The FM transmitter circuits are generates the output power depending on power amplifier . If any transmitter circuit have consist Rf amplifier stage, oscillator stage then the output power of that ckt provides good power signal. The transmitter circuit described here with an extra RF power amplifier stage, ,increase the power output to 200-250 milliwatts after the oscillator stage. This Long Range FM transmitter using two transistor can provide reasonably good signal strength up to a distance of about 2 kilometres With a good matching 50-ohm ground plane, antenna or multi-element Yagi antenna.
To change the frequency of the FM transmitter a varicap diode is included which provide frequency modulation by audio signals. BB109 is a Varactor Diode , varactor diode is a reverse biased p-n junction diode, whose capacitance can be varied electrically. As a result these diodes are also referred to as varicaps, tuning diodes, voltage variablecapacitor diodes, parametric diodes and variable capacitor diodes.
The circuit is make using transistor Q1 (BF494) is a low-power variable high-frequency VHF oscillator.The power at the output is about 50 milliwatts. Transistor Q2 (2N3866) ie asVHF-class A power amplifier. It make the signal powerfull by boosting the oscillator signal power four to five times. After that 200-250 milliwatts of power is generated at the collector of transistor T2 and then signal is feded through capacitor for output transmission.
Aassemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminium case for better quality,. Shield the oscillator stage using an aluminium sheet. Coil winding details are given below: