Antenna system for radiating multiple planar beams
United States Patent Hannan 1 Apr. 29,1975  
 1 1 ANTENNA SYSTEM FOR RADIATING 3.267.472 8/1966 Fink 343/854 MULTPLE PLANAR BEAMS 3.317.912 5/1967 Kclleher 343/840 3.341.151 9/1967 Kampinsky 343/840 [751 Inv nt Peter anna Smnhtown. 3.406.401 10/1968 Tillotson 343/100 3.414.904 12/1968 Ajioka 343/837 [73] Ass1gnee. gaeltlne Corporation. Greenldwm 3.568.184 3/1971 Drabowitch 343/754 3.775.769 11/1973 Hecren et a1. 343/854 [221 Filed: Apr. 3. 1973 1 l N0: PI&#39;I-IULU&#39;) EXIUTHVICI&#39;QEH Lieberman 1521 us. (:1 343/779; 343/840 1571 ABSTRACT it. CL Disclosed are antenna systems for imultaneously radi- [58] Fle&#39;d of Search 1 343/754. 779. 840. 776. aflng a plurality 0f planar beams into a region Of 3 3/ 7. 778 space. One such system includes a cylindrical reflector with a plurality of feed elements located near the focal 1 1 References Clted axis of the reflector. Each of the feed elements illumi- UNITED STATES PATENTS nates the reflector with a wave energy pattern. causing 2.437.231 3/1948 Tawncv 343 777 P beam be radiated in direction which is 2.663.016 12/1953 Coligney 343/777 unique to that element- 3 ()16.53l 1/1962 Tomiyasu et a1. 343/779 3.170.158 2 1965 Rotmun 343/754 4 Clam, 4 D&#39;awmg PATENIEDAPR29|975 SHEET 10F 2 FIG. 2  
 PATENTEUmesivS SHEET 2 BF 2 FIG. 3  
 FIG. 4  
 ANTENNA SYSTEM FOR RADIATING MULTIPLE PLANAR BEAMS CROSS REFERENCE TO RELATED APPLICATIONS The present invention relates to a multiple beam antenna suitable for use in an antenna system for radiating a Doppler coded pattern such as described in copending application Ser. No. 347.506. filed Apr. 3. 1973. entitled Antenna System For Radiating Doppler Coded Pattern Using Multiple Beam Antenna and assigned to the same assignee as the present application.  
 BACKGROUND OF THE INVENTION This invention relates to antennas for use in direction finding systems. In particular it relates to antenna systems capable of radiating multiple simultaneous planar beams.  
  Direction finding systems generally measure the angular components of a direction vector to a target in either planar or conical coordinates; for example, it is usual to specify azimuth direction of a target in planar coordinates and elevation angle in conical coordinates. When it is desired to use either planar or conical coordinates in a direction finding system which utilizes directional antennas. it is desirable that the antenna beam shape correspond to the desired coordinate system. This feature avoids the need for a coordinate transformation. Modern direction finding systems require antennas which have very rapid beam motion or can operate with many simultaneous beams. These features enable rapid movement of the beam between targets. or enable the antenna to have simultaneous beams on more than one target.  
  A well known antenna system for locating targets in a planar coordinate system is a mechanically rotated fan beam&#34; antenna commonly used for search radars and other applications. This antenna type may take the form of a specially shaped reflector and illuminating feed or a linear array of antenna elements with wave signals of equal phase coupled to each element. Either of these antennas radiates a single planar fan beam which is moved through space by mechanically rotating the antenna.  
  Prior antenna systems with multiple fan beams include linear arrays of antenna elements which achieve these beams by changing the phase of the energy coupled to each of the elements. Another multiple fan beam system is a paraboloid reflector with a large aspect ratio and multiple feeds. Both of these systems provide conical beams.  
  A circular array of antenna elements is capable of providing multiple planar fan beams. This antenna system requires. however, a large number of antenna elements and many complex circuit devices to achieve accurate direction finding. Another disadvantage is that the circular array must usually have a diameter larger than the useful aperture.  
