Patent Application
20050099350
1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for antennas and feed systems, and more particularly to ring focus antennas and feed systems that can operate in multiple frequency bands.
2. Description of the Related Art
It is desirable for microwave satellite communication antennas to have the ability to operate on multiple frequency bands. Upgrading existing equipment to such dual band capability without substantially changing antenna packaging constraints can be challenging. For example, there can be existing radomes that impose spatial limitations and constraints on the size of the reflector dish. The existing antenna location and packaging can also limit the dimensions of the antenna feed system. For example, the existing radome can limit the forward placement of the feedhorn and the subreflector. Similarly, modifications to the existing opening in the main reflector are preferably avoided. As a result, for small aperture reflectors, the feed horn and the subreflector must fit in a relatively small cylinder.
In view of these spatial limitations, special techniques must be used to maintain antenna efficiency. U.S. Pat. No. 6,211,834 B1 to Durham et al. (hereinafter Durham), concerns a multi-band shaped ring focus antenna. In Durham, a pair of interchangeable, diversely shaped close proximity-coupled sub-reflector-feed pairs are used for operation at respectively different spectral frequency bands. Swapping out the subreflector/feed pairs changes the operational band of the antenna. Advantage is gained by placement of the shaped sub-reflector in close proximity to the feed horn. This reduces the necessary diameter of the main shaped reflector relative to a conventional dual reflector antenna of the conventional Cassegrain or Gregorian variety. The foregoing arrangement of the feed horn in close proximity to the sub-reflector is referred to as a coupled configuration.
The coupled configuration described in Durham generally involves sub-reflector to feed horn spacing on the order of two wavelengths or less. This is in marked contrast to the more conventional sub-reflector to feed horn spacing used in a decoupled configuration that is typically on the order of several to tens of wavelengths.
Although Durham demonstrates how a ring focus antenna may operate at different spectral bands, sub-reflector-feed pairs must be swapped each time the operational band of the antenna is to be changed. Accordingly, that system does not offer concurrent operation on spectrally offset frequency bands.
U.S. Pat. No. 5,907,309 to Anderson et al. and U.S. Pat. No. 6,323,819 to Ergene each disclose dual band multimode coaxial antenna feeds that have an inner and outer coaxial waveguide sections. However, neither of these systems solve the problem associated with implementing dual band reflector antennas in very compact antenna packaging configurations.
The invention concerns a compact multi-band ring-focus antenna system. The antenna system includes a first and a second main reflector, each having a shaped surface of revolution about a common boresight axis of the antenna. A first backfire type RF feed is provided for feeding the first main reflector on a first frequency band. A second RF feed coaxial with the first RF feed is provided for feeding the second main reflector on a second frequency band spectrally offset from the first frequency band. Further a portion of the second RF feed passes through a first sub-reflector of the backfire feed. The second RF feed is terminated a distance from the first sub-reflector to illuminate a second sub-reflector.
According to one aspect of the invention, t at least a portion of the first main reflector can be substantially co-located with the second main reflector. For example, the colocated portion of the first main reflector can be located at an inner periphery of the main reflector closest to the boresight axis. Further, the first main reflector can advantageously be formed as a frequency selective surface (FSS).
The backfire feed is comprised of a first horn closely coupled to and directly interacting with the first sub-reflector. The first horn and the first sub-reflector together comprise a circular to radial waveguide transition section of the backfire feed. In contrast, the second RF feed is decoupled from the second sub-reflector. For example, a vertex of the second sub-reflector can be spaced along the boresight axis at least about four wavelengths from a vertex of the first sub-ref lector.
According to one aspect of the invention, at least one of the first and second main reflector has no continuous surface portion thereof shaped as a regular conical surface of revolution. According to another aspect of the invention, the second sub-reflector can be formed so as to have no continuous surface portion thereof shaped as a regular conical surface of revolution.
