G - Physics – 08 – B
Patent
G - Physics
08
B
340/124
G08B 13/22 (2006.01)
Patent
CA 1084137
ABSTRACT OF THE DISCLOSURE 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.
287373
Johnson Douglas S. Q.c.
Williamson Robert D.
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