Mini AM transmitter
For the whole MW band, 160m and shortwave
Designed by sv3ora

I have seen quite a few AM transmitters on the web, solid state and tube based. For this project I wanted something that would be able to cover the whole MW band from the bottom end to the top, the 160m and the shortware, with good audio fidelity and adequate frequency stability, so that I can use it to broadcast music to nearby radios and enjoy my favourite programs on them. The schematic of the transmitter I have ended up with, is shown below.

The oscillator is able to cover all frequencies from 480KHz to about 2MHz, split in two ranges, each one selected by the switch. I wanted the transmitter to be cheap and easy to be built and replicated, so I have used a 500pF varicap instead of a big and bulky variable capacitor. Frequency stability is more than adequate with the varicap in place. The oscillator output waveform is square wave, which is rich in harmonics and so it covers the whole shortwave and much more. The following amplifier stages work in class-C and the output of the transmitter is square wave as well. That way you can enjoy your favourite program at any frequency you like.

The transmitter is quite powerful and if a long antenna is connected to it combined with appropriate matching and filtering networks it can be used as a small broadcaster. However the intention was to use it for in-house usage so no such effort has been made.

AM modulation is strong, by listenning tests, because I also modulate the driver stage and not just the final. This technique is used in AM CB transceivers. By modulating only the final alone, I found the modulation to be usable but a bit low. By modulating the driver as well, some FM may present in the output waveform, but I did not consider it a big problem, since AM receivers are unaffected by it, so I did not add another buffer stage just for this.

The modulator is a simple series modulator that does not use a transformer, greatly reducing the cost and size of the transmitter. Set the potentiometer on it so that 6.6 volts are present at the collector of the BC549C in the modulator without any audio signal applied to it. Audio input sensitivity is quite good, it requires to set the sound blaster output to only 25% for a maximum undistorted modulation.

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