This is Micro-Spy circuit which designed using FET components..
- The FETs, T1 and T2, are BF245 N-channel FET’s model. It can be replaced with a NTE133 or ECG312. The BA121 varactor diode can be replaced with a ECG/NTE611. USW stands for ‘Ultra-Short-Wave’.
- The oscillator has a frequency of approximately 100MHz.
- L1 = 7 turns of 0.8mm Silverwire on a 5mm round Ferrit-core (adjustable).
- Feedback via 16pF capacitor. No interference from antenna on resonance-loop.
- Distance is at *least* 300 meters!
Micro-Spy circuit diagram by Tony Van Roon