The present invention generally relates to antenna systems and, in particular, relates to a stepped reflector antenna (“SRA”) for use in multiple beam antenna systems.
Dual-band antenna systems, operating simultaneously at both uplink and downlink frequencies of a multiple beam communication satellite, have the advantage of using half the number of reflectors and half the number of feed horns, when compared with a conventional multiple beam antenna (“MBA”) with a separate set of reflector antennas for each uplink and downlink band. Moreover, such dual-band antenna systems can increase usable space on the spacecraft for other payloads and cost less than conventional MBAs.
Although this type of antenna system is significantly better than conventional MBA systems, the receive (“Rx”) beams suffer from large peak-to-edge gain variations due to an electrically larger reflector size. For example, the reflector is about 50% larger for Rx beams when the reflector is sized for transmit (“Tx”) beams. One approach to compensate for this involves shaping the reflector surface such that it is heavily optimized for Rx frequencies and less optimized for Tx frequencies. Even with this compensation, such a dual-band antenna system suffers from peak-to-edge gain variation of about 5.0 dB to 7.0 dB at the Rx band with 1.0 dB to 2.0 dB gain loss due to pointing error and about 0.5 dB lower gain at the Tx band.
It is therefore considered highly desirable to provide for an antenna system which overcomes the deficiencies discussed above. In particular, it is desirable to provide an improved reflector antenna and to provide a novel MBA system that produces “flat top” Rx beams and more efficient Gaussian transmit beams.
In accordance with the present invention, a stepped reflector antenna is provided. The reflector has an annular region that is axially stepped a height h above or below the central region. The height h is chosen to create a desired 180° phase reversal at a receive frequency of the reflected phase front near the edge of the central region, to reduce peak-to-edge gain variation. When used in a multiple-band antenna system, this stepped annular region can improve the performance of one band without requiring the antenna be reshaped to heavily optimize for one band or another.
According to one embodiment, the present invention is a reflector for being fed by at least one antenna. The reflector includes a central region and at least one annular region surrounding the central region, axially stepped a height h above or below the central region.
According to another embodiment, the present invention is a reflector for being illuminated by at least one feed. The reflector includes a central region and a first annular region with an annular width of w1. The first annular region surrounds the central region, and is axially stepped a height h1 above the central region. Height h1 is approximately equal to
where m1 is a positive odd integer, Φ1 is a desired amount of phase shift of an outer region of a phase front for reflecting off of the reflector, φ is a feed phase contribution for an angle Θ, and Θ0 is an angle formed between an axis of the at least one feed and a line connecting a phase center of the at least one feed and an inner edge of the first annular region.
According to yet another embodiment, the present invention is a multiple-beam antenna system including a reflector having a central region and a first annular region, the first annular region having an annular width w1 surrounding the central region, the first annular region axially stepped a height h1 above or below the central region. The system further includes at least one multiple-band feed for illuminating the reflector. The at least one multiple-band feed is configured for providing transmission and reception of signals over respective transmission and reception frequency bands. Height h1 is approximately equal to
where m1 is a positive odd integer, Φ1 is a desired amount of phase shift of an outer region of a phase front for reflecting off of the reflector, φ is a feed phase contribution for an angle Θ, and Θ0 is an angle formed between an axis of the at least one feed and a line connecting a phase center of the at least one feed and an inner edge of the first annular region.
Additional features and advantages of the invention will be set forth in the description below, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings, which are included to provide further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:
In the following detailed description, numerous specific details are set forth to provide a full understanding of the present invention. It will be apparent, however, to one ordinarily skilled in the art that the present invention may be practiced without some of these specific details. In other instances, well-known structures and techniques have not been shown in detail to avoid unnecessarily obscuring the present invention.
As will be apparent to one of skill in the art, the scope of the present invention is not limited to stepped reflector antennas with particular physical dimensions, as the stepped reflector concept is applicable for any wavelength of radiation, which is one determining factor when choosing an antenna's dimensions. According to one embodiment of the antenna designed to operate in the Ka band (18 GHz-40 GHz), for example, the central region 101 of stepped reflector antenna may be between 60 inches and 120 inches. According to other embodiments, central region 101 may be larger or smaller, according to the various requirements of its design.
Annular region 102 may similarly be nearly any physical dimension. As will be apparent to one of skill in the art, the proportion of annular width w to the diameter m of central region 101 may determine what portion of the outer region of a reflected phase front will experience a phase shift. Accordingly, the selection of annular width w will depend upon the requirements of the design of reflector 100. According to one embodiment, annular width w may be between 5% and 15% of diameter m of central region 101. The scope of the present invention is not limited to annular regions of these dimensions, however, and may encompass annular regions of nearly any annular width.
Discontinuity region 104 may be configured in a number of ways. According to one embodiment, discontinuity region 104 is a smooth discontinuity, having an annular width wd of no more than 0.5 inches. In other embodiments, discontinuity region 104 may have a larger or smaller annular width, even of 0 inches (e.g., in an abrupt discontinuity where the discontinuity region is oriented parallel to axis 103).
