Patent Publication Number: US-6211834-B1

Title: Multiband ring focus antenna employing shaped-geometry main reflector and diverse-geometry shaped subreflector-feeds

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to communication systems, and is particularly directed to a new and improved multiband ring focus antenna architecture comprised of a common or shared pseudo parabolically shaped main reflector, and a plurality of diversely configured subreflector-feed pairs, that are interchangeable with each other to provide a reduced sidelobe envelope at a plurality of separate operational frequency bands. 
     BACKGROUND OF THE INVENTION 
     Satellite communication systems have customarily employed multi-reflector antenna architectures, often of center-fed Cassegrain configuration, in order to optimize the collection of electromagnetic energy within a prescribed frequency band transmitted over relatively long distances (e.g., earth station-satellite-earth station). Where the number and size of antenna components is not necessarily a major concern, such as a fixed, land-based facility that has ample room for the placement of one or more relatively large structures, it is common practice to employ a relatively large main reflector, and an associated subreflector that is on the order of several tens of wavelengths in diameter. Because of the substantial blockage associated with such a subreflector, the diameter of the main reflector may be in excess of five meters in diameter at C and/or X band. While such a large dimensioned subreflector—main reflector structure is capable of successfully performing its intended functionality for a given operational frequency band, if the earth station is to provide communication capability at separate bands, additional subreflector—main reflector pairs configured for operation at those bands must be installed. 
     In contrast, many communication systems, such as shipboard-mounted facilities, have only a limited amount of space for the installation of antenna components. In such spatially constrained environments, where antenna size is limited and its directivity pattern must typically comply with a very strict specification, it is not practical to install even one, much less multiple spatially large reflector structures. One proposal to deal with this space constraint problem is to employ a ring focus antenna, having a parabolic main reflector and a ‘shaped’ (i.e., ellipsoid) subreflector. 
     Advantageously, the conical properties of the ellipsoid-shaped subreflector provide a dual focus characteristic, with one of its foci displaced toward the vicinity of the aperture of the main reflector where a feed horn is installed. The other focus is symmetric about the antenna axis in the form of a ring, which enables the antenna to obtain a substantially uniform amplitude distribution in the aperture plane. As a consequence of this geometry characteristic, the antenna can is more compact than a conventional center-fed structure. 
     For non-limiting examples of documentation detailing the configuration and operation of a conventional ring focus antenna, attention may be directed to the following publications: “Amplitude Aperture-Distribution Control in Displaced-Axis Two-Reflector Antennas,” by A. Popov et al, Antenna Designer&#39;s Notebook, IEEE Antennas and Propagation Magazine, Vol. 39, No. 6, Dec. 1997, pp. 58-63; “The Theoretical Analysis of Shaped Dual-Reflector Antenna with Ring Focus,” by T. Wang et al, Conference Proceedings, 20th European Microwave Conference 90, pp 1553-1558; “Shaped Dual-Reflector Antenna with Ring Focus,” by R. Zhang et al, Science in China (Series A) Vol. 34, No. 10, Oct. 1991, pp 1243-1255; “Two-Reflector Antenna,” by Y. Erukhimovich et al, Radio Research Institute, Ministry of Posts and Telecommunications, USSR, pp. 205-207; and the Canadian Patent to Schwarz, No. 1,191,944, entitled “Improved Shifted Focus Cassegrain Antenna With Low Gain Feed,” and assigned to the assignee of the present application. 
     Now although a ring focus antenna, such as those described in the above literature, is intended to provide reduced subreflector blockage and thereby the overall size of the antenna structure to be smaller than a conventional Cassegrain architecture, its ellipsoid-shaped subreflector is still on the order of several tens of wavelengths in diameter, and is spaced apart from the antenna feed (horn) by similar electrical distance. 
     To minimize subreflector blockage, the size of the main reflector is still substantial; at C or X band, the main reflector may have a diameter on the order of three meters, depending upon gain and sidelobe requirements. This means that in order to provide communication capability at multiple spectrally separated bands, such as at each of C band and X band, the overall size of two ring focus antenna structures may extend to a diameter on the order of 16-20 feet. This not only places a strain on the space limitations of a facility such as a shipboard-mounted satellite communication system, but does not solve the hardware complexity and cost problems of having to install a separate ring focus pair for each operational band. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, these problems are effectively obviated by a new and improved, reduced size, multiband, shaped ring focus antenna architecture that employs a single pseudo parabolically shaped main reflector, and a plurality of diversely configured subreflector-feed pairs, that are designed for operation at respectively different spectral bands. The geometric optical properties of the subreflector-feed pairs are such that they may be used with the same shaped main reflector. This allows the operational band of the antenna structure to be readily changed by simply swapping out the subreflector-feed pairs. 
