Making A Mini AM Transmitter Better

The chances are that many of you will have made an FM “bug” style transmitter, a simple one-transistor oscillator usually driven by a small electret microphone. It’s also relatively straightforward to do the same for AM, and if you take a look through AliExpress you’ll find some modules which do just that. [Doz Television Workshop] has one, and he’s treated us to .

An AM transmitter is simple enough, in this case an oscillator and buffer driving a class C power amplifier. The modulation is applied by a transistor in series with the power amp, driven from an audio amplifier. Some attention has gone into the design of this one, with a proper output filter and plenty of room for tweaking to achieve proper levels and modulation density.



There are some problems though — The modulator transistor is mounted upside down for the heatsink, and the frequency stability leaves something to be desired. [Doz] fixes the heatsink mounting and incorporates a DDS frequency synthesizer with an Arduino for control. More after the break.

.. The resulting transmitter is better, but there’s still a problem.

The limitations of AM broadcasting demanded both limiting and pre-emphasis, which he applies in software through one of the more powerful Teensy boards. We have to admit we’d have tried to do the job the analogue way, but that’s merely preference. This board looks to be a good solution for an AM radio collector wishing to use their sets in an age of declining AM transmission.

It should be legal under Part 15 for Americans, but as he points out it’s not for Brits. We suspect such a low-powered device wouldn’t attract adverse attention though. The video is below the break.

If you don’t need so much quality, ..