  R. N. Assaly and L. .I. Ricardi. in an article entitled A Theoretical Study Of A Multi-Element Scanning Feed System For A Parabolic Cylinder. IEEE Transactions on Antennas and Propagation. Vol. AP-l4. No. 5. Sept. 1966. have considered the use of a cylindrically shaped reflector with multiple feeds for use in radiating multiple beams. However. in this article they assume a feed and reflector of infinite extent in the direction of the focal axis and they perform an analysis in two dimensions which does not distinguish between planar and conical beams. nor does it reveal the beam shapes available from a practical antenna operating in three dimensional space.  
 SUMMARY OF THE INVENTION An object of the present invention. therefore, is to provide an antenna system for simultaneously radiating a plurality of planar beams into an assigned region of space.  
  A further object of the present invention is to provide such an antenna system which will radiate focused wave energy over a desired radiation angle for each planar beam.  
  A still further object of the present invention is to provide such an antenna system in a simple configuration and at a low cost.  
  In accordance with the present invention. there is provided an antenna system for radiating a plurality of planar beams and having a desired radiation angle in the plane of each beam. The antenna system includes a cylindrical reflector for converting an incident spherical wave from a point source into a cylindrical wave. the reflector having an axial length which subtends an angle. viewed from a point on the focal axis of the reflector which is substantially equal to the desired radiation angle. Further included are a number of feed elements. equal to the number of planar beams desired and each oriented to efficiently illuminate the reflector with wave energy patterns. The feed elements are located in the vicinity of the point on the focal axis and have in a plane which is perpendicular to the focal axis a component of angle displacement from each other. viewed from the center of the reflector. When the reflector is illuminated by the pattern of any of the feed elements a corresponding planar beam will be radiated in a direction which is unique to that beam when observed in a plane perpendicular to the focal axis. and having the desired radiation angle in the plane of that beam.  
  For a better understanding of the present invention. together with other and further objects thereof. reference is had to the following description taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is one embodiment of an antenna system constructed in accordance with the present invention.  
  FIG. 2 illustrates. the planar beams radiated by the FIG. 1 antenna.  
  FIG. 3 and 4 are alternative embodiments of the present invention.  
 DESCRIPTION AND OPERATION OF THE FIG. 1 ANTENNA SYSTEM The antenna system of FIG. 1 includes focusing means for converting an incident spherical wave from a point source into acylindrical wave, namely, a parabolic cylindrical reflector l0. and means for illuminating the focusing means with wave energy patterns which in this embodiment comprises the three feedhorns 11a. b. c. The reflector l0 has a focal axis 12 and an axial length 13 such that the axial length 13 subtends an angle 14 when viewed from a point IS on the focal axis 12. The feedhorns II in the FIG. 1 embodiment are located in the vicinity of the point 15 on the focal axis 12 and have a component of angular displacement 16 from each other in a plane 17, which is perpendicular to the focal axis 12, when viewed from the center 18 of the reflector 10.  
  The height 19 of the reflector l and the&#39;distance to the focal axis 12 are chosen in accordance with standard principles well known to those skilled in the antenna art, and on the basis of desired beamwidth in the plane 17 perpendicular to the focal axis 12 and the characteristic patterns of the feedhorns II. The width 13 of the reflector is chosen such that angle 14 is substantially equal to the desired radiation angle of the planar beams to be radiated as measured in the plane of each beam.  
  The number of feedhorns used in an antenna system such as that of FIG. I is chosen to be equal to the number of planar beams desired. Feedhorns 11 may be of any suitable type that will radiate wave energy patterns which efficiently illuminate the reflector l0. Feed elements other than feedhorns 11 may very well be used as is evident to one skilled in the art. Other types of elements which may be used are dipoles and spirals.  
  It is evident that the wave energy patterns from each of the feedhorns have a radiation phase center at a different point in space. The radiation phase center is the point in space from which spherically diverging waves appear to originate. Other means for illuminating the reflector 10 with the necessary plurality of wave energy patterns are possible. One such means is the apparatus described in U.S. Pat. No. 3,710,388, which is an array of elements having the capability of radiating wave energy patterns from phase centers displaced in space.  
