The power output of many transmitter circuits are very low because no power
amplifier stages are incorporated. The transmitter circuit described here has
an extra RF power amplifier stage, after the oscillator stage, to raise the power
output to 200-250 milliwatts. With a good matching 50-ohm ground plane
antenna or multi-element Yagi antenna, this transmitter can provide
reasonably good signal strength up to a distance of about 2 kilometres.
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 signal
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.
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.
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