Patent Grant
8373597
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.
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'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.
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.
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
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'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.
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'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
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.
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
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
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.
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.
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
Note that 0 degree and −90 degree feed lines are provided to feed the patches 12 as shown in
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.
This application claims priority to a 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, which is hereby incorporated by reference in its entirety.
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| 4367474 | Schaubert et al. | Jan 1983 | A |
| 6067053 | Runyon et al. | May 2000 | A |
| 6239750 | Snygg | May 2001 | B1 |
| 6396442 | Kawahata et al. | May 2002 | B1 |
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| Entry |
|---|
| Richard C. Johnson, ed., Antenna Engineering Handbook, Third Edition, 1993, pp. 28.21-28.24, McGraw-Hill, Inc. |
| Number | Date | Country | |
|---|---|---|---|
| 20080036665 A1 | Feb 2008 | US |
| Number | Date | Country | |
|---|---|---|---|
| 60836398 | Aug 2006 | US |
