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Portable alarm device for detecting objects transgressing distance thresholds

  • US 5,367,288 A
  • Filed: 02/17/1993
  • Issued: 11/22/1994
  • Est. Priority Date: 11/15/1991
  • Status: Expired due to Fees
First Claim
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1. An alarm apparatus comprising an oscillator circuit (20) having an LC element (L1, C3) and an antenna (A) coupled to the LC element, the antenna and the oscillator circuit each having a resonance frequency, the resonance frequency of the antenna being slightly mismatched with respect to the resonance frequency of the oscillator circuit, the oscillator circuit having an oscillator current including an amplitude, the amplitude of the oscillation current being increased when electromagnetic waves emitted from the antenna and reflected by an object at a distance corresponding to a multiple of a half wavelength (λ

  • /2) of the emitted waves are received by the antenna, the increased amplitude being indicative of a transgression of a perimeter by the object, the perimeter comprising a circle having a radius corresponding to the multiple of the half wavelength of the emitted waves, the alarm apparatus also comprising a switch circuit (30) coupled to the oscillator circuit, the switch circuit being actuated by the increased amplitude of the oscillation current to provide an indication of the transgression.

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