Application of Gaussian beam in analysis of 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 with 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 LAR 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. In the proposed method of analysis the feed-reflector aperture field distribution is expanded into a set of Gaussian-Laguerre modes. Almost 99% of the power is carried in the fundamental Gaussian mode. These modes propagate from the feed-reflector aperture in a simple and well defined way. The feed-reflector near field radiation pattern is calculated at the sub-reflector location. The sub-reflector parameters in this system is found by maximizing the LAR aperture efficiency which includes phase and taper efficiencies, and minimizing the LAR spillover loss. This process is computationally more efficient than the physical optics current distribution method, and more accurate than the ray tracing approach.