. That is, a simple (linearly polarized) receive dipole is capable of receiving, and is substantially insensitive in orientation with respect to, a circularly-polarized broadcast signal. By contrast, with a vertically (linearly) polarized transmitted signal, the same receive dipole receives very little signal if placed horizontally, and likewise for a horizontally polarized signal and a vertically oriented receive dipole. This can be a significant consideration in ensuring robust and stable received-image quality in a mobile handheld imaging device, for example. Multipath issues, such as reflections from buildings that can reverse polarization handedness and delay time-critical signals, are often managed through signal processing. 
         [0005]    Omnidirectionality is frequently a desirable attribute of broadcast antennas, particularly in view of long-established FCC preference for azimuth uniformity in consumer-oriented broadcasting. A fundamental omni radiator, well understood in the art, is a vertical dipole (or a ground-plane-mirrored monopole), that cannot provide circular polarization and is limited regarding power, gain, beam tilt, and null fill. Some previous omni designs, such as that disclosed in U.S. Pat. No. 6,441,796 (&#39;796), issued Aug. 27, 2002, incorporated herein by reference, can provide circular polarization. 
         [0006]    In antennas according to the &#39;796 patent, a plurality of omni radiators (bays) are configured in a vertical array. Each radiator in the &#39;796 patent includes two or four arcuate, rod-section dipoles lying on quasi-helical paths around a vertical axis of the antenna common to all bays. As used herein, the term “quasi-helical” describes a radiator formed from material having a suitable shape, such as a cylindrical rod, effectively wrapped into a planar arcuate shape, then rotated without further forming to an orientation approximating a helical path. A projection into a plane perpendicular to the vertical axis of the antenna of a quasi-helical radiator is elliptical; a true helical radiator has a circular projection into that plane. A rod formed into true helical form also does not lie in any plane. The effect of using a quasi-helical radiator is to broaden the impedance bandwidth of the antenna compared to a true-helix equivalent. 
         [0007]    The dipoles in the &#39;796 patent are each driven near one end of one monopole, with the centermost ends of the monopoles (the midpoints of the dipoles) grounded to conductive radial structural components. A central hub of each bay is mounted to a strut; the struts project laterally with selected vertical spacing from a vertical bearing structure. Such a configuration is readily applied to a side-mounted antenna on a tower, for example. 
         [0008]    The radiative parts of antennas according to the &#39;796 patent emit a signal having a specific circular polarization in accordance with their arrangement—for example, a mirror-image arrangement (opposite direction of advance of the helical paths of the dipoles) would produce opposite circular polarization. 
         [0009]    In many other previous omnidirectional antenna designs, individual circularly-polarized radiators are strongly directional. For a multiple-bay antenna using directional radiators to broadcast with a reasonable approximation of azimuth uniformity, three or more separate radiators in each bay are needed, pointing radially outward around a vertical axis. The radiators can be mounted around a central member for top mounting, i.e., mounting of the antenna at the top of a structure. Antennas including such elements require more radiating devices and more power distribution devices than do intrinsically omnidirectional radiators. 
         [0010]    In addition to circular polarization, increasing transmitter power output to 5 KW is planned under the new bandwidth assignments in order to achieve effective radiated power (ERP) that approaches the FCC-permitted maximum. This power level is high compared to that of S-band transmitting systems currently used for purposes similar to those for which the auctioned upper-L band spectrum is intended. The new requirements also call for an economical antenna solution and a compact equipment package, both highly desirable attributes for implementation of a nationwide infrastructure. Small size in combination with a simple physical arrangement may result in low wind loading. Other considerations include capability to use a single product over the entire new spectrum without alteration, or to combine multiple signal channels on a single antenna. 
       SUMMARY OF THE INVENTION 
       [0011]    The foregoing considerations are addressed, to a great extent, by the present invention, wherein in one aspect a circularly polarized, corporate-feed antenna is provided that, in some embodiments, affords simplicity in mechanical construction, higher power capability, high gain, broad bandwidth, improved omnidirectionality, accommodation to vertical null, beam tilt, and null fill, and suitability for inconspicuous mounting. The present invention provides a low cost, broadband, high power, low wind load, circularly polarized omnidirectional pylon antenna. 
