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Method and apparatus for producing uniform electromagnetic fields in an article detection system

  • US 4,118,693 A
  • Filed: 05/09/1977
  • Issued: 10/03/1978
  • Est. Priority Date: 05/09/1977
  • Status: Expired due to Term
First Claim
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1. A system for detecting the unauthorized passage of specially targeted articles through an interrogation zone, said system comprising an interrogation antenna coil positioned in the vicinity of said interrogation zone, electrical oscillator means connected to said interrogation antenna for energizing same to generate electromagnetic waves in said interrogation zone, a receiver antenna coil also positioned in the vicinity of said interrogation zone, receiver means connected to said receiver antenna coil, said receiver means being operable to produce an alarm signal in response to predetermined electromagnetic disturbances caused by the interaction of a target on a protected article with the electromagnetic waves generated by said interrogation antenna coil, and a parasitic antenna circuit comprising a parasitic antenna coil also positioned in the vicinity of said interrogation zone at a location displaced from said interrogation antenna coil, and capacitor means connected to said parasitic antenna coil to form a resonant circuit therewith, said resonant circuit having a resonant frequency substantially the same as the frequency generated by said electrical oscillator.

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