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Shielded balanced loop antennas for electronic security systems

  • US 4,251,808 A
  • Filed: 11/15/1979
  • Issued: 02/17/1981
  • Est. Priority Date: 11/15/1979
  • Status: Expired due to Term
First Claim
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1. For use in an electronic security system having:

  • transmitter means for providing an electromagnetic field in a predetermined area at a frequency repetitively swept through a predetermined range of frequencies;

    a resonant tag circuit having at least one resonant frequency within said predetermined range of frequencies;

    receiver means for detecting the presence of said tag in said electromagnetic field and providing an alarm indication thereof, an antenna system comprising;

    a transmitting antenna coupled to the security system transmitter and a receiving antenna coupled to the security system receiver, said antennas being disposed in spaced parallel relationship and between which said resonant tag must pass for detection;

    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 each twisted loop antenna and including a shorted turn portion enclosing the periphery of the antenna and being grounded to provide a grounded shorted turn, and a crossover shield portion enclosing the crossed conductors of each pair of twisted loops, each cross-over shield portion being insulated from the shorted turn portion to prevent current flow in the crossover shield portion.

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