Abstract: In order to isolate microwaves of a selected polarization from incident microwaves collected by means of a dish reflector (10), a ferrite polarizer (16) is positioned at the focal point of the reflector. A ferrite body (26) of the polarizer extends axially of a wave guide (18) with a convergent receiving end (28) positioned outside the wave guide at the focal point and a convergent transmitting end (30) positioned within the wave guide. The ferrite body is mounted in an end portion of the wave guide by means of a plastics dielectric bobbin (32). The bobbin encases all but the transmitting end of the ferrite body, which projects beyond the bobbin into the wave guide. A controlling coil (36) of the polarizer, which creates an axial magnetic field in relation to the ferrite body, is wound around the bobbin within the end portion of the wave guide.