Patent Publication Number: US-2002005813-A1

Title: Shaped reflector antenna assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     [0001] This application claims the benefit of U.S. Provisional Application No. 60/177,254, filed Jan. 20, 2000. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] The present invention relates to the field of microwave antennas, and in particular to such antennas having offset-fed shaped reflectors.  
       [0003] In cell-based communication systems for point-to-multipoint transmission systems, a “hub” is located at the center of a usually round “cell.” Omni directional azimuth radiation is obtained by an arrangement of wide beam antennas, each covering a sector of the cell. Each hub transceiver antenna is generally mounted on an elevated tower or building roof, and transmits to and receives signals from customer-premise equipment in the form of transceiver and antenna devices.  
       [0004] As an example, the hub site may consist of four 90-degree azimuthal sectors which, when combined, service the entire 360-degree cell area. The antenna, which is attached to and many times integrated with the radio transceiver unit, must effectively provide uniform power coverage within its sector and suppress unwanted radiation that may tend to leak into adjacent sectors or neighboring cells. Further, the antenna must suppress energy above the horizon that may interfere with satellite-based communication systems. The ideal antenna also must be capable of operating over assigned bandwidths (such as 28 to 31 GHz) without degradation of performance, and must be highly efficient.  
       [0005] Historically, the radiation pattern has been formed by the use of antenna arrays, slot antennas and beam horns. These configurations tend to be large and generally complex in structure. Shaped reflectors are commonly used for satellite communication. Recently, it has been found that shaped reflectors may be used for point-to-multipoint terrestrial communication as well. The shaped reflectors that produce narrow beams appropriate for satellite communication are found to be inadequate for the wide azimuthal beams required for terrestrial hubs.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006] The present invention provides a shaped reflector antenna that provides improved performance characteristics, satisfying the stringent requirements of a sector-based terrestrial communication system.  
       [0007] Generally, a shaped offset-fed reflector antenna made according to the invention includes an antenna feed, a reflector having a reflector surface, and a support for the feed and reflector for providing a wave path between the feed and the reflector. At least a portion of the reflector surface is convex.  
       [0008] The preferred embodiment of the invention is a single offset-fed reflector antenna including a semi-cylindrical radome covering the reflector in the region of the beam produced by the reflector. Further, the support includes a base plate having an aperture. The antenna preferably includes a waveguide coupling the aperture to the feed, a waveguide support for supporting the waveguide relative to the base plate, and end caps covering the ends of the radome. The radome, end caps and base plate form an enclosure for the waveguide, waveguide support, feed, and reflector. The reflector has a focal length of about one-half of the diameter of the reflector, making the assembly very compact.  
       [0009] The preferred embodiment of the shaped reflector provides a 90-degree azimuth and 6-degree elevation beam at 38 GHz. The convex portion of the reflector surface is generally centrally located when viewed in cross section from a horizontal plane, and has a convex region near the top when viewed in cross section from a vertical plane. The reflector is symmetrical about a vertical plane and is formed of a cylindrical metal stock.  
       [0010] It is seen that the preferred antenna assembly includes an offset-fed shaped reflector mounted in a radome cover. The reflector shape is obtained by an iterative optimization process that produces a continuous compound concave/convex surface providing a radiation beam having a broad width in azimuth and controlled elevation profile that are typically realized by the use of antenna arrays or sectoral horns. A focal length to reflector diameter ratio of less than one is used to provide a compact structure made possible by a dramatic reflector shape. The antenna preferably provides null-filled pattern shaping in elevation, a broad, flat beam in azimuth, aggressive side lobe suppression in azimuth without dynamic adjustment or tuning, high efficiency and broad frequency bandwidth. These and other features and advantages of the present invention will be apparent from the preferred embodiments described in the following detailed description and illustrated in the accompanying drawings. 
     
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
     [0011]FIG. 1 is a front isometric view of an antenna assembly made according to the invention.  
     [0012]FIG. 2 is a rear isometric view of the antenna assembly of FIG. 1.  
     [0013]FIG. 3 is a rear elevation of the antenna assembly of FIG. 1.  
     [0014]FIG. 4 is an exploded rear isometric view similar to FIG. 2.  
     [0015]FIG. 5 is an exploded side isometric view showing the assembly of a radome cover of the antenna assembly of FIG. 1.  
     [0016]FIGS. 6 and 7 are exploded views of the feed assembly of the antenna assembly of FIG. 1 showing alternative orientation of a corrugated feed horn.  
