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| This page describes the design
and construction of the very simple circuit
used to convert audio signals to radio frequencies.
It is set to the same range used for public
FM broadcasting, so that its signals can be
easily picked up by inexpensive home receivers.
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| The Community FM Modulator is
designed as a three stage transmitter in the
simple Master Oscillator driving Power Amplifier
configuration. The line up consists of an
audio pre-amp modulating a power oscillator
that feeds a power amplifier. The total power
is kept under 50 mw. Simplicity is the hallmark,
and great care has been taken to over-engineer
to ensure repeatibility and stability.
The transistors used throughout the circuit
are completely interchangeable. Among the
transistors that work well are BF194, BF195,
BC147, BC148, BC149, 2N2218, 2N2219, 2N2222
and a host of others. Almost any high frequency
transistor will also work. In India, the
BF194 is the preferred choice since it is
cheap (under Rs 2/-), resilient to bad soldering,
and made locally by Indian manufacturer
Bharat Electronics Ltd |
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| The audio is fed directly into
the modulator. This resulted in varying load
on the modulator depending on the impedance
of the audio source and subsequent de-tuning
of the frequency of transmission. The common
collector audio pre-amp offers high isolation
between the modulator and the audio source.
It also reduces the possible hum from poor
quality power supplies. |
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| It is designed as a classic
Hartley oscillator, only departing from the
design in that the active element (the transistor)
is configured as a common base amplifier.
The frequency is determined by the air wound
coil and the 22 pf trimmer. The coil is made
of simple house wiring, without any necessity
for formers, and the latter is a standard
item available from any radio spares shop.
The coil is made by stripping the house
wire (about a foot can easily be procured
from any electrical shop) of its insulation
and winding the bared wire 4 turns around
a ball-pen refill or pencil, about 8 mm
thick. Leave leads about half a cm long
at either end to solder to the board, and
trim off the rest. Pull out the pencil/refill
and you are left with what looks like a
spring (the turns should not touch each
other). |
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Although 50 mw is hardly 'powerful'
from a transmission point of view, it was
chosen because this level of power output
is perfectly legal in all parts of the world.
A large number of consumer devices, including
walkie talkies, RC toys, telemetry and cordless
phones need to operate without special licenses,
hence they use this level of output.
The circuit has been designed as a Class C
amplifier with an RFC choke in the output
side. This means that no alignment or tuning
is needed, a major consideration as any person
adept in soldering can then assemble the circuit
without specialised equipment such as oscilloscopes
and signal generators.
The choke is made of simple enameled copper
wire, available from a motor rewinding shop.
Scratch the enamel off about 5 mm at one end,
leaving it shiny. Tin it with a dab of solder.
Attach this end firmly by wrapping it round
the leg of a 1/4 watt 100 k (100 kilo-ohm)
resistor and soldering it smooth. Now wrap
the enameled wire firmly round the body
of the resistor, making tightly wound turns
(the enamel prevents the coil from shorting).
Wrap it to the end and start back. Repeat
so that you have about 3 layers of coils
wrapped around the resistor, making about
15 turns in all. Leave about 1 cm of wire
extra and cut it. Scrape the enamel off
the lead and wrap it to the other resistor
leg, soldering it as before. |
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| Use a cheap standard battery
eliminator delivering about 9 or 12 volts.
In India, these cost no more than Rs 50. You
can also run the system directly off a 12-volt
car battery, and if you do this the results
will be really excellent, since there is no
possibility of mains power hum.
You also get the side benefit that the
system will work even when the mains electricity
is not being delivered, or when you are
going to a village and want to use your
vehicle as a temporary station. If you plan
a relatively short session (24 hours or
so of continuous output) you can use a bank
of 8 regular C or D batteries in a standard
holder, which is much lighter and easier
to handle. |
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| Keep all the parts ready, together
with a rectangular piece of copper laminate
about 10 by 15 cm. Try and ensure you use
standard connectors for the antenna, power
supply and audio input, so that you can connect
standard equipment later without hassle.
Assemble the circuit as per the diagram,
using the copper laminate as the earth.