Ring focus antenna architectures commonly make use of a dual reflector system as shown in
In a decoupled feed/subreflector antenna, the RF feed 100 is located in the approximate far field of the sub-reflector 102. For example, the aperture 106 of the RF feed 100 can be positioned spaced from a vertex 108 of the sub-reflector 102 by a distance at the frequency of interest, where s1 is greater than or equal to about four wavelengths. Since the RF feed is in the approximate far-field, the decoupled feed/subreflector configuration lends itself to optical design techniques such as ray tracing, geometrical theory of diffraction (GTD) and so on.
A second known type of ring focus antenna system illustrated in
According to a preferred embodiment, the diameter of the focal ring of the main reflector 204 and the diameter of the sub-reflector 202 at the aperture are advantageously selected to be about the same size. If they are not, the coupled feed focal ring will not be coincident with the focal ring defined by the main reflector 204. Further, the diameter of the subreflector 202 is preferably not much larger than the diameter of RF feed 200 at the aperture.
In a back-fire feed configuration, the RF feed 200 and the sub-reflector 202 in combination can be considered as forming a single integrated feed network. This single feed network is particularly noteworthy as it provides a circular to radial waveguide transition that generates a prime-ring-focus type feed for the main reflector 204. In this regard, the back-fire feed can be thought of as being similar to a prime-focus parabolic feed. The circular to radial waveguide transition is produced by the interaction of the horn portion of the RF feed 200 with the sub-reflector 202. Further, those skilled in the art will appreciate that the sub-reflector 202 in this feed configuration is not truly operating as a reflector in the conventional sense but rather as a splash-plate directly interacting with the feed aperture 206.
The ring focus antennas in
The present invention combines the concept of the decoupled feed/subreflector antenna in
As shown in
The first subreflector 302 and RF feed 300 can be arranged similarly to the (coupled) backfire feed system shown in
In contrast, second subreflector 303 and second RF feed 301 are preferably arranged in a conventional decoupled ring-focus configuration, meaning that aperture 318 of the second RF feed 301 is spaced at least about four (4) wavelengths from vertex 320 of the second subreflector 303 at the low end of the designed operating frequency of the feed. The second RF feed 301 passes through a vertex region of the first subreflector 302 and is terminated some distance from the first sub-reflector 302 for feeding the second sub-reflector 303 on a higher frequency band of the dual band system. Notably, the focal ring for the second sub-reflector is preferably located outside the second main reflector aperture to avoid distortion of the antenna beam produced by the second main reflector. This is because optical designs tend to perform poorly when the focal-ring (ring-focus antenna) or focal point (conventional parabolic antennas) is located inside the main reflector aperture.
Referring again to
For example, if the first subreflector and RF feed pair 300, 302 are designed to operate at C-band and the second subreflector and feed pair 301, 303 are designed to operate at Ku-band, then the FSS can have a stop band at low frequencies including C-band, and a pass band for higher frequencies including Ku-band. A suitable break point for the FSS band stop filter in this case could be selected at 6.425 GHz to accommodate these filter characteristics at C-band and Ku-band. Higher frequencies associated with feed 301 can be transmitted through the first main reflector 304 and are instead reflected by second main reflector 306.
An enlarged view of the first and second subreflector and RF feed pairs is shown in
First and second tapered horn sections 312, 316 can be provided for first and second RF feeds 300, 301. Horn 316 is preferably a conical type horn, it being understood that other horn profiles may also be adapted for use with the invention. Further, horn 316 can be selected to have an axial length and taper appropriate to improve impedance matching and beam shaping for meeting antenna selected performance specifications. Additional matching structure can be provided at the aperture 318 for controlling the gain factor and spillover efficiency if performance specifications so require. For example, conventional RF chokes (not shown) can be provided at the aperture 318 for this purpose. Similarly, horn 316 can have corrugations (not shown) formed along the axial length of the horn. Such corrugations are well known in the art for improving certain performance characteristics of the horn. The specific length taper, wall features and other characteristics of the horn 316 can be optimized using conventional computer modeling techniques.
