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There have been several stages in development of the horn antennae (HA). The initial period was characterized by employing straight HA. The antennae of this type are still used nowadays. They were followed by stepped-shape horns. Another type of horn was that with corrugated walls. As compared with similar flat-wall antennae these horns should be preferred for being able to provide symmetrical circular space patterns and practically stable main-lobe width even when the operating frequency changes two-fold. Also, their beam patterns practically do not have back or side lobes. And, finally, at the next stage, the employment of horns of curved configuration came into practice.