AbstractWhen a multibeam antenna uses a reflector antenna, the effect of blocking for a large feeder system cannot be neglected. An offset type becomes advantageous for such a case. However, the cross‐polarized waves increase due to asymmetry of the reflector. We considered an antenna in which a grid plate is inserted between the primary radiator and the reflector. We can suppress the components of cross‐polarized waves by choosing the position of the grid plate and by designing strip patterns. In this paper, we performed a full‐wave analysis for such an antenna. We clarified the cross‐polarization suppression effects of grids during beam scanning. It provides guidance in designing a multibeam antenna. We showed a design method using geometric optics for strip patterns according to scanning directions. We confirmed the effectiveness of this design method.