Method of designing circularly symmetric shaped dual reflector antenna using UTD/PO analysis and differential evolution algorithm

Abstract

A method of designing circularly symmetric shaped dual reflector antennas is presented in this thesis. The method is used to design shaped Cassegrain and shaped Gregorian reflector antennas. The two step design procedure includes a proposed method to define shaped dual reflector surfaces, and an optimization algorithm that determines the optimum shaped surfaces. The proposed method successfully describes practical shaped reflector surface using a small number of parameters compared to other commonly used methods found in literature. The reduction of design parameters reduces computational complexities. Differential evolution optimization algorithm is used to optimize the parameters that define the shaped geometry. Two separate sets of optimizations have been performed for each of the dual reflector geometries (Cassegrain and Gregorian). The first optimization concentrates on reducing beamwidth by creating uniform illumination of the main reflector while keeping main reflector size constant and spillover losses to a minimum. The second optimization concentrates on maintaining a small beamwidth by creating a uniform illumination over a smaller main reflector while keeping spillover losses to a minimum. The design requirements are incorporated into the definition of the cost function of the optimization algorithm. As the optimization requires evaluation of the radiation characteristics of the shaped dual reflectors, field analysis is inherent in the design procedure. Field characteristics of the feed, the subreflector, and the main reflector are formulated. A corrugated horn antenna is designed as the feed antenna and its field characteristics are evaluated using standard equations. Uniform theory of diffraction is used for analysis of the field scattered from the subreflector on to the main reflector. The field radiated from the main reflector is calculated using physical optics method. The beamwidth is computed for the shaped and unshaped dual reflectors from corresponding far-field patterns. It is found that the shaped reflectors outperform the unshaped reflectors in terms of beamwidth. Other performance characteristics of the designed shaped dual reflectors are also found to be within satisfactory limits. The obtained values are found to be in good agreement with numerical and experimental values reported in literature.