Electromagnetic Scattering by Arbitrarily Shaped Reflectors: Subreflector Efficiency.
MASSACHUSETTS INST OF TECH LEXINGTON LINCOLN LAB
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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 GH.z. 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 efficiences 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.
- Radiofrequency Wave Propagation