Abstract:
An apparatus and method for forming a cassegrain reflector antenna that allows an extended length feed horn to be employed without increasing an overall depth of the antenna. This enables the swept diameter of the antenna to be maintained at a minimum comparable to an antenna system using a standard length feed horn. The antenna system employs a hole at a vertex of the main reflector of the antenna system. The elongated feed horn is mounted at the vertex such that a major portion of its length projects outwardly form a rear surface of the main reflector. Antenna electronics components can be mounted on a neck of the feed horn or alternatively on a rear surface of the main reflector. Since the elongated feed horn does not increase the overall depth, and thus the swept arc of the antenna, the size of the radome needed to cover the antenna can be kept to a minimum size comparable to that required for reflector antennas employing conventional, standard length feed horns.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. patent application Ser. No. 09/965,668 filed on Sep. 27, 2001 now U.S. Pat. No. 6,861,994, entitled “Method and Apparatus For Mounting a Rotating Reflector Antenna to Minimize Swept Arc”, presently pending, the disclosure of which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to antenna systems, and more particularly to a method and apparatus for mounting a reflector antenna in such a manner as to minimize the swept arc of the antenna when the antenna is rotated about its azimuthal axis. 
   BACKGROUND OF THE INVENTION 
   The frontal surface area of an antenna mounted on an aircraft, under a radome, is of critical importance with respect to the aerodynamics of the aircraft. This is because of the drag created by the radome and the resulting effects on aircraft performance and fuel consumption. With reflector antennas that must be rotated about their azimuthal axes, the “swept arc” of the antenna is larger than the overall width of the main reflector of the antenna. This necessitates a commensurately wide radome, thus increasing the frontal surface area of the radome and consequently increasing the drag on the aircraft. 
   Referring to  FIG. 1 , the diameter of a swept arc “A” of a main reflector of a prior art antenna system can be seen when the azimuthal axis of rotation is located rearwardly, or behind, an axial center of the main reflector, as is conventional with present day reflector antenna systems. The outermost edges of the main reflector are also noted. This diameter is noted by dimension “B”. The diameter of the swept arc produced by the main reflector is considerably larger than the diameter of the main reflector itself when the azimuthal axis of rotation is located at, or rearwardly of, the center of the main reflector. 
   It is therefore extremely important that the height and width (i.e. depth) of a reflector antenna be held to the minimum dimensions consistent with the required electromagnetic performance of the antenna. More particularly, it is important for the main reflector of an antenna intended to be mounted on an outer surface of an aircraft, to be mounted in such a manner that the swept arc of the antenna is minimized when the antenna is rotated about its azimuthal axis. Minimizing the swept arc of the antenna would thus minimize the dimensions of the radome required to cover the antenna, and thereby minimize the corresponding drag created by the radome while an aircraft on which the radome is mounted is in flight. 
   Still another consideration in minimizing the swept arc is the physical length of the feed horn mounted at the axial center of the reflector (i.e., at the vertex). To maximize antenna performance, in some instances it would be desirable to use a longer feed horn on the reflector. However, using the longer than typical length feed horn necessitates increasing the depth of the reflector itself. Increasing the overall depth of the reflector means increasing its overall diameter or aperture size, and thus increasing its swept arc. Thus, there exists a need for a reflector antenna design that allows the use of an elongated feed horn which can be integrated into the reflector of the antenna without requiring an increase in the depth and the overall aperture size of the antenna. 
   SUMMARY OF THE INVENTION 
   The above drawbacks are addressed by an antenna system in accordance with a preferred embodiment of the present invention. The antenna system comprises a main reflector having an opening formed at its vertex. An elongated feed horn is disposed in the opening such that a major portion of the length of the feed horn extends outwardly of a rear surface of the main reflector. Antenna electronics components used with the antenna may be mounted on the portion of the feed horn projecting from the rear surface of the main reflector or on the rear surface of the main reflector itself. By mounting the feed horn such that a major portion of its length extends through the hole in the reflector, and thus outwardly of the rear surface of the reflector, the need to increase the depth of the reflector itself, and thus the overall aperture size of the antenna, is eliminated. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a simplified diagram of the swept arc produced by a prior art mounting arrangement wherein the azimuthal axis of rotation of the main reflector is disposed slightly rearwardly of the center of the main reflector; 
       FIG. 2  is a plan view of a prior art reflector antenna, wherein the main reflector of the antenna has center outermost edge portions. 
