This the Good Quality FM transmitter for your stereo or any other amplifier gives you a pretty good signal strength up to a range of 500 metres having a power output of about 200 mW. This circuit can be operated with a 9V battery.
The audio-frequency modulation stage is constructed close to transistor BF494 (T1), that is wired as a VHF oscillator and modulates the audio signal present at the base. Working with preset VR1, you’ll be able to alter the audio signal level.
The VHF frequency is decided by coil L1 and variable capacitor VC1. Decrease the value of VR2 to obtain a higher power output.
The next stage is designed close to transistor BC548 (T2), which serves to be a Class-A power amplifier. This stage is inductively coupled towards the audio-frequency modulation stage. The antenna matching network contains variable capacitor VC2 and capacitor C9. Fine-tune VC2 for the optimum transmission of power or signal strength at the receiver.
For frequency stability, use a regulated DC power supply and house the transmitter inside of a metallic cabinet. For better antenna gain, utilize a telescopic antenna in place of the simple wire. Coils L1 and L2 are to be wound over the same air core such that windings for coil L2 start from the end point for coil L1.
Here the coil winding specification:
This good quality 500M FM transmitter circuit absolutely works fine since it already tested.
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Could this be used as a metel pipe locator? Connect the aerial to the steel pipe and use a fm radio to locate the pipe underground