This circuit is used with low impedance microphones, such as small speakers, magnetic capsules of headphones up to 600-ohm. The circuit is from a 1989 publication but can be fitted with a BF494 transistor. The coil consists of 4 turns of 24 or 26 AWG wire in a form of 1 cm without core. The antenna can be 10 to 30 cm long and the range can reach a few hundred meters. Capacitors must be ceramic.
