This is a simple listening bug. The signal can be tuned on any FM radio.
The first transistor (in the circuit diagram on the left) works as an oscillator (in Colpitts connection), the frequency depends on:
trimmer capacitor, inductor (with 4 turns wound on 5mm diameter, no core), varicap and capacitor between
collector and emitter of the first transistor. Low frequency signal from the electret microphone affects
varicap voltage and thus its capacity. Varicap affectc the oscillator frequency and thus modulates the carrier wave.
The second transistor acts as an amplifier and also contributes to separation of the antenna from the oscillator, thereby improving the frequency
How to use the bug: Turn on the FM radio and connect the bug to voltage 9-12 VDC and try to tune the radio frequency bugs. If the bug is near the radio and the radio is well tuned, you can hear feedback whistling. Range of this bug is about 20 to 100m (66 to 330 feet). The antenna is cca 10 - 30cm (1/3 - 1 feet) wire.