Gain ripple in small offset Gregorian antennas

This paper investigates the effect of sub-reflector diffraction on gain ripple performance in small clear aperture offset Gregorian reflector antennas. A spherical mode expansion of an analytically specified feed pattern is used to illuminate a given sub-reflector, and method of moment simulations are performed to find the resulting radiation pattern. It is found that the diffracted rays from the sub-reflector edge can interfere with the main beam to produce a sinusoidal form gain ripple for the dual reflector system. The relative amplitude of the ripple is frequency dependent, and can be predicted from the sub-reflector diffraction results. The ripple frequency can be found from the antenna geometry. The effect on the ripple of changing some of the important parameters is investigated, and it is shown that accurate near field simulations of the feed pattern illuminating the sub-reflector is required to produce accurate results.