Portable alarm device for detecting objects transgressing distance thresholds
First Claim
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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Abstract
An alarm apparatus comprises an oscillator circuit (20) which provides an oscillating current to an antenna (A) from where a location (1) to be surveyed is irradiated with electromagnetic waves of in the RF frequency range provided by the oscillator circuit (20). The oscillator and the antenna are slightly mismatched with respect to their resonance frequency so as to avoid the oscillation current to go into resonance. Reception in the antenna (A) of an in-phase reflection of the emitted wave results in a superposition of the currents of the emitted and the received waves and increases thus the RF oscillation current. This current is coupled through a capacitor (C6) to a switch circuit (30) such as to trigger an alarm through actuation of the switch circuit if the RF current exceeds a defined threshold. Due to the arrangement of a non linear resistance energy sink (I) in series with the oscillator circuit (20), the switch efficiency is considerably increased by rapid variation of the supply voltage of the oscillator circuit in response to a variation of the oscillator current due to the action of the non linear resistance of the energy sink (I).
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Citations
15 Claims
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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.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
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15. A method for protecting a location against intruders comprising the steps of:
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installing an alarm apparatus essentially in the center of the location, the 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 oscillator circuit having an oscillator current including an amplitude,; and tuning the antenna to create a slight mismatch between the resonance frequency of the oscillator and the resonance frequency of the antenna such that the amplitude of the oscillation current increases 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 an intruder, the perimeter comprising a circle having a radius corresponding to the multiple of the half wavelength of the emitted waves.
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Specification