Aperture blockage by a subreflector in a Cassegrain antenna system

The problem of aperture blockage by a subreflector in a Cassegrainian antenna system is discussed. The Lorentz reciprocity theorem is used to derive an expression for the far field radiated by the Cassegrain system. It is suggested that the field scattered by the subreflector can be computed using Keller´s geometrical theory of diffraction.