Electronic surveillance of precisely defined control zone - using HF generator projecting wave to establish first field and FM generator for second field

摘要

The h. f 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 re-radiation of a signal comprised of the FM and the h. f signals which, in turn, activates an alarm circuit.