Effect of phase centre location on performance of an EHF personal communication offset reflector antenna

Recent advances in the EHF personal communications satellite industry requires highly efficient antennas with smaller aperture size of the order of 30 to 50 cm. The antenna assembly is constrained to fit in a portable briefcase and should receive/transmit orthogonal linear polarizations at 20/30 GHz, with bandwidth of 1 GHz, respectively. The antenna cross polarization must be of the order of -30 dB and its radiation patterns should meet the FCC recommendation envelope of 32-25 log /spl theta/. An offset reflector antenna is selected for the ground terminal which meets both the mechanical and electrical specifications. This configuration is selected to avoid the feed blockage. However, in offset reflector antennas the zero cross-polarization occurs only in the plane of symmetry, while in other planes low cross-polarization can in general be achieved only for large F/D ratios. For the selected offset configuration the antenna has the F/D ratio of 1.05 and its cross polarization is less than -30 dB in the plane of asymmetry. This paper presents the result on the performance of two designed feeds, namely the corrugated conical horn and the profile feed. These feeds are often used to improve the aperture efficiency and polarization characteristics. These horns are designed for an excellent pattern symmetry, low cross polarization and spillover. In practice, the phase centre of the feed may not coincide with the focal point of the reflector. It is therefore, useful to investigate the sensitivity of the reflector phase and gain efficiencies to their separation distance.<>