Abstract: An electromagnetic detector sensitive to the intrusion of a metallic mass (e.g., a road vehicle) into its field of radiated energy comprises, disposed on a metallic support, a transmitter coil and two strongly coupled receiver coils. These receiver coils are located at a distance from the transmitter coil and on opposite sides of the axis thereof. The axis of the transmitter coil and the common axis of the receiver coils are roughly perpendicular to each other. The presence of a metallic mass in the field around the detector induces signals in the receiver coils varying in relative phase according to the extent of the modification of the generated electromagnetic field. The output voltages of the receiver coils are combined to form a resultant square wave comparison with a reference square wave produces an output signal depending on the phase difference thereof.