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Theft detection apparatus using saturable magnetic targets

  • US 4,384,281 A
  • Filed: 10/31/1980
  • Issued: 05/17/1983
  • Est. Priority Date: 10/31/1980
  • Status: Expired due to Term
First Claim
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1. Article theft detection apparatus for signalling the passage of protected articles from a protected area through an interrogation zone, said articles having targets mounted on them in the form of thin elongated strips of easily saturable magnetic material which, when subjected to an alternating magnetic interrogation field in said zone, become repeatedly saturated and produce alternating magnetic response fields at frequencies which are harmonics of the frequency of said interrogation field, said article theft detection apparatus comprising transmitter and receiving antennas positioned across from each other on opposite sides of said interrogation zone, said transmitter antenna being formed of a single turn loop of a high electrical conductivity material, a transmitter connected to drive alternating electrical current in said loop to cause it to generate said alternating magnetic interrogation field in said zone, said receiver antenna being formed of a single turn rectangular outer loop of high electrical conductivity rigid metal pipe having its longer dimension vertical and bisected by a length of high electrical conductivity rigid metal pipe extending vertically along the center of said rectangular loop to form two bucking loops, a receiver connected to receive and detect altrnating currents at a selected one of said harmonic frequencies which flow more heavily in one bucking loop than the other and an alarm connected to operate in response to the reception and detection of said alternating currents by said receiver.

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