A high frequency generator projects an electronic wave into a control zone under surveillance to establish a first field. A frequency modulated (FM) generator is used for establishing a second field throughout the zone. A third field of the same FM carrier frequency, but with a different modulation, is established only at extreme ends of the control zone to define the limits of the control zone. Presence only within the control zone of a transponder causes the reradiation of a signal comprised of the FM signal and the high frequency signal, which in turn activates an alarm circuit.