Abstract: An antenna is constructed of an array of contiguous circular cylindrical radiators each of which extends forwardly of a radiator assembly producing two circularly polarized waves of opposite direction of rotation of their respective electric fields. The radiators measure one wavelength at the transmit frequency band, and approximately 1.5 wavelengths in diameter at the receive frequency band. A section of cylindrical waveguide in the back of each radiator assembly encloses a microwave structure for generating the circularly polarized waves, the microwave structure including an orthomode transducer at the back of the assembly and an electric field rotator disposed forward of the orthomode transducer. In each radiator assembly, there is disposed between the rotator and the radiator a transition between smaller diameter waveguide to larger diameter waveguide. The transition may have the form of a step or a flare for a more gradual transition. The transducer produces a higher order TM.sub.