This circuit is from an American publication translated from 1974, but it is feasible since modern equivalent modern transistors are used. The coil L1 consists of 100 turns of wire AWG 28 in a ferrite rod and L2 is a flat coil with approximately 20 turns of wire AWG 28 and a diameter of 15 to 20 cm. This is the exploring coil. The beat signal is heard on a high impedance phone connected to J1.