All other legs of the components can be
soldered to each other where required. This
allows you to assemble the entire FM modulator
without the need for a printed circuit board.
In fact, if you solder strips of laminate
about 5 cm high along the fours sides of
the main board, and one similar board along
the top, you have a closed box for no extra
cost. The input, antenna and power supply
connectors should be affixed to these vertical
strips easily (before soldering them to
the main board!!!!!). If this is not to
your taste, use a standard metal box and
fit the connectors to this, wiring them
to the indicated points on the circuit.
Once the side boards have been soldered
in place, switch on the power supply to
the modulator and turn on an FM receiver.
Tune it near the 100 Mhz band, but make
sure that you are not near any local FM
broadcast. A noisy sound is heard. Using
a fine screwdriver, turn the 100 pf trimmer
screw until the noisy sound suddenly dips
into silence. Turn it further and then back,
so that you are fairly sure that you have
the setting well defined. You are now ready
to modulate audio at this frequency through
the output connector. |
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| The antenna is a simple folded
dipole with 300 ohms impedance. It is easily
fashioned from a strip of 300 ohm TV antenna
cable (the standard ribbon that connects a
television to an antenna). Cut a length of
ribbon 56 " (1.2m) long (this works for
a nominal frequency of 100 Mhz, +/- 5%). Split
one wire exactly half way down the middle,
and expose the two cut ends. At both ends
of this strip, bare the two wires and join
them firmly, thus making a loop. Take another
length of ribbon, and join one end to the
two cut ends in the middle that you have already
exposed. You now have a T-shaped antenna with
two open wires at one end and the circuit
completely closed throughout.
We have found that stapling the ribbons
to a (damaged) 3.5" floppy disk case
is convenient and sturdy, with the case
itself providing a support that can be easily
stapled or nailed to a wooden pole. Stretching
a second piece of wood on the top of this
pole (to make a T) gives a support for the
T of the antenna. The ribbon can be attached
to this crosspiece with either tape or wire
or twine, or even nailed (making sure that
the plastic does not get cut).
Standard TV antenna baluns (75 to 300 ohm
impedance matching connectors - the name
is derived from balance-unbalance, the term
used for a simple circuit to isolate the
antenna earth from the system earth) are
available at any radio/TV spares shops,
and these have the advantage that they are
already fitted with the right sturdy connectors
at either end. They also serve the purpose
of ensuring the antenna is correctly balanced
for the output from the modulator.
The finished FM Modulator with its antenna
may not look very fancy, but it will deliver
a clean signal to a distance of about 450
m from the antenna. Hoisting the antenna
to the highest point in the vicinity will
result in a very clear signal being received
even in the narrow alleyways of the village.
If you are using the system inside an hall
or closed auditorium, make sure that you
have the antenna located well away from
the rear wall. |
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Good soldering iron
Solder
Fine screwdriver
Heavy duty stapler
Electrical Tape
A small piece of copper laminate (10 by 15
cm is enough)
(optional) side (5 cm high) and bottom (10
by 15 cm) pieces to make a closed box from
similar laminate
A small length of house wire (L1)
A small length of enamel wire (RFC)
Several meters of standard (300 ohm impedance)
ribbon antenna wire used to connect ordinary
household television antenna
1 Banana jack input socket connector (should
match the connector attached to your source
device, either a mic or audio player)
1 standard TV antenna socket connector
1 standard DC current socket (should be the
same size and type as the battery eliminator
jack)
1 12 volt Battery eliminator (or 12 volt car
battery or bank of 8 C or D battery cells)
Three transistors, BF149 preferred, other
choices listed above
Resistors
47k 3 R1, R2, R9
22k 3 R3, R5, R6
270 ohm 3 R4, R7, R8
22 ohm 1 R10
2.7k 1 R11
100k 1 Choke
Capacitors
1 microfarad 2 C1, C4 (electrolytic)
10 microfarad 2 C2, C6 (electrolytic)
100 microfarad 1 C10
0.001 microfarad 3 C3, C5, C11
33 pf 3 C7, C9, C12
22 pf trimmer 1 C8
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| Click on the diagram to view
in full size |
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