Aberration correcting subreflectors for toroidal reflector antennas

• US 3,922,682 A
• Filed: 05/31/1974
• Issued: 11/25/1975
• Est. Priority Date: 05/31/1974
• Status: Expired due to Term

• Current Assignee: Geoffrey Hyde
• Original Assignee: Geoffrey Hyde
• Inventors: Hyde, Geoffrey
• Primary Examiner(s): Lieberman, Eli
• Application Number:
  US05/475,226
• Time in Patent Office: 543 Days
• Field of Search: 343/781,837,840,761,779
• US Class Current: 343/761
• CPC Class Codes: H01Q 19/19 comprising one main concave reflecting surface associated with an auxiliary reflecting surface

Abstract

The correction of aberration in toroidal reflector antennas by a novel type of subreflector is disclosed. The specific shape of the subreflector ultimately depends on the geometry of the toroidal reflector. However, in any case, the effect of the subreflector is to achieve a point focus in a system which, without the subreflector, does not focus at a point. Considering the antenna system from a radiation point of view, this is equivalent to turning a non-planar equiphase surface in the aperture into a plane thereby eliminating the phase error about the aperture plane perpendicular to the desired direction of propagation. This is achieved while preserving the wide field of view characteristic of the torus antenna by designing the subreflector so that all pathlengths from a reference plane are constant and equal to a desired reference pathlength. In a practical case, the design of the subreflector is accomplished by developing a heuristic geometric optics model of the focusing properties of the toroidal reflector and using a programmable general purpose digital computer to generate the subreflector shape by numerically computing points on the surface of the subreflector for separate, individual rays intercepted by the toroidal reflector, for a bundle of rays incident from the desired direction. These points may then be used to machine the subreflector surface using well-known numerically controlled milling machines. Of special significance is an optimum antenna configuration using what may be termed a '"'"''"'"''"'"''"'"'Cassegorian'"'"''"'"''"'"''"'"' subreflector designed according to the principles of the invention having similarities to both Cassegrain and Gregorian subreflectors.

Claims

1. In a toroidal reflector antenna system including a main reflector having the shape of a surface section of a toruS of revolution and a feed-horn assembly positioned to illuminate said main reflector and thereby form beams in the desired directions of propogation, the improvement comprising an aberration correcting subreflector interposed between said main reflector and said feed-horn and forming a feed assembly with said feedhorn, the surface of said subreflector being nonconcentric with said main reflector and so designed that for the aberration correcting surface of said subreflector only one ray is incident on the surface at each point thereon, substantially all rays focus at a single point at said feed-horn and substantially all ray pathlengths from a reference aperture plane to said single point of focus are constant and equal to a predetermined reference pathlength, whereby said system is free of aberration and the efficiency of said system is independent of frequency, said feed assembly further being movable along an arc about the axis of revolution of said main reflector to provide substantially aberration free beams in scanning.

2. The improved antenna system as recited in claim 1 wherein said torus of revolution has a generating curve which is a conic section.

3. The improved antenna system as recited in claim 2 wherein said generating curve is a parabola.

4. The improved antenna system as recited in claim 3 wherein said main reflector is a surface section of a nonrectangular torus, said antenna system having a conical scan capability to scan along the geostationary arc.

5. The improved antenna system as recited in claim 4 wherein the angle between the axis of revolution of said torus and direction of said beam is 95.5*.

6. The improved antenna system as recited in claim 5 wherein said main reflector has the dimensional ratios f/R about 0.5 and R/D about 2 where f is the focal length of the parabola generating section, R is the radius of revolution, and D is the aperture of said main reflector measured parallel to the axis of revolution and said subreflector is hyperbolic along the axis of symmetry and departs from hyperbolic of the axis of symmetry in such a manner as to preserve the two essential properties of focusing all rays at the desired focal point with the desired pathlength.

7. The improved antenna system as recited in claim 6 wherein the dimensional ratio f/R <

0.5.

8. The improved antenna system as recited in claim 6 wherein the dimensional ratio f/R >

0.5 and the vertex of the subreflector is chosen to lie between the projection of the paraxial focus P and the focal point F and the feed point is chosen so that the subreflector formed by a circular pencil of rays incident on the aperture D approximates a circle when viewed from the feed point.

9. The improved antenna system as recited in claim 8 wherein said main reflector has the dimensional ratios of f/R about 0.56 and R/D about 2 where f is the focal length of the parabola generating section, R is the radius of revolution, and D is the length of said main reflector measured parallel to the axis of revolution, and said subreflector has a cross section which resembles an elliptic section along an axis chosen perpendicular to the axis of symmetry.