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Long-range
FM Transmitter
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| Several
circuits for constructing FM transmitters have been published
in EFY. The power output of most of these circuits are
very low because no power amplifier stages were incorporated.
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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.
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| 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. |
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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)
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| 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. |
