YOUR OWN LOW COST FM TRANSMITTER
show you how to build a long range FM
transmitter at world's lowest cost. This
transmitter will really work if you
construct it carefully and follow all the
have promised to offer this to all the
list members of Pirate radio list, we are
now publishing the full technical details
of the following FM transmitter.
request you to please mail us with your
feedback so that in future we can give
you more and more wireless and other
projects for free. If we receive enough
feedback, which will certainly encourage
us to develop newer projects for you.
is the specification of the transmitter:
of stage: 4
Frequency of operation: About 100MHz
Antenna type: Folded 300 ohms dipole.
obtained in free space: Up to 4km with
dipole antenna 30 feet above ground
level. More range with yagi antenna.
diagram of the Transmitter
transmitter is built on a Printed Circuit
Board. This board uses track inductor for
L1, L2 and part of L3. The section built
around Q1 is the oscillator section.
Oscillation frequency is determined by
L1, C4 & C5 which forms the tank.
Actually C5 is the feedback capacitor.
This is required to sustain oscillation.
This also influence the operation of tank
formed by L1 & C4. Modulation is
directly applied to the base of Q1 via
C2. A microphone is connected here to
serve this purpose. You can alternately
feed direct audio here after
disconnecting the microphone biasing
resistor R1. Q2, Q3 & Q4 gradually
raises the output power up to the desired
of the inductors are PCB etched, there is
practically very little frequency drift
provided you use a highly regulated and
ripple free power supply.
output from the transmitter is taken from
the junction of C11 & C12. This is
unbalanced output of around 75 ohms
impedance. But a folded dipole is a
balanced type antenna of around 300 ohms
impedance. So we need to use a 'BALanced
to UNbalanced transformer' or 'BALUN'. A
1:4 type BALUN is employed here for this
purpose. Antenna connection is taken from
this BALUN via a 300 ohms flat parallel
feeder cable commonly used in television
to receive terrestrial broadcast. No
coaxia is used to feed antenna. This
saves cost. Also a parallel feeder cable
provides much less signal loss compared
to a coaxial.
BALUN is made using a two-hole binocular
ferrite bead as shown above. You need to
use parallel insulated twin wire to
construct this. This wire is commonly
used to wind TV BALUN transformer. If you
want to get rid of this, then buy a
ready-made TV BALUN that is generally
used at the back of your television set
for interfacing with feeder wire.
prefer to build this yourself, the
circuit diagram is given above. You need
to carefully construct it keeping in mind
about the 'sense' & 'direction' of
turns. See there are four coils. Two
coils in the upper section, which are red
and blue, required to be wound on left
side of the BALUN and the remaining two
(blue & red) in the lower half to be
wound on right side. Connection marked
'A' and 'B' at the left side of the
circuit is reqired to be connected to the
PCB at the shown point. As dipole antenna
is balanced type, so you need not to
worry about its connection.
transmitter is built on a single sided
PCB. As mentioned earlier, this PCB has a
number of etched inductors. For this
reason, you need to very carefully
construct the PCB as mentioned below.
above drawing is the copper side and
below shown is the component mounting
copper side view, you can see that there
are three track etched inductors that
resembles 'RCL' Every
corner and track width/length are
calculated and then they are drawn so
that each 'RCL' section becomes an
inductor of required value. Never play
with this; otherwise, optimum result
could not be achieved.
to use a laser printer or a high quality
printer to get a printout of the
drawings. First, save the picture to
disk. Now try to print it from such a
software which permits you to control
print size. 'Paint Shop Pro' is such a
software. Of course you can use any other
software. Print the drawing so that
copper side drawing is exactly 59mm X
59mm. Few trial will give you the perfect
print. Now construct the PCB using
'Photo-etching' method so that all the
tracks becomes exactly same as you are
now seeing. Now drill the PCB carefully.
The PCB is now ready to populate.
population according to the component
mounting plan. You can also get a true
size copy of this plan printed and glued
to the PCB. This will help you work
fast.Part of L3 is required to be
constructed. This is described in parts
note that in the picture of the
transmitter kit, capacitor C1 & C10
are not mounted by mistake and the kit is
filmed. Please add these two capacitors.
Try to keep all component leads as short
need to design the dipole antenna to use
with the kit. In our web site we have a
complete manual of the above kit which
descibes in detail, the full antenna
construction procedure with tuning and
antenna alignment. Before you go there,
you need to remember that if you want to
come back here, please use your browsers
'Back' button. We shall provide you a
link below which will take you directly
at the dipole construction page.
it or not, a 2N2369 from Philips, used in
the final power amplifier section, can
give this much of range.
R1 - 22K
R9 - 1K
R4, R8 -
C10 - 1n
Q3 - BC548
PN2369 (Plastic casing) or 2N2369 (Metal
L3 - 7
turns, 22SWG wire, 3mm ID, Close wound,
binocular BALUN core, BALUN wire, 300
ohms TV feeder wire,
JP5 - All jumper wires.
completes the Project. Please mail us
with your feedback. It will really
encourage us to give you more & more
project like this.
Click Here to view Full Antenna
Construction Details and Tuning Procedure