     As will be described, the term ‘shaped’ as used to described the present invention is meant a subreflector and main reflector geometry that is defined in accordance with a prescribed set of (reduced sidelobe envelope) directivity pattern relationships and boundary conditions for a prescribed set of equations, rather than a shape that is definable by an equation for a regular conic, such as a parabola or an ellipse. As will be described, given prescribed feed inputs to and boundary conditions for the antenna, the shape of each of a subreflector and a main reflector are generated by executing a computer program that solves a prescribed set of equations for the predefined constraints. In a preferred embodiment, the equations are those 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. 
     While the boundary conditions may be selected to define a regular conical shape, such is not the intent of the shaping of the invention. The ultimate shape of each subreflector and the main reflector are whatever the parameters of the operational specification of the antenna dictate, when applied to the directivity pattern relationships and boundary conditions. As it turns out, because the main reflector produced by the shaping mechanism of the invention has a non-regular conical surface of revolution that is generally (but not necessarily precisely) parabolic, and its associated subreflector has a non-regular conical surface of revolution that is generally (but not necessarily precisely) elliptical, the shape of the main reflector may be termed ‘pseudo’ parabolic and the shape of the subreflector may be termed ‘pseudo’ elliptical. 
     Once the shapes of a subreflector and main reflector pair have been generated, the performance of the antenna is subjected to computer analysis, to determine whether the generated antenna shapes will produce a desired directivity characteristic. If the design performance criteria are not initially satisfied, one or more of the parameter constraints are adjusted, and performance of the antenna is analyzed for the new set of shapes. This process is iteratively repeated, until the shaped pair meets the antenna&#39;s intended operational performance specification. 
     This iterative shaping and performance analysis sequence is also conducted for another spectrally separate band, to obtain a set of subreflector and main reflector shapes at the second operational band. It turns out that the shape of the antenna main reflector produced for each of X and C bands can be made substantially the same, and performance analysis has revealed that the shaped main reflector produced for C band can also be used for X band, although their subreflector-feed pairs are different. As a result, all that is necessary to change operational bands is to interchange the subreflector-feed pairs for the same or common main reflector. This iterative design process can be extended to include any number of distinct frequency bands. 
     In addition to employing such non-regular conical surfaces of revolution, the shaped ring focus antenna architecture of the invention places the feed (horn) relatively close to the shaped subreflector, e.g., within two wavelengths of the vertex of the subreflector, as contrasted with the multiple tens of wavelengths spacing of a conventional regular conic ring focus antenna, in which the ellipsoid subreflector has a similarly dimensioned diameter. This placement of the shaped subreflector in close proximity to the feed horn provides a further decrease in aperture blockage, and enables the diameter of the main shaped reflector to be substantially reduced relative to that of a conventional ring focus configuration. As a consequence, not only does the shaped ring focus antenna of the invention provide for communication capability at multiple bands, but its reduced size and simplified hardware facilitates installation within the constrained space limitations of a facility such as a shipboard-mounted satellite communication system. 
     Each shaped subreflector also includes a single generally notch/wedge-shaped, edge current-limiting filter at its peripheral edge. In addition, respective antenna feed filter components are installed at the open ends of the antenna feed horns. For the C band configuration, the feed filter is configured as a conventional external choke contiguous with the outer edge of the forward open end of the feed aperture. For the X band configuration, the feed filter is configured as a set of internal circumferential corrugations that extend a prescribed distance along the interior wall of the feed from the outer edge of the forward open end of the horn (such as a standard corrugated horn with a parabolic flare). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an antenna geometry optics diagram for illustrating a ‘shaped’ multiband ring focus antenna architecture in accordance with the present invention; 
     FIG. 2 is a non-limiting example of a magnitude vs. angle Θ characteristic of an antenna feed; 
     FIG. 3 is a simplified antenna diagram of a multiband shaped antenna of the invention; 
     FIG. 4 is a partial boresight sectional diagram of a shaped antenna architecture of the invention for operation at C band; 
     FIG. 5 is a partial boresight sectional diagram of a shaped antenna architecture of the invention for operation at X band; and 
     FIG. 6 is a more detailed enlarged geometry optics version of FIG.  1 ; 
    
    
     DETAILED DESCRIPTION 
     As described briefly above, the ‘shaped’ ring focus antenna architecture of the present invention employs a single shaped main reflector, that is configured so that it can be used interchangeably with each of respectively differently configured subreflectors and associated feeds, to realize a composite optical geometry characteristic that satisfies the same set of antenna performance criteria (e.g., a directivity pattern having a reduced or substantially suppressed sidelobe envelope) at respectively different operational frequency bands. 
