Event detection device with reduced blind zone

摘要

An event detection device such as an intruder monitor comprises a microwave transmitting antenna 10 and receiver, the antenna emitting microwave radiation at a defined frequency and the receiver measuring the Doppler shift in reflected radiation. The device has a blocking strip or patch of conducting or microwave absorbing material 30 which distorts the microwave beam pattern to enhance the coverage area of the device in a desired direction. The strip may be metallic and act as a parasitic antenna radiating a field which in combination with the main field provides the desired coverage. Alternatively, beam pattern shaping is provided by a plurality of microwave emitting elements forming a phased array. The device may be used in combination with a passive infra-red sensor.