GB1017240A - Improvements in or relating to radio frequency antenna systems - Google Patents

Abstract

1,017,240. Aerials. WESTERN ELECTRIC CO. Inc. Sept. 26, 1962 [Oct. 5, 1961], No. 36677/62. Heading H4A. A Cassagrainian aerial system comprises a primary feed 10 filling an aperture 25 of diameter d in a primary concave paraboloidal reflector 20 facing a secondary convex reflecting surface 40 and spaced a distance x therefrom where x<d<SP>2</SP>/ 2 # (e.g. d = 14X, x = 30#) such that the secondary reflector 40 is in the near zone of the feed 10 thereby reducing spillover at the reflectors 20 and 40. As shown, the secondary reflecting surface 40 is paraboloidal, the primary feed 10 comprises a circular waveguide 14, conical horn section 18 and concave paraboloidal section 22 producing a substantially collimated beam having a plane wavefront, the diameters of the aperture 25 and secondary reflector 40 being substantially equal; alternatively, a plane wavefront may be produced by a small paraboloidal reflector fed in a conventional manner, or a diverging wavefront may be produced by a waveguide horn, or the paraboloidal section 22 of the feed 10 may be replaced by an ellipsoidal section to produce a converging wavefront, the contour of the secondary reflecting surface 40 being suitably modified. In order to reduce aperture blocking by the secondary reflector structure 30, radiation from the primary feed 10 incident on the central portion of the secondary reflecting surface 40 passes through an aperture therein and is directed on to a concave paraboloidal reflecting surface 42 of the secondary reflecting structure 30 via a conical horn 32, circular waveguide section 34 containing a dielectric phasing slab 38 and a reflecting cup 36 located at the focus 28 of the primary reflector 20. Suitable valves for the diameters of the primary and secondary reflectors 20 and 40 are 100# and 15# respectively. The aerial may be used for transmission to or reception from a communication satellite.