Gaussian-beam analysis of a large adaptive reflector antenna (LAR) using a feed-reflector

A novel approach for analysing the quasi-optical 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. In the proposed method of analysis the feed-reflector aperture field distribution is expanded into a set of Gaussian-Laguerre modes. 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 subreflector location. The subreflector parameters in this system are found by maximising the LAR aperture efficiency, which includes phase and taper efficiencies, and minimising 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. It also provides a new insight into the operation of the feed-reflector system.