         [0012]    In one embodiment, a broadcast antenna is presented. The antenna includes a structural support base, a support structure that includes a plurality of substantially vertical struts, uniformly distributed about a central vertical axis of the antenna, wherein each of the vertical struts extends upward from a point of attachment to the base, and a first substantially horizontal cross-brace that interconnects the vertical struts at a first elevation above the support base. The antenna further includes a first single-feed radiator, substantially omnidirectional with respect to azimuth, that radiates an elliptically polarized signal, wherein the first radiator is structurally integral with the first cross-brace, and resides physically within a prismatic volume that encloses the horizontal extent of the support structure. 
         [0013]    The first radiator includes at least a first conductive rod, joined to the first cross-brace proximal to a midpoint of a longest dimension of the rod, wherein the rod is on the order of a half-wavelength in physical length and substantially arcuate in form, and wherein the arc of the first rod falls along a quasi-helical path at a substantially constant distance from the central vertical axis of the antenna. The first radiator may also include a second conductive rod, substantially identical to the first conductive rod, wherein the first two rods are oriented with twofold rotational symmetry about the central vertical axis of the antenna. 
         [0014]    The first radiator may further include a second two arcuate, quasi-helically-disposed conductive rods, substantially identical to one another, wherein the second two rods are oriented with rotational symmetry about the central vertical axis of the antenna and are interstitially positioned with respect to the first two rods, wherein the length, angle of advance, and distance from the vertical axis of the antenna of the second two rods are independent of the corresponding dimensions of the first two rods. The antenna may further include a central hub wherefrom a plurality of structural parts, that the first cross-brace includes, extend to attachment points with the plurality of vertical struts, a central coaxial connector that includes an outer conductor joined to the hub and an inner conductor passing therethrough and terminating at a flange distal to the connection loci of the connector, and a manifold feed strap connecting the flange to at least the first rod. The central hub, the structural parts that the cross-brace includes, and the rods of the first radiator may be formed into a single conductive unit by a forming process, wherein the forming process includes casting, molding, forging, metal joining, solid freeform fabrication, pressing, machining, a combination of these processes, or another process. 
         [0015]    In another embodiment, a broadcast antenna is presented. The antenna includes antenna supports configured from a base position, capable of sustaining vertically-applied compression and tension loads and laterally applied bending, torque, and shear loads, originating at a plurality of locations uniformly distributed around a central vertical axis of the antenna, spacing apparatus for maintaining substantially constant spacing between the distributed load supports, one or more quasi-helically oriented dipole radiators for radiating a broadcast signal having elliptical polarization substantially invariant with azimuth from a location congruent with the spacing apparatus, and a radome for substantially barring air flow, water penetration, and access by airborne particulate matter from the interior volume containing the supports, spacing apparatus and the one or more radiators of quasi-helical form and orientation. 
         [0016]    In still another embodiment, a method for broadcasting electromagnetic signals is presented. The method includes accepting at least one broadcast-level signal having a bandwidth extent and a power level that fall within a prescribed range, dividing the accepted signal into a plurality of individual signals, wherein the respective individual signals have spectrum characteristics substantially identical to the accepted signal, and wherein the respective individual signals have substantially identical phase and signal strength, and applying the respective individual signals to a plurality of broadband radiative devices that each radiate with elliptical polarization and substantial azimuthal omnidirectionality. 