     For this purpose, given prescribed feed inputs to and boundary conditions for the antenna, the shape of each of a subreflector and a main reflector are generated by executing a computer program that solves a prescribed set of equations for the predefined constraints. In accordance with a preferred embodiment of the invention, the equations employed are those 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. 
     In particular, the equations are as follows:                  ∫       θ      1                   min         θ      1                   max                P   1          (     θ   1     )          sin                   θ   1                          θ   1           =       ∫     x     2      min         x     2                 max                  P   2          (     x   2     )            x   2                          x   2                   (   1   )                 R1   +   R2   +   R3     =   constant           (   2   )                          y   1              x   1         =     tan        (         θ   1     -     θ   2       2     )         ;              y   2              x   2         =     tan        (       θ   2     2     )                 (   3   )                         
     The parameters used with these equations and the associated antenna geometry defined thereby is diagrammatically illustrated in FIG. 6, which is a more detailed enlarged geometry optics version of FIG. 1, to be described. 
     For a given set of generated subreflector and main reflector shapes, the performance of the antenna is then analyzed by way of computer simulation, to determine whether the generated antenna shapes will produce a desired directivity characteristic, such as one that is compliant with Intelsat sidelobe envelope requirements at a prescribed operational band (e.g., C band having a receive bandwidth of 3.7-4.2 GHz and a transmit bandwidth of 5.9-6.4 GHz). 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 is iteratively repeated, as necessary until the shaped antenna subreflector and main reflector pair meets the antenna&#39;s intended operational performance specification. 
     This iterative shaping and performance analysis sequence is also conducted for another (spectrally separate) band, such as X band having a receive bandwidth of 7.25-7.75 and a transmit bandwidth of 7.9-8.4 GHz, to realize a set of subreflector and main reflector shapes at the second operational band. It turns out that the shape of the antenna main reflector produced for each of these spectrally diverse bands can be made substantially the same; as a result of performance analysis it has been determined that the shaped main reflector produced for C band can also be used for X band, although differently configured subreflectors are used for each band. 
     This means that all that is necessary to change operational bands (from C band to X band, or from X band to C band) is to swap out or interchange the subreflectors and their associated feeds. Moreover, although each set of subreflector and main reflector shapes may be derived separately, as described above, it is also possible to derive a first set of shapes for a first band, and then use the parameters for the (first band) shaped main reflector (which is also to be used for the second band) to derive the shape of the subreflector for the second band. 
     The manner in which a ‘shaped’ ring focus antenna architecture in accordance with the present invention may be obtained from the above set of equations may be understood by reference to the antenna geometry optics diagram of FIG. 1 (and its associated detailed geometry optics diagram of FIG.  6 ), which shows an antenna boresight axis  10  and an antenna aperture plane  12  that intersects and is orthogonal to the boresight axis  10 . The antenna includes a shaped main reflector  20 , that is symmetric about the boresight axis  10 . Main reflector  20  extends from some interior main reflector feed entry or opening  21  of radius rm from the boresight axis  10  to an outer or perimeter edge  22  in the aperture plane  12 . 
     An antenna feed  30 , such as a feed horn or section of open ended waveguide, having a forward open end  32  of a feed aperture  34 , is located in and is symmetric about the boresight axis  10 . The forward open end  34  of the feed, which may contain or be adjacent to a subreflector focal point FP is spaced from the intersection  14  of the axis  10  with the aperture plane  12  by a distance ‘a’. Also located in and symmetric about the boresight axis  12  is the vertex or tip  41  of a shaped subreflector  40 , that is symmetric about the boresight axis  10  and is spaced apart from the focal point by a distance ‘b’. The shaped subreflector  40  has a radius Rs, that extends orthogonally from the boresight axis  10  to an outer or perimeter edge  42 , where an edge current-limiting notch/wedge-shaped filter to be described is located. 
