200mW FM TRANSMITTER

Output daya banyak rangkaian pemancar sangat rendah karena tidak ada tahap power amplifier digabungkan. Rangkaian pemancar dijelaskan di sini memiliki daya RF amplifier tahap tambahan, setelah tahap osilator, untuk meningkatkan output daya 200-250 miliwatt. Dengan baik pencocokan 50-ohm bidang tanah antena atau multi-elemen antena Yagi, transmitter ini dapat memberikan kekuatan sinyal yang cukup baik hingga jarak sekitar 2 kilometer.

Circuit Notes

"class A" power amplifier. It boosts the oscillator signal power four to five times. Thus, 200-250 milliwatts of power is provided at the collector of transistor T2.

Coil winding details::

L2 - 2 turns of 24 SWG wire near top end of L1.
(Note: air core 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)

• Potentiometer VR1 is used to vary the fundamental frequency whereas potentiometer VR2 is used as power control.

• For hum-free operation, connect 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.

• Do not switch on the transmitter without a matching antenna.

• Adjust both trimmers (VC1 and VC2) for maximum transmission power.

• Adjust potentiometer VR1 to set the frequency to about 100MHz.

The circuit is built around transistor T1 BF494. It is a low-power variable-frequency VHF oscillator. A varicap diode is included to change the frequency of the transmitter and to provide frequency modulation of the audio signals. The output of the oscillator is about 50 milliwatts. Transistor T2 2N3866 forms a VHF

For best results, construct the circuit on a PC board with all components closely connected and put the transmitter in an aluminum case.

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