Horn 312 is also preferably a conical horn, it being understood that other horn profiles may also be adapted for use with the invention. The horn 312 is preferably positioned so that the aperture 314 of the first RF feed and the vertex 324 of the sub-reflector 302 can be spaced apart by a distance that is less than about 2 wavelengths at the frequency of interest. When arranged in this way, the horn 312 and the sub-reflector 302 are said to be coupled in the near-field to produce a “back-fire” feed as described above in relation to
As shown in
The integrated feed network generates a prime-ring-focus type feed for the main reflector 304 that is similar to a prime-focus parabolic feed. The sub-reflector 302 in this feed configuration is not truly operating as a reflector in the conventional sense but rather as a splash-plate directly interacting with the feed horn 312 and aperture 314. As shown in
The RF feed 300, horn 312, matching structure 315 and sub-reflector 302 can together form a single integrated coupled feed for illuminating the first main reflector 304 with RF at the lower one of the frequency bands. The shape of the first sub-reflector 302, the taper and aperture features of horn 312, and the shape of main reflector 304 can be selected using conventional computer modeling techniques.
In general, the shaped surfaces of the main reflectors 304, 306 and their respective sub-reflectors 302, 303 can be defined by an equation of a regular conic, such as a parabola or an ellipse. Alternatively, the shaped surfaces can be generated by executing a computer program that solves a prescribed set of equations for certain pre-defined constraints. For example, using techniques similar to those disclosed in Durham et al., each of the first and second sub-reflectors 302, 303 and the main reflectors 304, 306 can be advantageously shaped using computer modeling to achieve a desired set of antenna beam performance parameters.
According to a preferred embodiment, the precise shape of the first and second main reflectors 304, 306 and the first and second sub-reflectors 302, 303 can be determined based upon such a computer analysis. Given the prescribed positions of the apertures 314, 318 for RF feeds 300, 301 and boundary conditions for the antenna, the shape of the sub-reflectors 302, 303 and the main reflectors 304, 306 are generated by executing a computer program that solves a prescribed set of equations for the predefined constraints. Physical constraints drive some of the boundary conditions, such as the size of the subreflector and the size of the main reflector. Electromagnetic constraints drive other boundary conditions. For example, if the electrical spacing of the phase center for RF feed horn 316 to subreflector 302 is less than about four wavelengths at the high frequency band, then the operation of the subreflector 302 will no longer behave optically. Similarly, if the second sub-reflector 303 is too close to the first subreflector 302, then the low band feed will block the line-of-site between the subreflector 303 and main reflector, causing the system not to work properly.
Given the foregoing constraints, equations are employed which: 1—achieve conservation of energy across the antenna aperture, 2—provide equal phase across the antenna aperture, and 3—obey Snell's law. Details regarding this process are disclosed in U.S. Pat. No. 6,211,834 to Durham et al.
For a given generated configuration of RF feeds 300, 301, horns 312, 316, a given set of shapes for the sub-reflectors 302, 303 and the main reflectors 304, 306 the performance of the antenna is analyzed by way of computer simulation. This analysis determines whether the generated antenna shapes will produce desired directivity and sidelobe characteristics. RF matching components are used to achieve the desired return loss.
If the design performance criteria are not initially satisfied, one or more of the equations' parameter constraints are iteratively adjusted, and the performance of the antenna is analyzed for the new set of shapes. This process can be iteratively repeated, as necessary until the shaped antenna sub-reflector shape and coupling configuration, and main reflector shape, meets the antenna's intended operational performance specification for each band. Each of the feed configurations, and the shapes for the subreflector and main reflector may be derived separately, as described above.
Finally, it should be noted that while the antennas described herein have for convenience been largely described relative to a transmitting mode of operation, the invention is not intended to be so limited. Those skilled in the art will readily appreciate that the antennas can be used for receiving as well as transmitting.
