Actually I have a specially made device .33khz sine wave is applied to the ground with two cables laid on the ground. There is a tda2030 audio output circuit. only amp values are changed. The receiver part is divided into 3 with a 3-step switch and fine-tuned with a potentiometer. Between 16khz and 19khz between 19khz and 22khz between 22khz and 26khz. There is no connection between the receiver and the transmitter. The antenna in the receiver is about 40 cm long. a coil is wound on a core as an antenna.
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