Abstract: A single wire antenna placed in proximity to an area to be protected senses the presence of electromagnetic fields including the normal ambient field and modulation thereof resulting from the presence of a moving intruder. The antenna signals are demodulated and filtered to provide the input to circuitry for inspecting these signals and producing a signal based upon differentiation between normal transient signals and random modulation signals associated with the movement of an intruder. Digital counter circuitry provides the differentiation and various time-out features can be incorporated to control the generation and/or transmission of alarm signals. The system includes various elements to permit easy testing of system operations by the user.