The results of the design and testing of circular polarization splitters (polarizers) of the Ka-, X-, and S-bands, intended for operation as constituent parts of a three-band feed of a radio telescope reflector antenna, are presented. In addition, the results of developing a set of polarizers intended for connecting to waveguide outputs of band-measuring horn antennas are presented. These polarizers are developed to measure the antenna feed patterns on the co- and cross circular polarizations.