Minimization of Depolarized Currents of Offset Reflectors Using Multimode Pyramidal Horns PDF

Keywords

offset parabolic reflector, depolarization. horn feed

Abstract

The current distribution on the surface of an offset parabolic reflector antenna is analyzed for multimode pyramidal horn feeds. The diagonal horn feed, which gives good symmetric current distribution on the reflector surface, yields high levels of depolarized fields. These are in the order of -16 dB, and form four quadrants on the surface, alternating between positive and negative polarity, around the reflector center. Adequate compensation of this depolarization is not easy. For example, the use of large percentage mix from the TM21 mode (for vertical polarization), could only reduce these depolarization current levels to -26 dB. The depolarized currents are minimized, however, by selection of the mixture ratio of specific modes in regular pyramidal horns. Depolarization levels below -40 dB (from the main polarization level) are achieved for vertical polarization (which is parallel to the offset axis of symmetry), using the fundamental TE10 mode, with small percentage mix of the TE11 mode. For horizontal polarization, the small percentage mix needed is from the TE20 mode. In this situation the depolarization level is reduced to only -34 dB. (noting that the depolarization level of the fundamental mode, alone, is -26 dB)