Long Range FM Transmitter

The Long-Range FM Transmitter has an additional RF power amplifier stage after the oscillator stage, to raise the power output to 200-250milliwatts. This transmitter can provide good signal strength for up to a distance of 2 Kilometers, when coupled with a good matching 50-ohm ground plane antenna or multi-element Yagi antenna. The circuit of the Long Range FM Transmitter is built around transistor BF494 (T1), which is a basic low-power variable- frequency VHF oscillator. A varactor diode circuit is also included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50milliwatts. Transistor 2N3866 (T2) forms a VHF-class A power amplifier and boosts the oscillator signal power by four to five times. Thus, 200-250milliwatts of power is generated at the collector of transistor T2. For better results, the transmitter should be housed inside an aluminum case upon assembly and the oscillator stage house be shielded using an aluminum sheet. A potentiometer (VR1) is used to set the center frequency where as another potentiometer (VR2) is used for power control. For hum-free operation, the transmitter is operated using a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 is mounted on a heat sink. The transmitter should not be switched ON without a matching antenna. The circuit requires two trimmers (VC1 and VC2) to be adjusted for maximum transmission power while the potentiometer (VR1) needs to be set so that a center frequency of 100 MHz can be obtained. Disclaimer: This transmitter should only be used for educational purposes. Regular transmission using such a transmitter without a license is illegal in India. Learning: Application of transistors for amplification purposes. Use of varactor diodes. Concepts related to VHF Class-A power amplifier and Yagi antenna Brand: Kits'n'spares, Marketed by: Kits'n'spares