Middle Range FM Transimtter

The transmitter circuit described here consist of two parts. Its called as the Oscilator stage and an extra RF power amplifier stage to raise the power output to 200-250 milliwatts.

 

The circuit built around transistor T1 (BF494) is a basic low-power variable-frequency VHF oscillator. A varicap diode circuit is included to change the frequency of the transmitter and to provide frequency modulation by audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 (2N3866) forms a VHF-class A power amplifier. It boosts the oscillator signals’ power four to five times. Thus, 200-250 milliwatts of power is generated at the collector of transistor T2.

 

VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control. For hum-free operation, operate the transmitter on a 12V rechargeable battery pack of 10 x 1.2-volt Ni-Cd cells. Transistor T2 must be mounted on a heat sink.

Inductors Data

 

L1 - 4 turns of 20 SWG wire close wound over 8mm diameter plastic former.

L2 - 2 turns of 24 SWG wire near top end of L1.

(Note: No core (i.e. air core) is used for the above coils)

L3 - 7 turns of 24 SWG wire close wound with 4mm diameter air core.

L4 - 7 turns of 24 SWG wire-wound on a ferrite bead (as choke).

 

Notes

* Do not switch on the transmitter without a matching antenna. It is a must, or your transmitter (transistor) will get burn.

 

* Adjust both trimmers (VC1 and VC2) for maximum transmission power. Adjust potentiometer VR1 to set the fundamental frequency near 100 MHz.

Related Schematics