Non Linear Junction Detector locates eavesdropping devices during a technical surveillance countermeasures sweep and is equipped with a supersensitive receiver that detects second and third harmonic radiations, even if they are ultra low level. The Detector has circuitry that is designed to operate at low stress levels for longevity. The Detector is provided with ultra stable crystal control oscillators providing trouble-free operation without the need for factory calibration and periodic maintenance. Non Linear Junction Detector is easy to use and can give optimum performance.