Numerical analysis of large reflector system used as a ground station antenna

In this paper, we investigate a large dual-reflector (Cassegrain) antenna system, typically used as a ground station antenna in the space communication link. The study includes the blockage effects of the subreflector and the struts on the radiation performance of the antenna, as well as the estimation of the combination to the pattern degradation due to the panel gaps. The large electrical dimension of the reflector (thousands of wavelengths in diameter in the frequency range of interest) makes it impractical to analyze it via direct numerical simulation using the available computer codes. However, the problem becomes manageable if we first use the body of revolution version of the finite difference time domain code we have developed to model the axially-symmetric part of the structure viz., the feed, the reflector and the subreflector, and then modify the obtained aperture field by nulling out the fields in the regions that correspond to the projections of the gaps and the struts in the aperture. Finally, we carry out the numerical integration of the modified aperture field in order to obtain the radiation pattern. Then, by comparing the resulting pattern with the original one, we estimate the effects of the struts and the gaps on the sidelobe level and the relative gain loss. We validate the method by considering, as a test case, a simple reflector with annular ring-type gaps, and solving the problem by using both the direct and shadowing approaches. An excellent agreement is achieved between the two methods, and this serves to validate the proposed scheme