Abstract: A dual mode circularly polarized horn radiator is useful in the testing of radiation apparatus in an anechoic chamber. The horn has a buffered radiating aperture which permits a plurality of the horns to be arranged in an array with minimal mutual coupling between the horns. The radiating aperture is located at an end of a conic section, of low flare angle, and has a diameter of one free-space wavelength of the radiation to be radiated from or received at the horn. This permits use of the horn for measurement of near field radiation pattern of apparatus under test. The horn includes an orthomode tee having mutually perpendicular rectangular waveguides extending therefrom, and being coupled thereby to a circular port. A polarizer connects the circular port of the tee to the conic section, and includes two sets of diametrically opposed pins for conversion of both right-hand and left-hand circular polarizations to linearly polarized radiation.