This micro FM transmitter uses a tunnel diode as an oscillator. I found it in an English publication from 1989. See that the power is made with a 1.3 V mercury battery. The microphone is an ultra-miniature ceramic or even magnetic capsule. The range is only a few meters, but the circuit can be extremely small. L1 consists of 5 turns of wire 30 in the form of 0.5 cm without core.
