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 have an extremely small assembly. L1 consists of 5 turns of wire 30 in the form of 0.5 cm without core. The transistor can be the BC558.
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