Abstract: A proximity detection system for detecting the proximity of conventionally hard-to-detect materials having a complex permittivity and/or a complex permeability is disclosed. The system includes an oscillator for producing an output signal, the oscillator having a transistor with an open and a closed loop gain and a base-emitter junction that rectifies resonant signals at the collector to produce the output signal. The oscillator includes negative feedback including a resistive element coupled to the emitter for reducing the sensitivity of the closed loop gain to variations in the transistor's open loop gain and a capacitive element coupling the resistive element to the collector for stabilizing the frequency of the oscillator. The detection system further includes a sensor coupled to the oscillator for varying the level of the oscillator output signal in response to certain electrical properties of the material.