This Narrow Band FM VHF transmitter will output approximately 250mW of RF power using a 2N3866 O/P transistor and can operate between 75 and 146MHz. It utilises a variable high gain audio pre-amplifier, which can detect voices 40 feet away using an electret microphone. Using a NBFM scanner, ranges over 5KM have been acheived using a 48cm wire antenna. Coils are 22SWG. 7mm Air Core. L1 and L2 should be 6turns for 75 to 85MHz, 4turns for 85 to 100MHz and 3turns for 100 to 146MHz. For frequencies over 100MHz the Crystal will be higher than 20MHz hense the base emitter capacitor should be 47pF. L3 is a 4.7uH choke. It is Ideal to tune up this circuit using a wave detector meter placed a few inches away from the transmitter. I have a circuit on this site.
This page was updated on 27th June 2000.