The stepped design of stepped reflector 100 enables the reflector to modify the shape of a reflected phase front. For example, if h is approximately equal to (e.g., within 25% of) an odd multiple of one fourth of the wavelength of an incident wavefront, then the reflected phase front will be modified near its outer regions by a phase shift of approximately 180°. For a phase front which is substantially uniform over the stepped reflector 100, this phase reversal results in a “flat-topped” beam pattern with a greatly reduced peak-to-edge gain variation.
In a dual band multiple-beam antenna (MBA) system employing a stepped reflector of the present invention, this phase front modification can be used to improve the Rx performance of the system without significantly compromising its Tx performance.
Dual-band antenna 220 is characterized by a broadcast frequency band and a reception frequency band. Height h is selected to accomplish an integer multiple of 180° phase shift at the edge region of the beam reflected from reflector 210. For phase fronts which are uniform over the surface of reflector 210, h may be approximately equal to an odd multiple of one fourth of a reception wavelength corresponding to a reception frequency in the reception frequency band of dual-band antenna 220. Because the annular region that reflects the outer region of the phase front is axially stepped a quarter-wavelength multiple, the reflected phase front at the reception frequency will be modified near its outer regions by a phase shift of approximately 180°. This phase shift results in a “flat-topped” beam pattern at the reception frequency with a greatly reduced peak-to-edge gain variation.
If the feed phase pattern are not uniform within the reflector subtended angle corresponding to the diameter of the central region, (e.g., in MBA systems where a phase center of a feed antenna is not disposed in the focal plane of the reflector), the phase variation of the incident wavefront over the annular region may be taken into consideration when selecting the height h by which the annular region is to be stepped. One example of an MBA where height h has taken into account feed-induced phase variations is illustrated in
Discontinuity region 312 may be an abrupt discontinuity region (e.g., characterized by corners on either side), a smooth discontinuity region (e.g., not having corners), or a combination of the two (e.g., having an abrupt transition between the discontinuity region and the central region, and a smooth transition between the discontinuity region and the annular region).
High efficiency dual-band horn antenna 320 has a Rx phase center 324 and a Tx phase center 325. A MBA system of the present invention may exploit this phase center variation to minimize the height h of stepped reflector 310. In the present exemplary embodiment, Tx phase center 325 is disposed at the focal point F of stepped reflector 310. Because high-efficiency dual band horn antenna 320 of the present embodiment is not a frequency independent horn, Rx phase center 324 is located a distance d along axis 301 from focal point F. Thus, a wavefront at the reception frequency corresponding to Rx phase center 324 may be non-uniform over annular region 313 of stepped reflector 310. According to one aspect of the present invention, the phase variation from the phase on axis, Δ Phase, can be determined for a given angle Θ according to Equation 1, in which d is the distance between the Rx phase center 324 and focal point F, and k is the circular wavenumber (e.g., 2π/λ for radians or 360/λ for degrees) for a Rx wavelength λ:
ΔPhase=kd/(1−cos Θ0) [1]
For example, for an antenna system in which Θ0=30°, d=0.5 in., and λ=0.4 in.−1 (˜30 GHz), ΔPhase=1.05 rad or 60°.
While the phase variation may be determined by Equation 1, it will be apparent to those of skill in the art that the phase variation may, according to another aspect of the present invention, be determined with modeling or simulation software.
While the present exemplary embodiment describes an embodiment of the invention applicable to a stepped reflector fed by a multiple-band antenna, it will be understood by one of skill in the art that the present invention has application to antenna systems fed by single-band antennas, in which distance d can similarly be determined as the distance between a phase center of the single-band antenna and the focal plane (or focal point, if the antenna and reflector share an axis) of the stepped reflector.
In this manner, the phase variation at annular region 313 can be determined with reference to Equation 1, of with modeling or simulation software, by comparing the phase on axis with the phase at angle Θ0, where Θ0 is an angle between axis 301 and a line connecting Rx phase center 324 and the inner edge 313a of annular region 313. According to another aspect of the present invention, angle Θ1 between axis 301 and a line connecting Rx phase center 324 and the outer edge 313b of annular region 313 may be used to calculate the phase variation at a second annular region (not shown). This phase variation, which is introduced by the feed antenna, is hereinafter referred to as the feed phase contribution φ.
Returning to the exemplary embodiment, in which Θ0=30°, d=0.5 inch, and λ=0.4 inches (˜30 GHz), the feed phase contribution φ(Θ0) at the annular region is 60°. To accomplish the desired 180° phase shift at the outer region of the reflected phase front, h should be selected to accomplish an additional 120° (180°−60° of phase shift, according to Equation 2, in which m is a positive odd integer:
The feed phase contribution φ at the axis (when Θ=0) is 0, as can easily be seen with reference to Equation 1. Thus, Equation 2 solves to an odd multiple m of 0.067 inches to accomplish the desired 180° phase shift at the edge regions of the reflected phase front. Where minimizing the step height h is desired, a value of 1 can be selected for m. Where minimizing the step height h is not desired, m may be any positive odd integer.
