Intrusion warning system

An intrusion warning system for detecting the presence of an intruder in a given area comprising:

• an antenna provided around the given area, said antenna being insulated from ground;
• an oscillator for feeding an alternating current signal to said antenna;
• a coupling impedance means connected between an output of said oscillator and said antenna, said coupling impedance means including a means for varying impedance to obtain optimum detection sensitivity;
• a signal processing means for detecting the alternating current signal on said antenna and producing an output signal in response to a change in the alternating current signal on said antenna exceeding a predetermined level;
• said means for varying impedance comprising a feedback loop from said signal processing means to said coupling impedance, said feedback loop including an integrator circuit for integrating the alternating current signal on said antenna and an impedance control means for controlling the impedance value of said coupling impedance in accordance with integrated value, so that changing of the coupling impedance is carried out only for slow changes in the detected alternating current on said antenna and not for rapid changes due to the presence of an intruder in the given area.