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Article surveillance

  • US 4,063,229 A
  • Filed: 06/28/1971
  • Issued: 12/13/1977
  • Est. Priority Date: 03/30/1967
  • Status: Expired due to Term
First Claim
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1. An article surveillance system comprising transmitting means to establish an electromagnetic wave field within a surveillance zone, passive sensor-emitter means for application to an article susceptible of relative movement into said surveillance zone, said sensor-emitter means comprising a two-terminal nonlinear impedance element conductively connected directly to antenna means supported by a structure adapted to be secured to said article under surveillance, said nonlinear impedance element cooperating with said antenna means when in said zone to reradiate at least a portion of the energy in said wave field in the form of a signal which is different and distinguishable from any signal otherwise present in said wave field, and receiving means to detect said reradiated signal to the exclusion of any signals produced directly by said transmitting means.

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