The ± sign in Equation 2 indicates the need to consider the direction of the phase shift accomplished by the feed phase contribution when determining whether to add or subtract the contribution from the desired phase shift of 180°. The plus sign is used when the phase center is closer to the stepped reflector than is the focal plane, and the minus sign is used when the phase center is further from the stepped reflector than is the focal plane.
When a stepped reflector with multiple annular regions is designed, the height hn that a given annular region is stepped above the previous region (whether the previous region is an annular region or the central region) can be determined by a Equation 3, in which the feed phase contribution for a given annular region φ(Θn) is determined with reference to the phase of the previous region φ(Θn-1). When the previous region is the central region, the feed phase contribution φ(0) will of course be 0.
For some applications, it may be desirable to phase shift the outer regions of the reflected phase front by an amount other than 180°. In such an application, Equation 2 may be modified to select a height h to accomplish a desired phase shift Φ. Equation 4 may be used to determine a step height h by which to step an annular region to accomplish a phase shift of the outer regions of a reflected phase front by Φ degrees:
Thus, according to one embodiment, a stepped reflector of the present antenna may have an annular region axially stepped a height h above or below the central region, where h is determined by Equation 4. In other embodiments, h may be approximately equal to (e.g., within 25% of) the value determined by Equation 4.
While the present exemplary embodiment has illustrated a stepped reflector fed by only one antenna, it will be understood by those of skill in the art that a multiple beam antenna system of the present invention encompasses reflectors fed by more than one multiple-band antenna. In such an embodiment, the Tx phase center of each multiple-band feed antenna will be disposed at or near the focal plane of stepped reflector, rather than at the focal point of the stepped reflector. Moreover, while
In an alternate embodiment, a stepped reflector of the present invention may be illuminated by a single multiple-band feed in a contour antenna system, in which multiple contoured beams are generated by a single feed reflecting a phase front off of shaped regions of a stepped reflector.
One type of high efficiency dual-band horn antenna that may be used in conjunction with a stepped reflector of the present invention can provide signal transmission and reception over widely separated respective transmission and reception frequency bands. Referring back to
The diameter of the throat section of high efficiency dual-band horn antenna 320 may be selected to allow the throat section to propagate only the dominant mode over the transmission frequency band. The substantially conical wall 321 may contain a phasing section having a permanent slope. The phasing section may be configured to ensure that all modes add in a proper phase relationship with the dominant mode at the aperture. By contrast with conventional feed horns, the internal surface 326 of the substantially conical wall 321 is free from recesses, flares or corrugations all the way from the throat section 322 to the aperture 323 to maintain high horn efficiency (e.g., 85% to 90%) over widely separated transmission and reception frequency bands. For example, a frequency band from 18.3 GHz to 20.2 GHz may be used for transmission, and a frequency band from 28.3 GHz to 30.0 GHz may be employed for reception.
While dual-band horn antenna 320 has been described as having two frequency bands, in yet another embodiment of the present invention, a multiple-band feed with any number of frequency bands may be used to illuminate a stepped reflector. For example, a multiple-band feed may have one Tx frequency band and multiple Rx frequency bands, multiple frequency bands for both Tx and Rx, or one Rx frequency band and multiple Tx frequency bands.
While the stepped reflectors in
The performance advantages of stepped reflector antenna 600 are illustrated in Table 1, which summarizes the improved minimum edge-of-coverage (EOC) directivity in dBi of a stepped reflector antenna over a conventional reflector for a Rx frequency, both with and without accounting for pointing error (PE):
In
In
Because the stepped reflector of the present invention is able to improve the Rx performance of the MBA antenna system without requiring the reflector be oversized or otherwise heavily optimized for Rx performance, the Tx performance of the system of the present invention does not suffer the performance degradation of other approaches, and may in fact enjoy performance benefits in the Tx frequencies when both the annular region and central region of the stepped reflector antenna are shaped (e.g., with regions of non-parabolic curvature).
While the present invention has been particularly described with reference to the various figures and embodiments, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the invention. There may be many other ways to implement the invention. Many changes and modifications may be made to the invention, by one having ordinary skill in the art, without departing from the spirit and scope of the invention.
The present application claims the benefit of priority under 35 U.S.C. §119 from U.S. Provisional Patent Application Ser. No. 60/693,832 entitled “ENHANCED STEPPED-REFLECTOR ANTENNA SYSTEM FOR DUAL-BAND MULTIPLE BEAM SATELLITE PAYLOADS,” filed on Jun. 27, 2005, the disclosure of which is hereby incorporated by reference in its entirety for all purposes.
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