Patent Publication Number: US-6911953-B2

Title: Multi-band ring focus antenna system with co-located main reflectors

Description:
BACKGROUND OF THE INVENTION 
   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 sub-reflectors. 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 sub-reflectors 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-reflectors 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-reflectors 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. 
   SUMMARY OF THE INVENTION 
   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-reflectors. 
   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-reflector 
   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. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation of a decoupled ring-focus reflector antenna design that is useful for understanding the invention. 
       FIG. 2  is a schematic representation of a coupled-feed ring-focus reflector antenna design that is useful for understanding the invention. 
       FIG. 3  is a schematic representation of a hybrid antenna system that combines the features of the antennas in  FIGS. 1 and 2 . 
       FIG. 4  is an enlarged view of the feed system in FIG.  3 . 
       FIG. 5  is schematic representation of a dual band ring focus antenna that illustrates the compact nature of the antenna structure described in  FIGS. 3 and 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Ring focus antenna architectures commonly make use of a dual reflector system as shown in FIG.  1 . With the dual reflector system, an RF feed  100  illuminates a sub-reflector  102 , which in turn illuminates the main reflector  104 . RF feed  100  can be a simple conical horn arrangement or can include one or more additional features such as an RF chokes  107  to improve performance. For example, the introduction of the choke can improve the gain factor and spillover efficiency. Sub-reflector  102  and main reflector  104  are shaped surfaces of revolution about a boresight axis  110  and are suitable for reflecting RF energy. The arrangement of the feed horn and sub-reflector in  FIG. 1  is referred to as a decoupled configuration or a decoupled feed/subreflector antenna. 
   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 s 1  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  FIG. 2  is known as a coupled-feed/sub-reflector antenna. Similar to the antenna in  FIG. 1 , this type of antenna makes use of a sub-reflector  202  and main reflector  204  that are shaped surfaces of revolution about a boresight axis  210  and are suitable for reflecting RF energy. In this type of antenna, the RF feed  200  and the sub-reflector  202  are spaced more closely as compared to the decoupled configuration. The RF feed  200  can include one or more RF chokes  212  at an aperture  206  of the RF feed. The purpose of the chokes is to improve antenna pattern performance with respect to sidelobes. For example, such RF chokes can be used to meet a particular set of sidelobe specification curves and/or improve return loss matching. The aperture  206  of the RF feed and the vertex  208  of the sub-reflector  202  can be spaced apart by a distance s 2  that is typically less than about 2 wavelengths at the frequency of interest. When arranged in this way, the RF feed  200  and the sub-reflector  202  are said to be coupled in the near-field to generate what is commonly known as a “back-fire” feed. 
   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 subreflector  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  FIGS. 1 and 2  can employ a conventional geometry or may use shaped-geometry main reflector and a shaped-geometry sub-reflector feed similar to the arrangement described in U.S. Pat. No. 6,211,834 B1 to Durham et al., the disclosure of which is incorporated herein by reference. In Durham et al., interchangeable, diversely shaped close proximity-coupled sub-reflector/feed pairs are used with a single multi-band main reflector for operation at respectively different spectral frequency bands. Swapping out the sub-reflector/feed pairs changes the operational band of the antenna. Each of the main reflector and the sub-reflector in the system described in Durham et al. are respectively shaped as a distorted or non-regular paraboloid and a distorted or non-regular ellipsoid. 
   The present invention combines the concept of the decoupled feed/subreflector antenna in FIG.  1  and backfire type coupled feed/subreflector antenna in  FIG. 2  to provide multi-band capability in a very compact design. Ring focus antennas using the coupled configuration concept shown in  FIG. 2  tend to be more compact as compared to other comparably performing dual reflector antennas. Accordingly, two independent ring-focus reflector geometries can be located in approximately the same swept volume as a single Cassegrain or Gregorian system 
   As shown in  FIG. 3 , a pair of co-located first and second main reflectors  304 ,  306  can be used concurrently with first and second RF feeds  300 ,  301  for first and second RF spectrally offset RF frequency bands. In particular these can include a lower frequency band serviced by RF feed  300  and a higher frequency band serviced by RF feed  301 . First and second RF feeds  300 ,  301  can be circular profile waveguides having a coaxial configuration. Further each of the first and second RF feeds can have a respective corresponding subreflector for communicating RF energy between each of the RF feeds  300 ,  301  and their respective main reflectors  304 ,  306 . Specifically a first sub-reflector  302  is provided for first RF feed  300  and a second sub-reflector  303  is provided for the second RF feed  301 . 
   The first subreflector  302  and RF feed  300  can be arranged similarly to the (coupled) backfire feed system shown in FIG.  2 . In particular, the first subreflector  302  and RF feed  300  can be spaced one to two wavelengths apart so as to comprise essentially a single backfire feed network. The first sub-reflector  302  and the RF feed  300  provide the feed system for a low frequency band of the antenna. 
   In contrast, second subreflector  303  and second RF feed  301  are preferably arranged in a conventional decoupled ring-focus configuration, meaning that aperture  31   8  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  FIG. 3 , it can be seen that the first and second main reflectors  304 ,  306  at least partially overlap one another and can be substantially coincident at a point  308  closest to the RF packaging  310 . In order to prevent first main reflector  304  from shielding the second main reflector  306 , the first main reflector  304  can be formed from a frequency selective surface (FSS). Frequency selective surfaces are well known in the art and can be formed from one or more layers of various geometric patterns of wires or apertures that are usually defined on a dielectric substrate. The FSS used to form the first reflector  304  can be selected to reflect RF energy at the design frequency selected for the first subreflector and feed pair  300 ,  302 , but pass RF energy at the design frequency selected for the second subreflector and RF feed pair  301 ,  303 . 
   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 FIG.  4 . As illustrated therein, the RF feeds  300 ,  301  can be arranged coaxially about a boresight axis  322 . RF energy can be communicated through each of said coaxially configured first and second RF feed elements  300 ,  301  as is known in the art. 
   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 FIG.  2 . 
   As shown in  FIG. 4 , the diameter of the focal ring of the first main reflector  304  and the diameter of the first sub-reflector  302  at the aperture are advantageously selected to be about the same size. Further, the diameter of the subreflector  302  is preferably not much larger than the diameter of RF horn  312  at the aperture  314 . In the back-fire feed configuration, the RF feed horn  312  and the sub-reflector  302  in combination can be considered as forming a single integrated feed network that provides a circular to radial waveguide transition. The circular to radial waveguide transition section includes the horn  312  and the sub-reflector  302 . 
   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  FIG. 4  additional matching structure  315  can be provided at the aperture of the horn. The matching structure is typically a choke ring or rings of a number, width, and depth determined through an iterative computer modeling process where the cost function is one or more of the following:
         a. improved antenna pattern performance with respect to sidelobes;   b. improved directivity; and   c. improved return loss.       

   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&#39;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&#39; 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&#39;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. 
     FIG. 5  is schematic representation of a dual band ring focus antenna that illustrates the compact nature of the antenna structure described in  FIGS. 3 and 4 . The antenna system illustrated in  FIG. 5 , is designed for operation at C-band and Ku-band. It has a main reflector of 98.5 inches, and a pair of sub-reflectors that are each about 12.4 inches in diameter. The antenna achieves an equivalent focal ring distance (F/D) from vertex of main reflector (F) to diameter of main reflector (D) of 0.29. The antenna has an extremely small swept volume compared to other designs of equal performance. For example, equivalent performance from conventional Cassegrain/Gregorian co-located antenna designs would require substantially more volume. 
   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.