         [0017]    The respective radiative devices are integral with cross-bracing structures. Each of the respective radiative devices includes a plurality of quasi-helically-disposed, conductive, arcuate rods operable to radiate in a common frequency band, arranged with approximate n-fold rotational symmetry, where n is the number of rods included in a radiative device, the respective rods are joined to the cross-bracing structures at respective rod midpoints, and the signals applied to the respective radiative devices are so coupled to the respective rods as to radiate therefrom with substantially uniform phase. The method further includes providing vertical load bearing capability, from a locus above the topmost radiative device to a locus below the bottommost radiative device, sufficient to support not less than the full weight of and climatic loading applied to the structure. The method further includes providing junction between the cross-bracing structures and the load bearing capability, wherein mechanical interaction therebetween is sufficient to reduce tendencies for the load bearing capability to deform under load, and providing weather shielding, wherein a weather protective enclosure includes at least a tubular sleeve of substantially continuous, cylindrical, nonconductive material, external to the load bearing and radiative components. 
         [0018]    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 that will form the subject matter of the claims appended hereto. 
         [0019]    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. 
         [0020]    As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized 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 
         [0021]      FIG. 1  is a perspective view of a complete antenna according to the present invention. 
           [0022]      FIG. 2  is a perspective view of a single radiator of an antenna according to the present invention. 
           [0023]      FIG. 3  is a schematic diagram of a signal broadcasting system incorporating an antenna according to the present invention. 
           [0024]      FIG. 4  is a measured pattern showing signal strength versus azimuth for a steel-framed prototype antenna according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    The present invention is shown in the figures, wherein like numerals refer to like elements throughout. Earlier designs for circularly polarized, high-gain, omnidirectional antennas for high L-band generally have high wind loading, weight, and complexity, and are generally not designed for ordinary broadcast applications. The present invention overcomes these disadvantages at least in part, having instead the characteristics described below. 
         [0026]    Regarding bandwidth issues, S-band development provides an instructive archetype for antennas according to the present invention. S-band begins at 1.5 GHz, immediately above L-band; the present invention addresses primarily the latter band, previously unavailable for this type of use. Typical S-band antennas have very narrow bandwidth. The present invention provides antennas with an impedance and pattern bandwidth capable of covering the entire lower 700 MHz band (698 to 746 MHz, former television channels 52 through 59, near the upper end of L-band). This capability is realized by arranging broadband circularly polarized radiating elements in a multiple-bay, single-axis vertical array. 
         [0027]    Regarding issues of high power,  FIG. 1  shows an embodiment of an antenna  10  according to the present invention, including a dedicated power divider  12  driving a set of semirigid coaxial signal distribution lines  14  to deliver broadcast energy to a plurality of individual radiators  16 , an arrangement that allows for high power capability. Each of the distribution lines  14  is a helically-corrugated coaxial transmission line in the embodiment shown. For graphical simplicity, the helical corrugations are omitted from the drawing, but may be preferred in order to permit ease of manufacture while assuring low impedance error, since the outer-conductor construction and dielectric material in such lines assure low flattening (cross-section distortion) during such manufacturing steps as coiling and stowing surplus line in a reserve area  18  at the antenna base  20 . 
         [0028]    Regarding wind loading, a simple, cylindrical radome envelope  22 , shown in phantom in  FIG. 1 , and preferably scaled specifically for the lower 700 MHz band, encloses the entire radiative assembly  10  in a single, low-drag, “pylon” shaped body. A simple cylinder offers appreciably lower drag than more complex arrangements, such as multiple, independently enclosed, directional radiators of comparable total cross-sectional area, with an improvement on the order of 40% in some embodiments. 
         [0029]    Despite low material cost and simplicity, the present invention may be configured with increased mechanical strength compared to that required merely to allow the antenna to be self-supporting. This strength extends even to the extent of supporting a high dynamic load, such as that applied by a flagpole, above the radiating portion of the antenna  10 . 
         [0030]    The power divider  12  shown in  FIG. 1  distributes applied signal power to the individual circularly polarized radiators  16 . The power divider  12  accepts a broadcast signal from a single coaxial input port  24 , and provides multiple outputs at coaxial ports  26 , which outputs may be uniform in phase and power level. The power divider  12 , like the radiators  16  discussed in greater detail below, may have a broad passband in some embodiments, and can exhibit low dissipative (heat) loss in keeping with known methods for providing broad-band, high-power RF signal dividers. Each of the power divider output ports  26  includes a pressure barrier (not shown) in accordance with known practice, so that the interior of the radome  22  is not pressurized in the embodiment shown. Configuring the radome  22  as nonpressurized should not be viewed as limiting. Signal output power level to each port  26  may be unequal in some embodiments, for such purposes as tailoring beam characteristics. 