     [0017]FIG. 8 is a partial exploded view of the feed assembly of the antenna assembly of FIG. 1 without the radome cover or mounting arm.  
     [0018]FIG. 9 is an isometric view of a cylinder of metal stock used for manufacturing a shaped reflector according to the invention with a potential reflector surface shown in dashed lines.  
     [0019]FIG. 10 is a top right isometric view of the microwave antenna shaped reflector included in the antenna assembly of FIG. 1 and made according to the invention; the top left isometric view being a mirror image.  
     [0020]FIG. 11 is a rear left isometric view of the shaped reflector of FIG. 10, the rear right isometric view being a mirror image.  
     [0021]FIG. 12 is a front elevation of the shaped reflector of FIG. 10.  
     [0022]FIG. 13 is a rear elevation of the shaped reflector of FIG. 10.  
     [0023]FIG. 14 is a right side elevation of the shaped reflector of FIG. 10, the left side elevation being a mirror image.  
     [0024]FIG. 15 is a top plan view of the shaped reflector of FIG. 10.  
     [0025]FIG. 16 is a cross-section taken along the line  16 - 16  in FIG. 15, corresponding to the plane X=2.5 defined in FIG. 10.  
     [0026]FIG. 17 is a cross-section taken along the line  17 - 17  in FIG. 15, corresponding to the plane X=4.0 defined in FIG. 10.  
     [0027]FIG. 18 is a cross-section taken along the line  18 - 18  in FIG. 15, corresponding to the plane X=5.5 defined in FIG. 10.  
     [0028]FIG. 19 is a cross-section taken along the line  19 - 19  in FIG. 15, corresponding to the plane Y=1.5 defined in FIG. 10, the cross-sectional view taken along the plane Y=−1.5 being the same.  
     [0029]FIG. 20 is a cross-section taken along the line  20 - 20  in FIG. 15, corresponding to the plane Y=0 defined in FIG. 10. 
    
    
     DETAILED DESCRIPTION  
     OF THE PREFERRED EMBODIMENT OF THE INVENTION  
     [0030] Referring initially to FIGS.  1 - 5 , the design of a microwave antenna assembly  10  made according to the invention is shown. Antenna assembly  10  provides a beam having a half-power width in azimuth of 90 degrees and a height in elevation of 6 degrees at 38 GHz. The invention also applies to other beam patterns and frequencies. Assembly  10  includes a radome cover assembly  12  mounted to a base plate  14 . An antenna mounting arm  16 , for mounting the antenna assembly to a pole-mounting assembly is rigidly mounted to the backside of the base plate, as particularly shown in FIGS.  2 - 4 .  
     [0031] As shown, base plate  14  has an elongate rectangular shape. Radome cover assembly  12  includes an elongate semi-cylindrical radome cover  18  and semi-circular ends  20  and  22  that provide a full enclosure  23  of an antenna  24  mounted to the base plate under the cover. As shown in FIG. 3, the radome cover is seen to have a longitudinal axis  25  that is perpendicular to ends  20  positioned horizontally in the preferred embodiment. The radome cover thus provides a continuous curved surface for the wide-angle beam to pass through. Alternative implementations may include other custom shapes, and the shape may be made with a fully formed or molded surface. Microwave communication signals are fed to antenna  24  via a waveguide coupler  26  mounted in base plate  14 , as shown.  
     [0032] Referring now to FIGS.  6 - 8 , antenna  24  includes a waveguide  28 , a corrugated feed horn  30 , also referred to simply as a feed, and a shaped reflector  32 . As shown particularly in FIG. 8, the feed horn is offset from the central axis  33  of reflector  32  by an offset angle A. The central axis is also referred to as the bore sight of the antenna or the axis of the beam produced by the antenna. The feed horn in FIG. 7 is rotated in orientation about the feed axis of the horn 90 degrees relative to the orientation shown in FIG. 6. The dimension F represents approximately the focal length of the antenna. The actual focal length, as it is conventionally understood, corresponds to the distance from the center of the feed horn aperture to a point on plate  38  along the axis of a parabola approximately containing the reflector surface. The axis of this parabola is the Z-axis at X=Y=0 in the coordinate system of FIGS.  10 - 14 .  
