Combined radio and magnetic energy responsive surveillance marker and system
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United States of America Patent
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-
Nov 1, 1983
Grant Date -
N/A
app pub date -
Sep 4, 1981
filing date -
Sep 4, 1981
priority date (Note) -
Expired
status (Latency Note)
Importance
Abstract
A surveillance marker has a semiconductor connected between arms of a dipole antenna, one arm being of non-ferrous conductive material and the other of high permeability low coercivity material such as permalloy. Higher coercivity magnetizable pieces are disposed adjacent the low coercivity arm, and all is sandwiched between paper or other insulating layers. The marker is detected by a combination of RF field and low frequency magnetic field generating and receiving units that energize a master alarm only when both units simultaneously detect the marker signals. Magnetizing the higher coercivity pieces suppresses a signal that would otherwise be reradiated by the marker from the high permeability arm thereby rendering the marker undetectable.
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