Many electronics enthusiast love transmitters. It is very hard to imagine that a hobbyist in his lifetime has never tried to build a transmitter. Remember your childhood when you were fascinated about building a small, one transistor based medium-wave transmitter. And how thrilling it was to hear the voice or song played back to your neighbour’s radio set few hundred feet away.
When I was a school-going student, this ‘transmitter’ bug bite me and I tried my best to build a medium wave transmitter using just a single transistor, and the MW antenna coil with the ferrite rod, as the oscillator part, and just a stick antenna to radiate the signal. It was giving very good sound, but the transistor was getting too hot despite using a heat sink.
So, what I did, is very funny. I put few droplets of water on the transistor and it kept evaporating. And I kept putting drops after drops to keep running the transmitter and tried to mimic a local radio station.
To my amazement, one of my neighbour picked up the signal. And it was a ‘Joy’ moment for me in my life – it felt like one of the biggest achievement in my life. And indeed it is. I still remember it vividly.
Then comes the FM transmitter era. To broadcast to more and more of my neighbours, I started hunting for many long range FM transmitter circuit. And as time passes on, I built myself several. One of such circuit is still on Tripod website. Internet was at its infancy stage back then, Google, Facebook etc. were not popular. And free sites like ‘Tripod’ allowed common internet users to build a free page. (And they all were non-existent when I built that medium-wave transmitter).
That was around 1998 when I already have built several FM transmitters. Just two years before this website was born. Yes, this website, Electronics Infoline, you are in right now, is 15 years old at the time of this writing in 2015. At that time, our company ‘Radio Communication Lab’ was new. So, I myself developed a transmitter circuit for my company and put the complete details on ‘Tripod’ webpage.
This is a long range fm transmitter circuit. This transmitter uses four transistors and a BALUN transformer (Balanced to Unbalanced Transformer) to use with a dipole antenna, instead of commonly used Ground Plane antenna. Exactly like old days television receiver set that used a yagi antenna, a booster, and a flat feeder cable. See the circuit below. This transmitter will really work if you construct it carefully and follow all the information honestly.
1. No. of stage: 4
2. Frequency of operation: About 100MHz
3. Antenna type: Folded 300 ohms dipole.
4. Range obtained in free space: Up to 4km with dipole antenna 30 feet above ground level. More range with yagi antenna.
This was made to give it a little directional bias, and indeed, it transmitted very directionally, boosting its range to several kilometers in free space.
Our intention, as a small startup company, to sell this as a finished kit, or do-it-yourself kit. In those days, people were tending to copy any transmitter design and claim as their own. So, we made a tricky decision. In the PCB, I changed the dimension of the three coils so that they mimic something like our company name ‘RCL’. See the PCB design below:
This actual design was made using a very early version of ‘Protel‘, called ‘Protel 99SE’. I designed this on a 486 desktop having just 4MB RAM and a 256KB video RAM. Today when I see this, I find myself that, to create brilliant things, we may always not need the latest gadgets and software.
Because, this first design, and all subsequent improvement of this version (that I designed on the same computer using same software), was best-seller for our company back then.
So after selling many thousand units across the globe, I decided to offer this for free to the internet community and posted that to ‘Tripod’.
The 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 level.
As most of the inductors are PCB etched, there is practically very little frequency drift provided you use a highly regulated and ripple free power supply.
RF 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 coaxial is used to feed antenna. This saves cost. Also a parallel feeder cable provides much less signal loss compared to a coaxial.
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