  In the FIG. 1 embodiment, the feedhorns 11 are located substantially in a plane 17 which passes through the center of the reflector l0 and is perpendicular to the focal axis 12. Alternatively, the feed locations may be displaced to one side of center. This would be likely in a system wherein asymmetrical coverage is required. It is also not required that the feed elements be located in plane 17, so long as they have the required component of angular displacement 16 in the plane 17.  
  The operation of the FIG. 1 antenna is illustrated in FIG. 2. When the reflector 19 is illuminated by the wave energy pattern of each of the feedhorns 11, a corresponding planar beam 20 is radiated in a direction which is unique to that feedhorn when measured in a plane 17 perpendicular to the focal axis 12. Each of the planar beams 20a, 12, c so radiated has a desired radiation angle 21 in the plane of that beam corresponding to the angle 14 subtended by the axial length 13 of the reflector 10 as shown in FIG. 1. These planar beams 20 radiated by the antenna system of FIG. 1 are suitable for use in a direction finding system using planar coordinates.  
  In accordance with other embodiments of the present invention, the reflector 10 in FIG. 1 may be replaced with any one of many devices that are capable of converting an incident spherical wave from a point source into a cylindrical wave, including transmissive lenses and arrays. Such focusing means have a substantial focusing effect on incident wave energy in one plane which includes the direction of propagation and no substantial focusing effect in a perpendicular plane which also includes the direction of propagation. The plane in which focusing occurs is perpendicular to the focal axis of the focusing means. Hence, spherically diverging waves originating from a point source in the vicinity of the focal axis are converted by such focusing means to cylindrically diverging waves as would originate from a line source of radiation.  
  The parabolic reflector 10 in the FIG. 1 embodiment has only one focal axis 12. A transmissive focusing means for converting an incident spherical wave from a point source into a cylindrical wave, such as a dielectric lens, has two focal axes, one on each side of the lens. A circularly symmetrical focusing means such as a circular cylindrical reflector or a cylindrical Luneberg lens has an infinite number of focal axes by reason of its circular symmetry.  
  In a case where a focusing means has more than one focal axis, a point on any focal axis may be used as a reference for locating the necessary illuminating means. The required axial length of such a focusing means at the surface from which wave energy would be radiated. if originating on the selected focal axis, must subtend the desired radiation angle viewed from the point on the selected focal axis.  
 DESCRIPTION OF THE FIGS. 3 AND 4 ANTENNA SYSTEMS FIG. 3 is an embodiment of the present invention wherein the focusing means is transmissive and is a cylindrical Luneberg lens 22. The most commonly known Luneberg lens is a spherical structure of material whose propagation velocity is a function of the radius at which the material is located, such that incident plane wave energy is focused to a point on the surface of the sphere diametrically opposite the point corresponding to the direction of incidence. The cylindrical Luneberg lens 22 causes incident plane wave energy to be focused to an axial line on the surface diametrically opposite the axial line corresponding to the direction of incidence. The cylindrical Luneberg lens 22 is therefore capable of focusing incident wave energy with respect to an infinite number of focal axes due to its circular symmetry.  
  In the FIG. 3 embodiment the feedhorns llu, b, c are located on the surface of the lens 22 in the vicinity of a particular focal axis 12&#39;. The feedhorns 11 have an angular displacement component 16&#39; from each other in a plane 17, which is perpendicular to the focal axis 12, viewed from the center 18 of the lens 22. The lens 22 has an axial length 13&#39; which on the surface diametrically opposite the focal axis 12 subtends an angle substantially equal to the desired radiation angle, viewed from a point on the focal axis 12&#39; in the vicinity of the feedhorns 11.  