       FIG. 3  is a side view of an antenna system in accordance with a preferred embodiment of the present invention illustrating the azimuthal axis located within a plane extending between the outermost edges of the main reflector of the antenna; 
       FIG. 4  is a diagram illustrating the swept arc produced by locating the azimuthal axis of rotation as shown in  FIG. 3 ; 
       FIG. 5  is a side view of the antenna system of the present invention located with the azimuthal axis disposed in a plane located forwardly of the outermost edges of the main reflector of the antenna system; 
       FIG. 6  is a diagram of the swept arc produced by the antenna system shown in  FIG. 5 ; 
       FIG. 7  illustrates a present day, low profile cassegrain reflector having a feed horn with an antenna electronics components mounted at the rear surface of the main reflector; 
       FIG. 8  illustrates the antenna of  FIG. 7  but with an elongated feed horn, and also illustrating the increase in overall depth of the antenna; 
       FIG. 9  illustrates a cassegrain reflector antenna in accordance with a preferred embodiment of the present invention; 
       FIG. 10  illustrates only the main reflector and subreflector of the antenna of  FIG. 9  but showing a hole formed at the vertex of the main reflector; 
       FIG. 11  shows the feed horn projecting through the hole in the main reflector of the antenna; and 
       FIG. 12  is an enlarged side cross sectional view of a portion of the main reflector showing the attachment of the feed horn thereto. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   Referring to  FIG. 2 , a prior art antenna system  10  well suited to be mounted on an external surface of an aircraft is shown. The antenna system  10  includes a main reflector  12  having a center  12   a  and outermost edge portions  12   b . A subreflector  14  is positioned forwardly of a feed horn  16  located at the center  12   a  of the main reflector  12 . A pair of low noise amplifiers (LNA)  18  and  20  are used, as are a pair of diplexers  22  and  24 , for performing signal conditioning operations on the received and transmitted signals. An elevation motor  26  is used to position the main reflector  12  at a desired elevation angle, while an azimuth motor  28  is used to rotate the main reflector  12  about an azimuthal axis to position the main reflector at a desired azimuth angle. An encoder  30  is used to track the azimuth angle of the main reflector  12  and to provide feedback to the azimuth motor  28 . 
   Referring now to  FIG. 3 , an antenna system  100  in accordance with a preferred embodiment of the present invention is illustrated. The antenna system  100  is similar to antenna system  10  by the use of a main reflector  102  having an axial center  102   a  and outermost lateral edge portions  102   b . A feed horn  104  is disposed at the center  102   a  of the main reflector  102 . The main reflector  102  is supported on a platform  106  which places the azimuth axis of rotation  108  of the main reflector  102  in a plane which extends through the outermost edges  102   b  of the main reflector. The platform  106  is rotated about the azimuthal axis of rotation  108  by an azimuth motor  110  to thus position the main reflector  102  at a desired azimuth angle. A two channel coaxial rotary joint  112  is preferably employed to enable the necessary electrical connections between the feed horn  104  and a transmission line  112   a  which extends through an outer surface  114  of an aircraft. For simplicity, the radome which would ordinarily enclose the entire antenna system  100  has not been shown. 
   Referring to  FIG. 4 , a swept arc  116  is shown which is produced by rotational movement of the main reflector  102 , shown in highly simplified form, of the antenna system  100 . When the azimuthal axis of rotation  108  is located such that it extends through the outermost lateral edges  102   b  of the main reflector  102 , as described in connection with  FIG. 3 , the radius of the swept arc  116  is approximately one-half that of the overall length  118  of the reflector  102 . Thus, locating the azimuthal axis of rotation  108  forwardly of the center  102   a  of the main reflector  102  (i.e., to the right of center point  102   a  in  FIG. 3 ) dramatically reduces the swept arc produced by the main reflector. This reduction in the overall area, and volume, of the swept arc is also visible from a comparison of  FIGS. 1 and 4 . 
   The antenna system  100  shown in  FIG. 3 , however, in some applications, may result in an unacceptable degree of blockage of the signal being transmitted and/or received by the antenna system  100 . Accordingly, it may be desirable to locate the azimuthal axis of rotation  108  shown in  FIG. 3  forwardly of the outermost edges  102   b  of the main reflector  102 . Such a mounting arrangement is shown in  FIG. 5 . Antenna system  200  shown in  FIG. 5  is identical with antenna system  100  shown in  FIG. 3  with the exception that mounting platform  206  has a longer overall length to allow the azimuthal axis or rotation  108  to be located forwardly (i.e., to the right in  FIG. 5 ) of the outermost edges  202   b  of the main reflector  202 . It will also be appreciated that components of the antenna system  200  in common with those of antenna system  100  have been designated by reference numerals increased by a factor of  100  over those used to denote the components of the antenna system  100 . The swept arc produced by the antenna system  200  is shown in  FIG. 6 . The swept arc is designated by dashed circle  220 . The maximum, effective frontal width of the main reflector  202  is thus represented by arrow  222 , which is only slightly larger than a diameter  226  of the main reflector. The radius of rotation of the reflector  202  is represented by line  224 . Comparing the swept arc  220  of  FIG. 6  with the swept arc  116  illustrated in  FIG. 4 , it can be seen that the swept arc produced by the mounting arrangement of antenna system  200  is slightly greater than that produced by antenna system  100 . However, the location of the azimuthal axis forwardly of the outermost edges  202   b  of the main reflector  202  helps to eliminate a degree of the blockage produced by the mounting platform  206  and the rotary joint  212 . 
   Referring to  FIG. 7 , there is shown a conventional cassegrain reflector antenna for the purpose of illustrating the problem of increasing the depth of the antenna when the feed horn length is increased. The antenna  300  includes a main reflector  302  having a feed horn  304  mounted at a vertex  306  of the main reflector  302 . A subreflector  308  is mounted at an outermost edge  310  of the main reflector  302  that forms the aperture of the antenna  300 . An antenna electronics subassembly or subassemblies  312  may be mounted on a rear surface  314  of the main reflector  302 . The overall depth of the antenna  300  is designated by arrow  316 . 