         [0031]    A flanged, pressurized feed line  28  (the portion connecting to the antenna input is shown in phantom in  FIG. 1 ) connects to the flange  30  of the input port  24  of the power divider  12  in the embodiment shown. Although flanged connections and pressurization are shown and described, other mechanism may also be used. 
         [0032]    The distribution lines  14  are coaxial lines that carry power from the power divider  12  to the radiators  16 . The distribution lines  14  in the embodiment shown are equal in length, with excess coaxial line length coiled in the reserve area  18  below a bottommost radiator  16  so that radiators  16  successively farther from the power divider  12  are nonetheless fed by lines  14  of equal length. In other embodiments, the distribution lines  14  may vary in length, such as with each higher radiator  16  fed by a longer feed line  14 . Such arrangements tend to degrade antenna bandwidth to a greater or lesser extent, but may be preferred in some embodiments, for purposes such as cost and/or weight reduction. 
         [0033]    Small adjustments in the relative lengths of the individual distribution lines  14  allow beam tilt and/or null fill to be provided. The individual radiators  16  generate circularly polarized signals independently of one another, and are fed with delay that depends in large part on the lengths of the respective distribution lines  14  and the properties of the power divider  12 . As a consequence, it is possible to drive the respective radiators  16  simultaneously, generating a main beam that has no deliberate tilt. This means that the far-field signal in a plane  32  passing through the middle of the antenna  10  aperture (the extent from the top radiator to the bottom radiator), and perpendicular to a central vertical axis  34  of the antenna, is most strongly reinforced. According to this description, the signal strength at angles above or below the perpendicular plane  32  is reduced in proportion to the deviation of the angle from zero degrees, so that a primary beam in the shape of a flattened toroid is formed. The gain of the beam (flatness of the toroid) is a function of, among other factors, the aperture size, the number of radiators, and the vertical spacing between radiators. 
         [0034]    It is further possible to alter the lengths of the respective distribution lines  14  in such a way as to cause far-field signals to be most reinforced at an angle other than zero degrees—that is, to introduce beam tilt. Similarly, a pronounced null immediately below the main beam may degrade close-in reception. To offset this, it may be helpful to deviate the lengths of the distribution lines  14 , such as by altering one or more lines to an extent different from that required by beam tilt. This can broaden the main beam to improve close-in reception, while decreasing peak beam strength (and range) only slightly, a process termed null fill. 
         [0035]    Vertical placement of the radiators  16  can be used to establish beam shape, but is not used in the embodiment shown to effect beam tilt or null fill. The term “antenna aperture” as used herein relates to the effective extent from the highest to the lowest point of the radiative parts of the antenna. Aperture in general determines gain, referenced to a point source radiator (0 dB) or a dipole (+2 dB) in free space. The number of radiators within the aperture establishes a limit on emitted power capacity, and, in conjunction with gain, height above average terrain, and details of radiator design, determines effective broadcasting range of a signal with a given power level. 
         [0036]    It is desirable in many applications (including for safety in low-mounted systems) to have an emission pattern that includes a null directly below the antenna. As is readily derived, a highly effective vertical spacing for providing both a vertical null and high gain in proportion to the number of radiators uses a spacing between radiators that is slightly less than one wavelength, namely (n−1)/n wavelengths, where n is the number of radiators. For example, for a single radiator, there is no spacing; for two, they are approximately one-half wavelength apart, for eight, they are approximately ⅞ of a wavelength apart, and so forth. If i is an integer less than n, all values of (n−i)/n produce such a null except i=0. For negative values of i (spacings greater than one wavelength), there is a tendency to produce banding, and for positive values of i greater than 1, the aperture decreases, so that gain as a function of signal power is sacrificed. Unless an embodiment is vertically constrained, therefore, the preferred spacing between radiators remains (n−1)/n wavelengths for many antennas according to the invention herein disclosed. 
