Optimum Paraboloid Aerial Design

摘要

A computer study was made of the spillover, cross polarization and illumination efficiency of paraboloids of various F/D ratios (focal length/paraboloid diameter) fed by rectangular, square and circular waveguides in their lowest order modes. The feed radiation patterns used were those derived by simple diffraction theory, from the incident waveguide-mode field distribution. The approximations made in this derivation give unnaturally low spillover for very small aperture feeds and thus favour short F/D systems which would use such feeds. The feed dimensions were optimised to give maximum merit (gain/total noise temperature) for each combination of feed, receiver noise temperature and F/D ratio. The optimum horn dimensions and resulting illumination tapers, excess aerial noise temperatures, gain efficiencies and merit, produced by the computations are presented. The results show amongst other things that the merit of a low F/D system is about 0.8 dB less than that of a large F/D system. They also show that, if the system has a low receiver temperature, the feed dimensions should be larger than conventional design procedures indicate, to give up to 3 dB more edge illumination taper. (Author)