Reflector feeds for large adaptive reflector antennas
Mousari Bafrooei, Seyed Pedram
A novel Feed-Reflector system for large Cassegrain antennas for Radio Astronomy and Deep-Space Communication applications is investigated. This Feed-Reflector is used to illuminate a hyperboloid sub-reflector with 5-10 m diameter located 500 m above the ground. Because the sub-reflector is located in the near field of the Feed-Reflector antenna, a theory based on the near field focusing properties of paraboloid reflectors is established. The focusing at near distance is formed by moving the feed horn away from the focal point of the Feed-Reflector. In this theory the properties of axial defocused paraboloid reflectors at near distance are investigated in some detail. By using equivalence path law, sub-reflector shape is obtained. It is found that the hyperbola can approximate the sub-reflector well. A detailed ray tracing analysis is performed on the entire system which reveals that some part of the sub-reflector receive three rays per point from the feed. The performance of the system over the operating band (1-22 GHz) is also studied and shown that the lower frequency limit is dependent on sub-reflector and Feed-Reflector sizes. To obtain higher efficiencies, three sets of shaping techniques, based on the genetic algorithm and Jacobi Fourier surface expansion, are performed. An efficiency of 78.5% for a 5 m sub-reflector is obtained. In another method of analysis, the Feed-Reflector aperture field distribution is expanded into a set of Gaussian-Laguerre modes. These modes propagate from the Feed-Reflector aperture in a simple and well defined way. The Feed-Reflector near field radiation pattern is calculated at the sub-reflector location. The sub-reflector parameters in this system are found by maximizing the Large Adaptive Reflector (LAR) aperture efficiency which includes phase and taper efficiencies, and minimizing the LAR spillover loss. An exact equation for the offset LAR surface is obtained in this thesis. To scan the beam up to 60, which is one of the LAR requirements, the concept of the dual offset LAR with the Feed-Reflector is introduced. In this design the cross-polarization is eliminated by proper orientation of the sub-reflector. The parameters of the configuration are obtained by utilizing generalized Gauss-Laguerre beam modes and matrix representation of beam mode transformation factor. In this design the blockage effect due to the Feed-Reflector is totally removed.