A multimode feed for worst case single offset reflector antenna with wide cross polar bandwidth

This paper presents the design of a conjugate matched feed to get significant cross polar level reduction in secondary field radiation pattern of an offset parabolic reflector antenna system having very low F/D. This kind of feed will be advantageous in many practical situations, especially when the space and weight is concern. The conjugate feed is implemented in cylindrical waveguide and two axial corrugations with higher order modes, TE 21 and ТМ 11 in addition to fundamental TE 11 mode with proper amplitude and phase. The designed conjugate horn was then used as a primary antenna at the focus of the offset parabolic reflector antenna having focal length to diameter ratio of 0.5, with offset angel 90 deg, which is considered as a worst case. The secondary far field radiation patterns were computed with the given feed. From the results, large reduction in the cross polarization level is observed over more than 10% percentage bandwidth at center frequency, compared to that of illumination of offset reflector by conventional feed.