     [0033] The feed horn thus defines a wave path, shown generally at  31 , between the feed horn and reflector. The reflector may be made of a cylindrical metal stock  34 , shown in FIG. 9 having a diameter D, or it may be cast. Reflector  32  shown in FIG. 8 was cast and is supported at a fixed orientation relative to base plate  14  on legs  36 . Dashed line  34   a  in FIG. 9 represents the initial position for the shaped reflector surface. The metal body of the shaped reflector, whether it was cast or made from a stock  34 , is also referred to as a unitary body.  
     [0034] Waveguide  28  is supported in a fixed position relative to plate  14  by a mounting plate  38  having a waveguide opening  38   a  aligned with waveguide coupler  26 . Coupler  26  serves as a base plate/waveguide transition that converts the electromagnetic linear fields present within waveguide  28  to linear fields within the waveguide (not shown) attached to the other side of the base plate. Waveguide  28  has a base end  28   a  aligned with opening  38   a,  and a suspended or feed end  28   b.  Feed horn  30  is mounted to waveguide end  28   b  by a circular plate  40  that functions like coupler  26  to provide a rectangular to circular waveguide transition. This transition is not necessary if the feed waveguide is circular. The waveguide follows a serpentine path from plate  38  and is supported in the suspended position by an upright  42 . It will be understood that other waveguide sources and shapes may also be used. Waveguide  28 , upright  42  and base plate  14  are included in what is referred to as a support assembly  44 . If the waveguide is sufficiently rigid, upright  42  is not necessary.  
     [0035] The corrugated horn is designed to optimally illuminate the surface of the shaped reflector. The phase center of the horn, as determined through conventional mode-matching techniques, is positioned at the virtual focus of the offset reflector, and illuminates the reflector with the proper (primary) illumination pattern to provide low spillover energy. The horn preferably provides −25 dB of roll-off at the edge of the reflector boundary.  
     [0036] Reflector  32  has a shaped surface  32   a  having a contour illustrated in FIGS.  10 - 20 . The position and grid for the X, Y and Z axes used to define the shaped surface are shown in the figures. This same convention is followed for the definition of surface points given in the table of Appendix A, which table defines the shape of the reflector surface shown in the figures. The cross-sectional views show that the surface is symmetrical about a plane  106  corresponding to Y=0 and generally has a convex contour for cross-sections taken normal to plane  46 , as shown in FIGS.  16 - 18 .  
     [0037]FIG. 16 illustrates a cross section taken along line  16 - 16  in FIG. 15. The plane of view of this figure is represented by the plane  50  identified in FIG. 20, corresponding to Y=2.5. In FIG. 16 it is seen that the entire curve of surface  32   a  in plane  50  lies above a line  52  of construction extending between peripheral points  54  and  56  on the outer rim or periphery  32   b  of reflector  32 . The surface in this view is thus seen to be generally convex, particularly in the central portion  58 . It is seen, though, that a line, such as line  60  in plane  50 , connects points on the surface, below which the surface is concave in the side regions, such as region  61  adjacent to the periphery of the surface. It is seen, then, that surface  32   a  is both convex and concave in this cross section.  
     [0038]FIG. 17 illustrates the cross section through the center of reflector  32  as viewed in plane  46  and taken along line  17 - 17  in FIG. 15. Again the surface lies entirely above a straight line of construction  62  extending between two points  64  and  66  on the surface periphery. As in the cross section of FIG. 16, the surface is seen to be generally convex, particularly in a central region  68 . The surface is also concave in the peripheral regions, such as region  70  below a line of construction  72  extending between two spaced-apart points  64  and  74  on the reflector surface.  
     [0039]FIG. 18 illustrates the cross section through reflector  32  as viewed in a plane  76  identified in FIG. 20 corresponding to Y=5.5, and as taken along line  18 - 18  in FIG. 15. Again the surface lies entirely above a straight line of construction  78  extending between two points  80  and  82  on the surface periphery. As in the cross section of FIG. 16, the surface is seen to be generally convex, particularly in a central region  84 .  
     [0040]FIG. 19 is a cross section taken along line  19 - 19  in FIG. 15, which line corresponds to a plane  86  shown in FIG. 17. The cross section plane  86  corresponds to the grid value Y=1.5. The cross section for the grid value Y=−1.5 is the same since the reflector surface is symmetrical about the plane containing the grid value Y=0 as shown in FIG. 15. In FIG. 19 it is noted that most of the surface lies above a line of construction  88  extending between periphery points  90  and  92 . A central region  94  that extends up to adjacent point  92  on the surface periphery is seen to be convex.  
