Cancellation of Polarization Rotation in an Offset Paraboloid by a Polarization Grid

Current interest in frequency reuse through orthogonal polarizations creates a strong incentive for improving polarization properties of antennas. Cross-polarized radiation from an offset reflector 1 is often regarded as a blemish on an otherwise excellent antenna, which offers both low sidelobe level and good impedance matching. 2 Although the cross polarization can be minimized using a large effective f/D ratio, the corresponding requirements of small offset angle and large feed aperture are not always convenient in applications. Recently a trimode feed horn 3 has been discussed as a means of reducing the cross-polarized radiation of offset reflectors; however, a multimode arrangement is inherently narrow in bandwidth. Our purpose here is to propose a broadband method for reducing the cross-polarized radiation of the offset reflector antenna. This scheme is based upon cancellation of polarization rotation due to reflector curvature 1 by opposite rotation generated by a polarization grid.4 In Section II we examine the similarity between cross polarization in the aperture of an offset reflector and that of a polarization grid. The condition of cancelling the first-order cross-polarization terms is then 977