         [0037]    Since the outer conductors of the respective distribution lines  14  are at roughly the same (ground) potential as the main input  24  outer conductor, the distribution lines  14  act as vertically oriented parasitics—known in the art as directors—that are long compared to a wavelength. Like the vertical struts  36 , these may have negligible effect on the horizontally polarized component of antenna output versus azimuth, while causing the vertically-polarized component to exhibit gain variation. A graphical representation  120  of this phenomenon as shown in  FIG. 4 , and as discussed in greater detail below, is described in the art as a “propeller” shape; the effect in the embodiment shown can be calculated and measured to be on the order of 3 dB. In the presence of conductive vertical struts  36 , also discussed below, the distribution lines  14  may not be appreciable contributors to signal propagation characteristics. 
         [0038]    Note that the distribution lines  14  for the elements  16  in  FIG. 1  rise in multiple groups at multiple azimuths. In some embodiments, the individual distribution lines  14  may rise at a common azimuth. The distribution lines  14  are shown with their vertical portions positioned near the outermost extent of the antenna  10 . In this arrangement, each line or group of lines  14  subtends a relatively small arc of the radiating pattern, and is not significantly intrusive in the feed arrangement at each radiator  16 . In some embodiments, it may be preferred to position the vertical portions of the distribution lines  14  nearer the central vertical axis  34  of the antenna  10 . 
         [0039]    Regarding tradeoffs between use of conductive and nonconductive support structure, the embodiment shown in  FIG. 1 , which uses four vertical support struts  36 , has been tested at least in glass-fiber reinforced polymer (FRP, commonly referred to as fiberglass) and in steel. In embodiments wherein the vertical struts  36  of the support structure are metallic, such as aluminum or steel of suitable dimensions, high strength can be achieved at low material cost. In embodiments wherein the vertical struts  36  are a dielectric material, such as FRP, weight can be lowered with minimal cost impact, but may result in reduced stiffness and/or load bearing capacity of the overall structure. In still other embodiments, higher performance materials such as carbon fiber, which has moderate conductivity, or other relatively exotic reinforcing fibers, such as aramid or blends of fibers, may be used as reinforcing filler for matrix-forming polymers such as epoxies, polytetrafluoroethylene (PTFE), high-density polyethylene (HDPE), or polyvinyl chloride (PVC), for blends, or for other matrix materials. Vertical struts  36  that are nonconductive and/or exhibit a low dissipation factor can reduce interaction between the structure and the radiated pattern in at least some embodiments. 
         [0040]    Perimeter cross members—that is, structural elements that join the vertical struts  36  to one another without significantly intruding into a prismatic volume whereof the faces are defined by the extents of the vertical struts  36 —are generally preferred to be nonconducting for embodiments wherein the diagonal cross members  38  and any horizontal cross members proximal to the faces of the vertical strut  36 -defined volume (none are shown in  FIG. 1 ) may potentially interact with the radiated signal. A material having properties generally comparable to FRP may be preferred in at least some embodiments. For example, FRP can be thermosetting, relatively low in cost, available off the shelf in familiar sizes and shapes based on standard steel construction shapes, and moderately easy to work with. FRP can also have acceptable electromagnetic properties, lifetime, strength-to-weight ratio, and stability over temperature. Other nonconductive materials, such as aramid reinforced polyester, filled thermoplastics, and the like, may be preferred in some embodiments. Conductive or semi-conductive materials may be less effective as cross members  38  to the extent that the materials absorb or reflect signals or exhibit electrolytic interaction with other parts of the antenna. 
         [0041]    High mechanical strength in the vertical struts  36  can allow the antenna to serve an additional purpose, such as bearing another antenna, or a flagpole, weather vane, traffic monitoring camera, or the like. Such use, or the appearance of the antenna to be an anonymous gray cylindrical pylon, may allow the high-value device—the antenna and its associated transmitter—to be less conspicuous than, for example, an open framework bearing one or more cavity-backed directional radiators with their feed coaxes and specialized radomes. 
