Stable wireless microphone 140 MHz
Unlike many others, represented in the figure the transmitter frequency drift, even when touched by hand, due to the original inclusion of feedback.
When powered 1.5 V range transmitter is 150 metres away, while frequency drift in temperature range -20...+30 deg. With is 500 kHz.
The RF generator is performed on the transistor VT2. High stability is ensured thanks to the introduction of positive feedback ( sequential chain L2C4 ).
Coil L1 consists of 6 turns of wire PEL - 0,53 wound on the frame with a diameter of 3.5 mm; L2 - 3 turns of wire PEL - 0,25 on top of the coil L1.