| 摘要 |
<p>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). <IMAGE></p> |
