PURPOSE: To facilitate the acquisition of a satellite by providing a driver dividing radially a sub-reflection mirror face and folding an mirror face at the peripheral part so as to change the half-amplitude level of a secondary radiation pattern while keeping a blowing pattern to the sub reflection mirror the same.

CONSTITUTION: A main reflection mirror 1, sub reflection mirrors 2a, 2b divided radially and a driver 4 to fold the peripheral are provided. The sub reflection mirror face is divided to relect only a part of the radiation pattern irradiated to the sub reflection mirror and to reduce the effective aperture area of the main reflection mirror thereby changing the beam half-amplitude level of the secondary radiation pattern.