         [0042]    In the embodiment shown, diagonal  38  elements of the support structure are nonconductive and low-loss, so that their interaction with the radiated signals—reflection, absorption, reradiation—is low. In embodiments having a high-strength support structure, the radome  22  may be thin or low in strength, required only to provide sun and/or ice protection, wind load management, and the like in a radio-transparent structure; in embodiments having a radome  22  with high strength and bearing negligible external load, the support structure may be made less robust to the extent that it is required to do little more than stabilize spatial placement of radiators  16 . 
         [0043]    Use of fewer than four vertical support struts  36  has also been evaluated. For many embodiments other than the simple four-strut  36  configuration of  FIG. 1 , the radome  22  may be required to be at least self-supporting, and adding of loads above the antenna may be restricted. Depending on the cross section and strength of the support struts  36 , use of fewer support struts  36  can result in a less rigid overall structure. Use of three conductive struts  36  at uniform intervals (120 degrees) is compatible with three-dipole configurations if it is desired to avoid pattern distortion that may result from having each of the struts  36  subjected to and interacting with a different field gradient. With two or four struts  36 , each may be positioned in a substantially equivalent position in a four-dipole configuration, as shown in  FIG. 2 , discussed below. 
         [0044]    The radome  22  shown in phantom in  FIG. 1  may be a simple cylindrical segment of PVC construction pipe, with “small schedule”—i.e., thin wall—and suitable for prolonged exposure to daylight and weather—i.e., resistant to ultraviolet (UV) light, heat, cold, rain, ice, and typical pollutants. Comparable materials having acceptable structural integrity and extent of transparency to radio waves in the band of use may be preferred in some embodiments. The thin wall and cylindrical form of the radome  22  shown are advantageous for assuring low loss, low effect on azimuth uniformity, and inconspicuousness of the antenna  10 , although other designs may also be used. The radome  22  can be attached to a top plate  42  above, and can be attached to, resting upon, or suspended above the antenna base  20  below. In such arrangements, if the top plate  42  is strongly attached to the vertical struts  36  as an upper terminus therefor, the antenna  10  may be capable of supporting significant mechanical loads, such as compression, bending, shear, and torque. The radome  22  may be sealed to a closed, substantially horizontal top plate  42  with one or more O-rings (not shown) within  0 -ring grooves  44 , for example, as shown in  FIG. 1 . In other embodiments, the radome  22  may use a sealant such as room temperature vulcanizing (RTV) adhesive (not shown) in lieu of O-rings and O-ring grooves  44  in the top plate  42 . The radome  22  may be provided with drain cutouts  46  at the bottom, as shown in  FIG. 1 . 
         [0045]    The base  20  provides attachment for the vertical struts  36 , and further provides mounting ears  48  whereby the antenna  10  can be fixed to an external structure (not shown), such as a tower top, a building, or a lateral strut or base plate projecting from a structure. Many alternative mounting provisions are possible, such as a flare at the base  20  similar in appearance to the mounting ears  48  shown, but continuous around the base  20 . Such a configuration may provide more attachment options. 
         [0046]    In embodiments with a mechanically robust base  20 , strut  36 , cross member  38  and top plate  42  configuration, the radome  22  may have no more strength than is needed to perform one or more functions such as retaining shape under wind load, shielding against sun and ice over the anticipated product life, and facilitating sealing against water intrusion over anticipated climate conditions. In other embodiments, the radome  22  may be further required to be self-supporting, to perform a sealing function without aid from the support structure, or to provide at least some load bearing capability. 
         [0047]    The antenna input shown in  FIG. 1  is a short segment of coaxial line  24  terminated at a flange  30 , with provision for pressurization. A typical embodiment can use an Electronic Industry Association (EIA) standard flange  30 , welded or brazed to the input coax  24 , with provisions for bolting to the broadcast transmission line  28  and sealing with an O-ring (not shown), for example. Various pressurization methods are known in the art for maintaining a transmission line  28  above atmospheric pressure and in a dry condition, at least in those parts of the line  28  that are exposed to weather, although other methods may also be used. 
