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Shielded, closely spaced transmit-receiver antennas for electronic article surveillance system

  • US 4,509,039 A
  • Filed: 07/05/1983
  • Issued: 04/02/1985
  • Est. Priority Date: 07/05/1983
  • Status: Expired due to Term
First Claim
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1. For use in an electronic article surveillance system having transmitter means for providing an electromagnetic field in a predetermined area at at least one of a plurality of frequencies extending through a predetermined range of frequencies, a resonant marker circuit having at least one resonant frequency within said predetermined range of provided frequencies, and receiver means for detecting the presence of said marker circuit in said electromagnetic field and providing an alarm indication thereof, an antenna system comprising:

  • a transmitting antenna coupled to said transmitter means and a receiving antenna coupled to said receiver means, said antennas being disposed in a closely spaced, generally parallel relationship on one side of an exit-way along which a said marker circuit must pass for detection, said transmitting and receiving antennas each having substantially the same area and being magnetically nulled to minimize mutual inductance, at least one of said antennas having at least two twisted loops lying in a common plane, each loop being twisted 180°

    to be in phase opposition with each adjacent loop; and

    a conductive shield enclosing substantially all of both of the closely spaced antennas and including a discontinuity in each outer portion enclosing a said twisted loop to provide a discontinuous shield for both the transmit and receive antennas to prevent external electric fields from inducing current flow in either antenna which could otherwise destroy the null created therebetween and hence adversely effect the detection of said marker circuit.

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