Performance of offset large adaptive reflector Cassegrain configuration

A new type of radio telescope, large adaptive reflector (LAR), was proposed by Legg (1998) which may be used in high gain antenna applications such as deep space communication. It consists of a large and almost flat paraboloid reflector of the diameter in excess of 200 m, that is slightly adjustable in shape and made up of identical flat panels supported by actuators on the ground. A very large focal length-to-diameter ratio (2.5) imposes the unusual condition that the receiver or sub-reflector be carried by an airborne vehicle such as tethered aerostat. The LAR is in general an offset paraboloid which is intended to have a wide angle beam scanning of around /spl plusmn/60/spl deg/. A novel approach for analysing the quasi-optical offset large adaptive reflector Cassegrain system is described. In this system a feed-reflector is used to illuminate a hyperboloid sub-reflector with 5-10 m diameter, located 500 m above the ground. An exact equation for the offset LAR surface is given in this paper. To scan the beam up to 60/spl deg/ which is one of the LAR requirements, the concept of the dual offset LAR with the feed-reflector is introduced. In this design the cross-polarization is eliminated by a proper orientation of the sub-reflector. The parameters of the configuration are obtained by utilizing generalized Gauss-Laguerre beam modes and matrix representation of beam mode transformation factor. In this design the blockage effect due to the feed-reflector is totally removed.