         [0048]    Each bay includes a single circularly-polarized radiator  16 . Each radiator  16  emits an elliptically polarized signal that is substantially omnidirectional with respect to azimuth and toroidal with respect to elevation, with an axial ratio near unity at all azimuths—i.e., effectively circularly polarized. A limitation on azimuthal uniformity of axial ratio, namely the presence of conductive vertical struts  36 , has been discussed. Strut  36  materials that are substantially nonconducting and low-loss may provide somewhat higher uniformity, particularly in the distribution of vertical signal strength with azimuth. 
         [0049]      FIG. 2  shows a single radiator  16 , including a multi-arm cross-brace  50  that forms a structural component of the radiator  16 . The cross-brace  50  may be able to contribute radial mechanical strength sufficient to reduce tendencies for the peripherally-mounted vertical struts  36  and diagonal struts  38 , shown in  FIG. 1 , to bow outward, twist, buckle, or otherwise deform or fail in response to mechanical loads. A coaxial feed line  14  from the power divider  12 , shown in  FIG. 1 , is provided to each radiator  16 . Each feed line  14  may terminate in a connector half  52  that mates with a corresponding connector half  54  on the radiator  16 . In the embodiment shown, the feed line  14  terminates in a standard Type-N cable-end connector  52  (male center conductor, female-threaded outer conductor), and mates with a common Type-N threaded bulkhead-style connector body  54  (female center conductor, male-threaded outer conductor) that is screwed into the hub  56  of the radiator  16 . The extended center conductor (not shown in  FIG. 2 ) of the bulkhead connector opposite the connector  54  mating face is attached to a “mushroom,” i.e., a terminating flange  60 , that provides an attachment point to a single X-shaped feed strap  62 , termed herein “manifold” in view of the plurality of radiating components whereto signal energy is coupled by the feed strap  62 . 
         [0050]    Four blades  64  of the feed strap  62  extend outward, lying approximately in a strap plane  66  generally parallel to the plane  68  of the structural brace  50  portion of the radiator  16 , with the blades  64  directed toward upper extents of the radiative components, or dipoles  70 , of the radiator  16 . The ends of the blades  64  are formed to wrap around and make electrical contact at near-tip attachment points  72 . The blades  64  in the embodiment shown are creased to broadly match the angle of advance  74  of the dipoles  70 . The blades  64  tilt upward out of the strap plane  66  as a consequence of being creased. In some embodiments, such as those wherein the dipoles  70  differ from one another in length or in angle of advance  74 , the form of the respective blades  64  may vary, such as by being nonorthogonal within the feed strap  62 , having differing crease  76  locations or extent of bending, attaching to the respective dipoles  70  at differing distances along the respective dipoles  70 , and the like. Such variations fall within the scope of the invention, although other configurations may also be used. 
         [0051]    The blades  64  in the embodiment shown include conductive tuning paddles  80 . The paddles  80  can be positioned radially (by design change) or in tilt (by bending) to adjust radiator  16  impedance. The shapes, dimensions, and orientations of the respective paddles  80  tune the radiators  16  as viewed at the input connector  54 , while the paddles  80  emit negligible additional or spurious radiation in at least some embodiments. In particular, final settings of bandwidth, impedance, axial ratio, and like properties of each radiator  16  may be established by altering configuration of the paddles  80 . 
         [0052]    The four dipoles  70  in the embodiment shown are cast as a single part with the arms of the structural cross-brace  50  and with the associated hub  56 . The upper monopoles  82  of the respective dipoles  70  extend about a quarter-wavelength from the braces  50 , so that the overall combination of dimensions, along with load splitting by the manifold feed strap  62  to the near-tip attachment points  72  provides termination in a preferred impedance at the antenna  10  frequencies. The lower monopoles  84  are not separately excited, but function with the driven monopoles  82  to form dipoles  70 . 
