Investigations on Low Cross Polarized Antenna Systems at Microwave Frequencies

Abstract

In view of the widespread applications of the offset parabolic reflector antennas in satellite communications, microwave radiometers, radio astronomy, monopulse tracking radar, etc., the research work described in the thesis is mainly focused on improving the performance of offset parabolic reflector antennas. Although, the offset parabolic reflector configuration offers significant advantages as compared to the front-fed parabolic reflector antenna, it suffers from two serious drawbacks. Due to the structural asymmetry, when illuminated by a linearly polarized primary feed, it generates high cross-polarization. While in case of a circularly polarized feed, it results in beam squinting. The presence of high cross-polarization in the antenna radiation patterns implies the loss of energy in the undesired polarization, which ultimately results into reduction of the antenna efficiency. High degree of cross-polarization degrades the performance of the communication channel and can cause measurement errors in case of remote-sensing applications. In monopulse tracking radars, the high cross-polarization creates boresight-jitter (boresight uncertainty), which severely affects the tracking accuracies. Considering these undesirable effects of high cross-polarization, it becomes necessary to develop a suitable technique to suppress the unwanted cross-polarization of the offset parabolic reflector antenna. In the present thesis, the high cross-polarization of the offset parabolic reflector has been suppressed by using a multi-mode matched feed. The concept of matched feed is thoroughly described at the beginning of the thesis. The multi-mode matched feed has been realized in three different waveguide structures, i.e., rectangular, cylindrical and corrugated structure. The detailed designs of all the three matched feed viii structures have been presented in the thesis. Through extensive numerical as well as experimental results, it has been verified that the proposed matched feeds effectively suppress the undesired high cross-polarization of the offset parabolic reflector antenna. The feeds also remove the beam squinting in the secondary radiation pattern of the reflector, when circular polarization is employed. Finally, a few practical applications of the proposed feeds are discussed and the corresponding results are summarized in the thesis.