A long range FM Transmitter circuit

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:

2kmtx_ckt

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:

2kmtx_pcb

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’.

Sadly, I can not copy paste the entire post here, despite it is my original and own creation, because of search engine penalty, but Tripod still holds the entire post where you will be able to see the full description, construction details, parts-list etc.

You can see the original Tripod Article here (opens in a new window or tab).

The original article has a link for full antenna construction and tuning details at the bottom. Unfortunately that link is broken now. So, I am planning to write a fresh article for those, who really want to construct this transmitter. For the time being, see the original article which was our first creation for the internet back in 1999.

2kmtx

Thank you very much for reading this article. Please keep visiting our site and this article section in future for more similar interesting things.

 

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1 Response

  1. Pam94 says:

    Reading your content is pure pleasure for me, it deserves to go viral.

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