Abstract: A corrugated horn radiator which includes a transition zone made up of a number of parts between the smooth-wall feed wave guide 1 having a cross-section which is constant and a horn radiator with regular periodic corrugations 10 with the depth of the corrugations being less than one-quarter wave length relative to the lowest frequency to be transmitted. The transition zone starting at the constant diameter feed wave guide comprises a first section which is conical shaped and then joins a second section which is cylindrical in shape and which serves as a phase drift space and then connects to a second conical smoothed walled portion and then connects to an exponential horn having a pair of corrugations with re-entrant ring-shaped ends (6, 7) which are narrower than the regular periodic corrugation structure 10.