Factorization of Antenna Efficiency of Aperture-type antenna: Beam Coupling and Two Spillovers

Antenna efficiency is one of the most important figures-of-merit of a radio telescope, even for a multibeam radio telescope. To analyze a system where each beam illuminates only a part of the aperture, a lossless antenna consisting of two apertures in series is considered in the frame of the scalar wave approximation. We found that its antenna efficiency is factorized into three factors: efficiencies of beam coupling, transmission spillover, and reception spillover. We numerically confirmed our factorization with an optical simulation. The factorization is applicable to general aperture-type antennas and is useful in design of multibeam radio telescopes.