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The power
output of most of these circuits are very low because no power
amplifier stages were 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
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 aluminium case.
Shield the oscillator stage using an aluminium 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.
This transmitter should only be used for educational purposes.
Regular transmission using such a transmitter without a licence is
illegal in India
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