     Given a phase center of electromagnetic energy emitted from (or received by) the subreflector focal point FP, a first (boresight—perimeter boundary) multi-segment ray  50 -1 may be defined as a spatial sequence or arrangement of a set of linear ray segments as follows: a first linear ray segment  51 -1 (extending from the focal point FP along the boresight axis  10  to the subreflector vertex  41  and having a first phase angle Θ-1 =0 relative to boresight axis  10 ); a second linear ray segment  52 -1 (extending from the subreflector vertex  41  to the main reflector perimeter edge  22 ); and a third linear ray segment  53 -1 (extending from the perimeter edge  22  to a point  12 -1 in the aperture plane  12 ). As the perimeter edge  22  is coincident with point  12 -1 in the aperture plane  12 , the length of the ray segment  53 -1 is zero. 
     For a prescribed antenna feed pattern, such as one having the magnitude vs. angle Θ characteristic shown in FIG. 2, as a non-limiting example, at successive increments of phase angle Θ-2, Θ-3, . . . , Θ-N relative to the boresight axis  10  from the focal point FP (one of which is shown at Θ-2), additional multi-segment rays  50 -2,  50 -3, . . . ,  50 -N (one of which is shown at  50 -2), having the same length as ray  50 -1 to provide equal phase across the aperture plane  12 , and satisfying Snell&#39;s law with respect to points  20 -i on the main reflector  20  and points  40 -i on the subreflector  40  to ensure equal angles of incidence and reflection relative to the subreflector and main reflector surfaces, may be defined. These additional rays are comprised of successive linear ray segments  51 -2,  51 -3, . . . ,  51 -N (one of which is shown at  51 -2), extending from the focal point FP to successive subreflector points  40 -2,  40 -3, . . . ,  40 -N (one of which is shown at  40 -2), linear ray segments  52 -2,  52 -3, . . . ,  52 -N (one of which is shown at  52 -2), extending from subreflector points  40 -2,  40 -3,  40 -N to successive points  20 -2,  20 -3,  20 -N (one of which is shown at  20 -2) along the main reflector  20 , and linear ray segments  53 -2,  53 -3, . . . ,  53 -N (one of which is shown at  53 -2) extending from successive points  20 -2,  20 -3,  20 -N along the main reflector  20  to points  12 -2,  12 -3,  12 -N in the aperture plane  12  (one of which is shown at  12 -2). 
     Each of the multi-segment rays  50 -i that establish the optical properties of the ring focus antenna of FIG. 1 is not only defined in accordance with equations (2) and (3), as described above, but also with the conservation of energy relationship of equation (1). While equation (1) serves to confine and substantially evenly distribute all of the energy emanating from or received by the feed horn within the reflection geometries of the subreflector and main reflector surfaces, the parameters of equation (1) may be tailored to realize a modified energy distribution characteristic associated with an intended adjustment of the antenna&#39;s directivity pattern. For example, equation (1) may be adjusted as necessary to provide a prescribed ‘tapering’ of the energy at peripheral portions of the reflector surfaces, in order to provide substantial suppression of the sidelobe envelope. 
     As pointed out above, as equations (1), (2) and (3) are solved and respective associated sets of subreflector and main reflector shapes are generated for iteratively adjusted values of input parameters to the antenna feed and boundary conditions of the antenna (including main reflector outer radius Rm, main reflector interior opening radius rm, subreflector radius Rs, focal point-aperture plane spacing ‘a’, and focal point to subreflector tip spacing ‘b’), the resultant directivity characteristic is analyzed for each of a plurality of spectrally separate frequency bands (e.g., C band and X band, as non-limiting examples), until it tentatively satisfies a prescribed design specification. In the present non-limiting example, the intended directivity characteristic is defined to be compliant with Intelsat sidelobe suppression requirements. As noted previously, performance analysis has revealed that the same main reflector shaped for C band can also be used for X band, although differently configured subreflectors are produced for each band. 