     [0041] The convexity drops off dramatically at the upper edge or periphery, forming a pronounced protuberance  96  particularly identifiable in the isometric views of FIGS.  10 - 13 . A short construction line  98  connecting point  92  to the surface at the protuberance shows that the surface is still slightly concave immediately adjacent to the surface periphery at a region  100 . The surface adjacent to periphery point  90  is seen to be much more broadly concave, as indicated by the surface line passing below a line of construction  102  extending along a region  104  between point  90  and central region  94 .  
     [0042]FIG. 20 is a cross section taken along line  20 - 20  in FIG. 15, which line corresponds to a plane  106  shown in FIG. 17, which plane is perpendicular with plane  46 , as shown in FIG. 15. Plane  106  is the plane of symmetry of the reflector surface and corresponds to the grid value Y=0. A line of construction  108  extending between reflector periphery points  110  and  112  shows that the reflector surface is disposed predominantly above the line and is primarily convex along a region  114 . The surface adjacent to periphery point  110  is seen to be broadly concave, as indicated by the reflector surface line passing below a line of construction  116  extending along a region  118  above point  110 .  
     [0043] Planes  46 ,  50  and  76  are parallel to each other, and they are perpendicular to planes  86  and  106 . Planes  86  and  106  are accordingly parallel to each other. All of these planes are parallel to the beam axis  33 .  
     [0044] As has been discussed, reflector surface  32  radiates a beam  120 , represented by arrow  120  in FIG. 20, along axis  33  that nominally has an azimuth beam width of 90 degrees and an elevation beam width of 6 degrees at 38 GHz. Reflector shapes that provide other beam patterns or to operate at other frequencies may be used. As shown in the figures, reflector surface  32   a  is preferably formed as one end  122 a of a unitary body  122  having a circular cylindrical form, as particularly shown in FIG. 15. Body  122  may be cast, as shown in FIG. 8 or formed from stock as shown in FIG. 9. It will be appreciated, though, that the reflector surface could be formed as part of a material or body that extends outwardly from periphery  32   b.    
     [0045] An alternative embodiment of the antenna is as a dual offset reflector antenna. This geometry makes use of a feed and feed horn that illuminates a shaped subreflector. This energy is then reflected onto the surface of a shaped primary reflector. The primary reflector is shaped to reflect the energy with the desired pattern characteristics. In this embodiment, the primary reflector is shaped to generate cross polarization energy that exactly compensates for or cancels undesirable cross polarization energy generated by the subreflector.  
     [0046] The data points given in the table in Appendix A may be used to form the shaped reflector shown in the figures. The data in this table was derived using commercially available optimization computer software. By the use of the optimization routine, the reflector surface was designed so that, when illuminated by the energy radiated by the feed horn (primary radiation), it provides the desired radiation pattern (secondary radiation). Conventional shaped reflector surfaces generally provide “contoured” patterns that encompass land mass (satellite applications). The aspect ratio of these patterns (ratio of azimuth angle extent to elevation angle extent) generally ranges from 1:1 to perhaps 4:1. The preferred antenna provides an aspect ratio of about 15:1, corresponding to 90-degree azimuth by 6-degree elevation. The resulting reflector surface is generally convex in azimuth and concave/convex in elevation. The reflector surface is preferably symmetric about the vertical (azimuth) axis and highly asymmetric about the horizontal (elevation) axis, as required, to provide asymmetrical elevation pattern shaping. Although not shown, an absorber may be applied to edges of the reflector surface, in order to reduce or eliminate the effects of unwanted diffracted energy. The reflector surface can be machined or cast for low cost high volume manufacture.  
     [0047] An inherent feature of the preferred reflector is that residual cross-polarized energy is generated as an artifact of the reflector surface and offset geometry. This effect tends to be increasingly pronounced with increasing azimuth beam width. To eliminate the effect of this resultant cross polarization, external polarizer “cleansing” grids, not shown, are attached to the inner surface of the radome. These parallel conductive traces or wires are generally etched on a substrate sheet (carrier) and the sheet is bonded to the inner radome surface. The angular orientation of the grids is dependent upon the polarization of the antenna. For example, a vertical antenna provides transmission and reception of vertically linear polarized energy. A small amount of horizontal linear polarized energy is generated which needs to be suppressed. To accomplish this, the grids are oriented horizontally such that the horizontal energy is generally incident on and reflected by the grids, rather than being transmitted through the radome.  
     [0048] Although the present invention has been described in detail with reference to a particular preferred embodiment, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims as written and as judicially construed according to principles of law. The above disclosure is thus intended for purposes of illustration and not limitation.