1,047,374. Aerials; duplex radio signalling. HUGHES AIRCRAFT CO. May 13, 1965 [June 1, 1964], No. 20241/65. Headings H4A and H4L. In a microwave aerial feed arrangement a flared receive horn contains a coaxial higherfrequency flared transmit horn which projects from its mouth. It is stated that a substantial de-coupling is provided between the transmit horn and the receive horn by this arrangement. As described, a flared pyramidal horn 10 is fed from a transmitter 20 by a waveguide 18 operating in the TE 01 mode and projects from a similar larger receive horn 12 which is connected to a waveguide 22. A receiver 34 is connected to one terminal of a stripline hybrid ring 30 and connections are taken from two other terminals thereof, between which there is a phase difference of 180 degrees, to two probes 26, 28 in the waveguide 22, which is in consequence also caused to operate in the TE 01 mode. The feed arrangement may be associated with a paraboloidal main reflector 38, Fig. 3 (not shown), when the angles of flare of the horns and their dimensions are made such that their phase centres either coincide with the focus of the reflector or, that the phase centres of the transmit and receive horns are on opposite sides of, and small distances from, the focus, with the beam angles of the horns corresponding with the angle subtended by the main reflector at the focus. The feed arrangement may comprise sectoral horns instead of pyramidal horns, when a main cylindrical parabolic reflector may be used.