     In the more detailed geometry optics diagram of FIG. 6, equations (1), (2) and (3), set forth above, are solved given the following boundary conditions: 
     α, β, Θ min , Θ max , x 2min , x 2max , and the form of P 2 (x 2 ). Two additional constraints are: i) to allow P 2  to be uniform in the aperture of the main reflector  20 , which results in maximum gain with approximately 17dB sidelobes; and ii) forcing P 2  to taper to −80dB at the outer edge of the main reflector  20 , which results in an extreme loss of gain with very low sidelobes. A practical (real time) trade-off between constraints i) and ii) is employed to develop the desired directivity pattern (within limits imposed by the feed, and the sizes of the main reflector  20  and the subreflector  40 ). 
     In addition to being preliminarily shaped in accordance with the geometry optics-based process described above, the subreflector in accordance with the present invention has two additional features that enable the antenna of the present invention to achieve its intended performance criterion (e.g., Intelsat specification-defined directivity pattern). First, it has been found that reducing the focal point-subreflector vertex spacing ‘b’, so as to bring the terminal end  32  of the feed horn  30  within very close proximity (i.e. within two and preferably less than one and one-half wavelengths) of the vertex  41  of the subreflector  40 , effectively creates a very compact or ‘close proximity-coupled’ feed-subreflector radiating structure, that effectively obviates the problem of phase center migration with frequency of a conventional subreflector—feed spacing, which is typically on the order of several to tens of wavelengths, as referenced previously. 
     Moreover, this compact structure allows the subreflector to main reflector spacing along the boresight axis to be reduced to a value on the order of tens of inches, as shown diagrammatically in the antenna geometry diagram of FIG.  3 . It has been found that, at either X band or C band, a main reflector  20  shaped in accordance with the invention may have a radius Rm on the order of only forty-five inches or so. This allows a multi (dual) band capability antenna to be installed within a space that is only half the size of a conventional ring focus architecture, making the antenna of the invention readily installable within a constrained space facility such as a shipboard-mounted satellite communication system. 
     A second additional feature of the subreflector of the present invention is the placement of a generally notch/wedge-shaped filter at a peripheral edge thereof. More particularly, FIGS. 4 and 5 diagrammatically show half-portions of boresight symmetrical antenna structures, that are obtained in accordance with the shaping process described above for C band and X band operation. As shown therein, in the radial direction outwardly from the boresight axis  10 , the outer peripheral edge  42  of the shaped surface  40  of each (C band and X band) subreflector terminates or is bounded by an edge current limiting filter  44  having a single generally deep V-shaped notch  46 . Notch  46 , in turn, is contiguous with a single wedge  48  at the circumference  49  of the subreflector  40 . Wedge  48  projects generally in a direction parallel with the boresight axis toward the main reflector  20 . Filter  44  is operative to reduce radial currents at the peripheral edge of the subreflector. The shapes and dimensions of the filter  44  are determined empirically. 
     Also shown in FIGS. 4 and 5 are respective antenna feed filter components of conventional construction that are installed at the open ends of the antenna feed horns. For the C band configuration of FIG. 4, a filter  35  is configured as a conventional external choke that is contiguous with the outer edge  38  of the forward open end  32  of the feed  30 . For the X band configuration of FIG. 5, a filter  36  is configured as a set of internal circumferential corrugations that extend a prescribed distance along the interior wall  37  of the feed from the outer edge  38  of the forward open end  32  of the feed  30 . 
     As will be appreciated from the foregoing description, the spatial and performance constraints of conventional Cassegrain and regular conic ring focus antenna geometries, described above, are effectively obviated by the multiband shaped ring focus antenna architecture of the present invention, which employs only a single or common main reflector, that is shaped such that it can be shared by each of a pair of diversely configured but interchangeable, close proximity-coupled, subreflector-feed pairs designed for operation at respectively different spectral bands. Since the subreflector-feed pairs may be used with the same shaped main reflector, the operational band of the antenna is readily changed by simply swapping out the subreflector-feed pairs. Also, placement of the shaped subreflector in close proximity to the feed horn helps reduce the diameter of the main shaped reflector relative to that of a conventional ring focus configuration. Consequently, not only does the shaped ring focus antenna of the invention provide for communication capability at multiple bands, but its reduced size and simplified hardware facilitates installation within the constrained space limitations of a facility such as a shipboard-mounted satellite communication system. It should also be noted that the invention is not limited to use with any band or groups of bands. X and C bands have been given for purposes of providing a non-limiting example. Other antenna applications, such as those designed for use at Ku band and Ka band, as well as X band and C band, may also benefit from the present invention. 
     While we have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as are known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.