Another low power FM transmitter idea.

Hi Ruud,
That was a really interesting description.

At the moment, there are NO local FM radio stations whatsoever in Spain. The Spanish telecom authorities will not issue licenses for commercial radio stations.

I have done some work for Coast FM Gold (Nerja) and the authorities also turn a blind eye.
On one occasion they had a complaint from Malaga Airport that they were interfereing with aircraft communication, so they gave advice on positioning the antenna to minimise interference.

They are using a commercial 500 Watt transmitter in a shed at the top of a 480m high mountain. The antenna is directional and pointing down to the coastal town. The studio is behind Lidl, and they have a custom uWave link from the studio to the transmitter site.

If one operates sensibly and take it seriously, then it looks as though one may use a "pirate" radio station in many countries. It is only when you compete with licensed stations (money involved) that can cause consequences. It's allways about money.

BR Harry

In the 80's some of my friends were active in pirate radio.
At the time, I had developed my own PLL, with a ridiculous expensive 95H90 (:10) prescaler.
So, it was no problem to build a 'rock steady' FM transmitter.
They wanted real power! 10 Watts wasn't interesting enough for them.
At the border of the village where I lived at the time, there was a very high silo.
On top of that, they placed a mast with four stacked dipoles...
They wanted at least 150 Watts, the more, the better!
At the time, there were BLW96 transistors for sale, at very low prices.
The funny thing was, that they weren't stamped. (Although I have an idea why the stamp was missing...) With these BLW96 transistors, I created a power amplifier that would deliver 250 Watts continuously. (They transmitted 24/7.) The transistor was on a big 30 cm. long and thick 'H' aluminium profile, with blowers on both sides. The result of this setup was the the station could be heard in a big part of the country!
Of course they weren't waiting for the authoriteis to get them, so they also needed a 'studio-transmitter link'. In my country, there were two television channels, on CH27 and CH30.
So, if you would use the same antenna, this would not be easily spotted.
The 'uplink' consisted of a small 177.4 MHz transmitter, followed by a tripler, creating an output frequency of 532.2 Mhz, just 'between' the official television channels.
At the receiving end a downconverter was used to receive the 'link' frequency on an ordinary domestic radio. The audio was tapped at MPX level and that went straight into the modulator of the main transmitter. The squelch was used to activate a relay, so everything could be switched on or off from a distance. For some strange reason, this pirate activity was more or less tolerated, as long as there were no complaints or disturbances of 'official' transmissions. (Rumour has it that even in the police office they listened to the radiostation...)
The whole setup has worked for many years without any problems.
Good memories to the 'good old times'!

Ruud wrote:Personally I think that we have more than enough FM transmitter designs nowadays.
Any 'new' design will be more of the same, unless it is 'something special'... (Which I doubt.)

Hi Ruud,
I think you are right. 20 years ago there was a strong interest in FM transmitters, but today there are 433MHz and 2400MHz modules available for simple communications. Not so many households have FM radio: it is all DAB or Internet-streamed, at least here in Sweden.

But a well designed project can be great for stimulating interest and is more fun to build rather than useful.

In about 1980 a friend asked me to make him an FM transmitter, so I built one using germanium transistors. The oscillator was very low power (AF114) but the final stage was a 1-Watt Germanium device ripped out of an old Storno military field unit. It gave about 50mW to 100mW out. The guy lived on top of a hill and he stuffed is cassette player into it, 1/2 wave on the roof and went for a walk. Signal was booming out, so he went for a drive. At 5 km he got frightened of detection, so he raced home, switched if off and has never used it since.

I built that one in a tobacco tin and gently soldered the oscillator transistor to the inside of the tin (using solder mixed with mercury to get a low melting point).

I once built a larger power FM transmitter (10 Watts if I remember rightly) for a music radio station. I powered it from a car battery and hid it up a tree. I sent audio to it via a 44MHz link. Loads of people listened to it and word soon got around. Never got caught. Soon after that I got my ham-radio license.

In those days you could have a lot of fun.

Best regards Harry

After much playing around here's the new design.
Output is very low though.. around 0.2mW as no RF transistors have been used. A 1/4 wave wire antenna should cover a small room.

L is around 150nH. It consists of 9T of 25SWG on 2mm plastic former. It's secured with several layers of superglue.

Choose C to set your desired frequency. It should be on the lower end of the band or 88-91MHz if you want any usable power. The RF gain of 2n3904 drops sharply after that.

First buffer is common base amplifier for it offers better input output isolation. Second buffer is common emitter amplifier. 10K resistor isolates the Oscillator and superb frequency stability has been obtained.

The Oscillator circuit has been taken from the internet.
I added a 10K resistor from base to ground to slightly increase the output power and for better biasing of the Oscillator transistor. A PN2222A worked fine.

Schematic https://ibb.co/7btPVPg

It works best when built ugly bug style on a piece of copper clad board. Keep all leads short and components as close to the ground plane of the board as possible. The 100n coupling capacitor between the buffer transistors can be of a lower value for improved stability. Try 10p. Gain will be lower then but stability should improve further.

For more power, reduce the 10K coupling resistor to 1K or so.

So many designs available on the internet and all of them suffer from frequency drift.

Personally I think that we have more than enough FM transmitter designs nowadays.
Any 'new' design will be more of the same, unless it is 'something special'... (Which I doubt.)

Schematic and prototype pic.

https://ibb.co/LdyCJPv (Prototype on copper clad board. It's feeding my 2 X 2n3866 amplifier for 0.3W output.

https://ibb.co/n1gQZRm (Schematic)

The 9pf capacitor is NP0. Coil is 10T of 25AWG (26SWG) on 2-3mm diameter plastic former. I used a small piece of an empty ballpen ink tube. The coil is secured with several layers of superglue.

Frequency is stable at room temperature. The other two transistors heavily isolate the Oscillator from antenna. A pre-emphasis network at the input will improve audio quality.

Output power is around 1-2mW at 103MHz. Audio quality is good.