This circuit sends signals to a shortwave radio (range 3 to 7 MHz) at a distance of a few tens of meters when powered by a voltage of 9 V. It was found in a 1980 publication. The L2 coil consists of 15 turns of thread 25 in a 1cm diameter coreless form, while L1 consists of 3 loops of thread 34 wrapped around L2. The microphone used must be of good sensitivity crystal to obtain the desired modulation and the antenna is of the telescopic type, at least 1 m long.
