FM Transmitter


When I a little kid I always thought it would be hilarious to have a portable radio transmitter to interfere with the neighbour's radio channel. All in good fun, of course.

I guess, in some ways, that interest in radio lead to my acquisition of an amateur radio license, call sign VK8NPF. A word of warning - an amateur radio license does not mean that you may do whatever you like on air!

This is a radio!

I thought we should cover some legal issues before we dwelve into FM transmitters themselves. The communications authority allows the use of the FM broadcast band for anyone, provided any transmissions are low power and don't "cause harmful interference".

In other words, transmitting at 500W right outside your neighbour's shed is not a great idea. But transmitting at 1mW for 'personal' use may be OK - depending on where you live.

There are schematics for FM transmitters all over the internet, ranging from low power devices which can fit in your pocket to huge monsters which draw more current than your household supply can provide.

I decided to go with a small FM transmitter kit, which saves the hassle of RF issues on homemade PCBs, winding inductors etc.

This FM transmitter has an output power of around 5mW from what I can tell. Depending on terrain, that's probably sufficient to cover about 50m of ground.

I wanted this baby to be as portable as possible, so managed to jam everything in this small jiffy box along with a 9V battery and a frequency adjust knob. I also managed to find an old transistor radio extendable antenna, which I mounted in the jiffy box at the lower right.

Here is the completed transmitter, with extendable antenna (~70cm). The kind of shonky black wheel is the frequency adjust knob which allows quick changing of frequencies.

This little guy works rather well. Once, with a little tuning, it managed to transmit over the neighbour's radio. Needless to say, this was a bit of fun for all!




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