Intrusion sensor and aerial therefor

An intrusion sensor comprising a microwave transmitter and associated microwave aerial;

• and a microwave receiver and associated microwave aerial for receiving a radiation transmitted by said microwave transmitter and its associated aerial, the transmitter, receiver, and associated aerials being operable at a predetermined microwave frequency, the receiver including means responsive to a variation of the received radiation from an established level to give an intruder-indicative signal, wherein the transmitter and receiver aerials each comprise a vertical array of slotted waveguide radiator elements having a vertical aperture dimensioned to provide at said predetermined frequency a beam pattern in a vertical plane having a half-power beamwidth not greater than about 2.degree., said vertical aperture being not less than 0.75 meters providing a beam pattern in a vertical plane, and said transmitter and receiver aerials are mounted adjacent the ground at opposite ends of a path to be monitored for the presence of intruders, each slot in each slotted waveguide array being offset from the longitudinal axis of the waveguide wall in which the slot is formed to provide circular polarization, each slotted waveguide array having a feed aperture substantially halfway along the waveguide for center-feeding the waveguide array, the slots located on one side of the feed aperture being disposed to one side of the longitudinal axis of the waveguide wall in which they are formed and the slots located on the other side of the feed aperture being disposed to the other side of the longitudinal axis of said waveguide wall, thereby providing in operation an intruder-sensitive zone of radiation which extends along said path contiguous to the ground substantially to each of said aerials.