   Referring to  FIG. 8 , when an elongated, moderate flare angle feed horn  304   a  is employed, the subreflector  308  must be moved outwardly of the main reflector  302 . The subreflector  308  is typically held by two or more struts  318  so as to be concentric with the vertex  306  of the main reflector  302 . The overall depth of the antenna  300  is represented by arrow  320 . As will be appreciated from  FIGS. 7 and 8 , the depth of the antenna  300  increases significantly when an elongated feed horn  304   a  is employed. This increases the swept arc of the antenna, which in turn necessitates a larger radome for covering the antenna when the antenna is employed on an external surface of a high speed mobile platform. The larger radome contributes to reduced aerodynamic efficiency of the mobile platform. 
   Referring to  FIG. 9 , an antenna  400  in accordance with a preferred embodiment of the present invention is illustrated. Antenna  400  includes a main reflector  402  having an elongated feed horn  404  disposed at an axial center (i.e., vertex)  406  of the main reflector  402 . A hole  408  is formed in the main reflector to allow a major portion of the length of the feed horn  404  to project outwardly from a rear surface  410  of the main reflector  402 . A subreflector  412  is disposed at the vertex  406  of the main reflector  402  and supported by one or more struts (not visible). An antenna electronics subassembly  414  may be supported on the rear surface  410  of the main reflector  402  or on a neck portion  405  of the feed horn  404 . The antenna electronics  414  may comprise an ortho mode transducer, low noise amplifiers, or other components. 
   With brief reference to  FIGS. 10 and 11 , the hole  408  in the main reflector  402  can be seen in even greater detail. The hole  408  should be of sufficient diameter to permit a desired portion, preferably about 50%, of the feed horn  404  to project therethrough. The larger the diameter of the hole  408 , the greater the portion of the feed horn  404  that will be able to project through the hole  408 . In one preferred form the feed horn comprises an overall length of about six inches (152.4 mm) and has a diameter at its forward end  404   a  of about three inches (76.2 mm). A more traditional feed horn, such as feed horn  304  in  FIG. 7 , has a diameter of about 3–5 inches (76.2 mm–127 mm) at its forward end and an overall length of about three inches. The hole  408  in the main reflector is preferably made slightly larger than what might be actually needed to permit a degree of longitudinal adjustment of the feed horn  404  relative to subreflector  412 . 
   The use of an elongated feed horn with a narrower forward end produces a more focused, near-field illumination of the subreflector  412 . In practice, the overall length of the feed horn  404  will typically be between 20%–100% greater than the length of a standard, wide angle feed horn such as feed horn  304 . 
   Referring to  FIG. 9 , arrow  416  represents the overall depth of the antenna  400 . The depth  416  is significantly less than the depth indicated by arrow  320  in  FIG. 8 , and substantially the same as the depth indicated by arrow  316  in  FIG. 7 . Thus, the overall swept volume of the antenna  400  will be less than that produced by the antenna of  FIG. 8 , and substantially the same as that produced by antenna  300  in  FIG. 7 . 
   The use of the hole  408  in the main reflector  402  thus allows an elongated feed horn  404  to be employed that even better disperses electromagnetic wave energy onto the subreflector  412 , but without incurring the penalty of increasing the overall depth of the antenna  400 . This allows the swept arc of the antenna  400  to be minimized, which contributes to maintaining aerodynamic efficiency when the antenna  400  is covered by a radome and disposed on a fast moving mobile platform. 
   Referring to  FIG. 12 , an enlarged portion of the main reflector  402  and the feed horn  404  is shown. The reflector hole  408  includes a counterbored area  408   a  which houses a flange  404   b  of the feed horn  404 . A plurality of screws  418  are used to secure the flange  404   b  in the counterbored area  408   a . The screws  418  engage in blind threaded holes  420  formed in a boss portion  422  that surrounds the vertex  406  of the main reflector  402 . One or more washers or shims can be placed over the threaded screws  418  to adjust the longitudinal positioning of the feed horn  404  relative to the subreflector  412 . 
   It will also be appreciated that both the main reflector  402  and the subreflector  412  are preferably “shaped” as needed to achieve the desired performance for the antenna  400 . The overall length of the feed horn  404 , its diameter at the forward end  404  and its spacing from the subreflector  412  are all factors that are taken into account in determining the optical shape of the main reflector  402  and the optimal shape of the subreflector  404 . 
   The preferred embodiments of the present invention thus provide a means for supporting a reflector antenna in a manner which minimizes the effective frontal area of the reflector antenna, and thus allows a radome having a smaller frontal area to be employed in covering the antenna when the antenna is located on an outer surface of an aircraft. The preferred embodiments do not significantly complicate the construction of the antenna system nor do they complicate the mounting of the antenna system on the outer surface of an aircraft. Furthermore, the preferred embodiments do not significantly add to the costs of construction of the antenna systems. 
   Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.