  FIG. 4 is another embodiment of the present invention wherein the focusing means is a transmissive array structure 26. The array structure 26 includes a first array of receiving elements 23 for receiving the wave energy patterns from the feedhorns 11. The structure 26 also includes means 24 for shifting the phase of the wave energy received by each of the elements 23 of the first array and a second array of elements 25 for radiating the phase shifted wave energy. The phase shifting means 24 provide an appropriate predetermined amount of phase shift for each of the elements 23, such that the structure 22 performs the function of converting an incident spherical wave from a point source into a cylindrical wave. The phase shifting means 24 may be any of the commonly used devices such as variable lengths of transmission line or transmission line with variable amounts of dielectric loading. The array elements 23 and may be any of those commonly used in the art, such as dipoles, horns or spirals.  
  It will be evident to those skilled in the art that the phase shifting means 24 used in the HO. 4 embodiment may be reflective phase shifters which cause energy to re-radiate from the first array of elements 23, in which case the second array of elements 25 is not required. The result is a reflective focusing means comprising an array of reflective elements.  
  In describing the various embodiments above, reference has been made to transmitting antennas. but it will be recognized by those skilled in the art that the principles of the present invention can also be applied to receiving antennas. Accordingly, the appended claims shall be construed as covering both transmitting and receiving antenna systems regardless of the descriptive terms actually used therein.  
  An antenna constructed in accordance with the present invention can be used as a scanned beam antenna system as well as a multiple beam antenna system. The scanned beam antenna is achieved by causing motion of the phase center of the illuminating means or by sequentially coupling each of the multiple feeds to the transmitter or receiver.  
  There are design criteria applicable to any antenna which may also be applied to antenna systems constructed in accordance with the present invention, as will be apparent to one skilled in the art. Various tradeoffs in relation to aperture size and shape, and feed type and location may be made in designing antenna systems embodying the invention described herein. These tradeoffs will be affected by the angular extent of the region of space which is to be illuminated by the antenna system, the desired polarization. the number of beams required and the orientation of the antenna with respect to those beams.  
  While there have been described what are at present considered to be the preferred embodiments of this in vention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention and it is therefore aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.  
 What is claimed is:  
  1. An antenna system for radiating a plurality of planar beams and having a desired radiation angle in the plane of each beam, comprising:  
 a cylindrical reflector for converting an incident spherical wave from a point source into a cylindrical wave, said reflector having an axial length which subtends an angle, viewed from a point on the focal axis of said reflector, which is substantially equal to said desired radiation angle;  
 a number of feed elements, equal to the number of planar beams desired and each oriented to efficiently illuminate said reflector with a wave energy pattern, said feed elements being located in the vicinity of said point on said focal axis and having in a plane which is perpendicular to said focal axis a component of angular displacement from each other, viewed from the center of said reflector;  
 whereby, when said reflector is illuminated by the pattern of any one of said feed elements, a corresponding planar beam will be radiated in a direction which is unique to that beam when observed in a plane perpendicular to said focal axis, and having said desired radiation angle in the plane of that beam.  
 2. An antenna system as specified in claim 1 in which said reflecting means is a parabolic cylindrical surface.  
 3. An antenna system as specified in claim 1 in which said reflecting means is a circular cylindrical surface.  
 4. An antenna system for radiating a plurality of planar beams and having a desired radiation angle in the plane of each beam comprising:  
 a parabolic cylindrical reflector for converting an incident spherical wave from a point source into a cylindrical wave, and having an axial length which subtends an angle when viewed from a point on the focal axis of said parabolic reflector, which is sub stantially equal to said desired radiation angle;  
 a number of waveguide feedhorns, equal to the number of planar beams desired and each oriented to efficiently illuminate said reflector with wave energy patterns, said feedhorns being located in the vicinity of said point on said focal axis and having in a plane which is perpendicular to said focal axis substantially equal components of angular displacement from each other, viewed from the center of said reflector;  
 whereby when said reflector is illuminated by the pattern of any one of said feedhorns, a corresponding planar beam will be radiated in a direction which is unique to that beam, when observed in a plane perpendicular to said focal axis, and having said desired radiation angle in the plane of that beam.