Abstract: A microwave transmitter and receiver are placed at a distance from each other to define between them a survey area in which the field of transmission is modified by reflection and/or absorption by objects in the survey area. A modulation generator is used for controlling modulation in the transmitter and for controlling the cycling period of a commutative filter in the receiver in accordance with the modulation period, thereby automatically tuning the receiver to the transmitter. The receiver further comprises an alarm unit which analyses the signal admitted through the commutative filter and strikes an alarm in response to such changes of that signal as are caused by the presence of moving objects in the survey area.