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.
For better results, assemble the circuit on a good-quality glass epoxy board and house the transmitter inside an aluminum case. Shield the oscillator stage using an aluminum sheet.
Schematic
Coil winding details are given below: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)Potentiometer
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. 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 fundamental frequency near 100 MHz.This transmitter should only
be used for educational purposes.
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