TEST OF A RAY-TRACE METHOD FOR FIELD CALCULATIONS IN ANTENNA SYSTEMS DESIGN

Abstract

A numerical method of beam propagation is based on decomposition of the beam into a set of geometric rays in the Gabor Representation, followed by ray-tracing. The method was developed for the optical design field, whereas here its applicability to MM-wave problems is demonstrated, as are its advantages over other methods for describing diffractive propagation in cases where the components used produce significant aberrations and/or beam clipping. Such cases arise particularly in space borne instruments, due to the limited aperture size (relative to wavelength) and other accommodation constraints. In order to validate this ray-trace method a previously published result, from a paper on the beam-mode method, is used as a test case. This involves the transformation of a gaussian beam by oblique reflection at an off-axis parabolic mirror. The results for ray-trace computation of the aberrated output beam pattern are presented and compared with those of the beam-mode method. At short wavelengths (mirror in the far-field) where the main aberration effect is from beam size variation along the mirror surface, the ray-trace method agrees well with the beam-mode method. At longer wavelengths (mirror in intermediate-field) there is an additional effect of the wavefront centre of curvature varying its position along the mirror, and the ray-trace method is shown to accurately incorporate this effect, whereas the beam-mode method does not. This feature of the ray-trace method should overcome previous restrictions whereby the only designs which could be analysed were those with components either in far-field or collimated regions of the beam.