Electromagnetic scattering by arbitrarily shaped reflectors: Subreflector efficiency

A general expression for the electromagnetic scattering by an arbitrary shaped reflector is developed and applied in a computer model of offset hyperboloid reflectors. The computed scattering is shown to be in excellent agreement with scattering measurements made on an offset reflector of projected diameter = 24 cm, at frequencies of 10.35 GHz, 20.7 GHz and 44.5 GHz. Next, using the computer model, the efficiency of the subreflector in a dual-reflector antenna is calculated as a function of subreflector diameter and for two values of illumination taper. For subreflectors truncated at the ray-optics boundary the calculated efficiency is 0.83 and 0.91, respectively, for truncation diameter of 7.7 lambda and 30.4 lambda, with 5 dB of illumination taper; these respective efficiencies increase to 0.91 and 0.95 with 12 dB of illumination taper. However, subreflectors of diameter about two wavelengths larger than the ray-optics diameter have very nearly unit efficiency.