         [0053]    Because of the geometry of the components, even a single one of the dipoles  70 , driven as shown by a single blade  64 , in the absence of the other three dipoles  70 , will emit a circularly polarized signal. An opposed pair of dipoles  70  will also emit, and will exhibit greater pattern uniformity than the single. As discussed in  Antenna Engineering Handbook, Third Edition,  R. C. Johnson, ed., McGraw-Hill, 1993, section 28-3, “Circularly Polarized Antennas,” herein incorporated by reference, a four-dipole shunt-fed helical radiator, similar to the quasi-helical radiator shown in  FIG. 2 , having uniform dipole lengths, helix angles, and feed points, may have a preferred circumference—in this instance the effective path length of a projection parallel to the antenna axis  34  of the dipoles  70  onto a plane  32  perpendicular to the axis  34  (see FIG.  1 )—of about one wavelength. A three-dipole equivalent is preferably about three-fourths of a wavelength in circumference, while a two-dipole equivalent is preferably about one-half wavelength in circumference, and a one-dipole equivalent is preferably about one-quarter wavelength in circumference. An antenna configured according to the present invention and dimensioned approximately according to Johnson will behave similarly with respect to pattern, and may exhibit improved bandwidth. 
         [0054]    The diagram in  FIG. 3  shows in schematic form a more complete view of a system  90  of which an antenna  92  according to the present invention forms a part. In the embodiment shown in  FIG. 3 , an antenna  92  is fed from a coaxial line  94  that mates with the input feed line  96  of the power divider  98 . The coaxial line  94  provides a signal from a transmitter or group of transmitters  100 , and may be fed by way of output filters  102 , combiners  104 , circulators  106 , pressurizing apparatus  108 , and the like in some embodiments to form the transmitting system  90 . The source apparatus  100 ,  102 ,  104 ,  106 ,  108  may be positioned within a transmitter house  110 . The antenna  92  may be configured to bear a flagpole  112  or other external structural load; for such functions, the top plate  42 , shown in  FIG. 1 , may accommodate mounting provisions of any appropriate type, such as blind threaded holes. 
         [0055]      FIG. 4  shows a set of overlaid test plots  120  representing antenna signal strength versus azimuth for a prototype 8-bay antenna according to the present invention, wherein the vertical struts  36  of  FIG. 1  are fabricated from a good conductor, such as structural steel. In keeping with conventional practice in the art for representing circularly polarized waveforms, the figure includes, as a first curve  122 , a boundary limit for horizontally polarized signal strength, measured by orienting a linearly-polarized receiving antenna horizontally at far field and rotating the antenna under test about its vertical axis  34 , shown in  FIG. 1 , through at least 360 degrees, while transmitting.  FIG. 4  also illustrates, as a second curve  124 , a boundary limit for vertically polarized signal strength, measured similarly, but with the linearly-polarized receiving antenna oriented vertically. A representation of circularly-polarized signal strength  126  at each azimuth, as developed by rotating the antenna under test at a low rate with respect to the receiving antenna, while the receiving antenna is rotated at a high rate about an axis radial to the antenna under test, is also shown. 
         [0056]    The jagged appearance of the signal strength plot  126  is an artifact of the relative rotation rates. The greater the magnitude of the excursions, the greater the difference between vertical and horizontal signal magnitudes in the elliptical emission pattern as detected in the test procedure. This plot shows instantaneous voltage measurements as a radial distance from the center of the chart, roughly normalized, so doubling displacement from the center represents a  6  dB increase in signal strength. Using the horizontal  122  and vertical  124  plots, the worst-case voltage axial ratio is around 2 (6 dB) at 224 degrees and 320 degrees, and is generally highest at the intercardinal nodes, here located around 45, 135, 225, and 315 degrees referenced to the chart. The axial ratio decreases to unity at several azimuths, and has a greater vertical component  124  over some azimuths. 
         [0057]    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.