WHY WOULD YOU want a broadcast band AM transmitter with a power output so low that it can only be received inside a radius of about four metres? Well, let’s say you’ve just finished building a replica of a classic 1940s’ era AM radio, which you’re entering into a club competition. Wouldn’t it be great if you could tune it into an "authentic" old time radio program, to recreate the way it might have sounded back then?

With this little transmitter you’ll be able to do just that, by rebroadcasting historic radio programs like those available on CD from Screensound Australia (see sidebar).

Alternatively, you might want to play the music from your personal MP3 or CD player through your car radio when you’re driving to and from work – but the radio lacks direct audio inputs. With this little transmitter, that’s no problem – although you will need to modify it slightly so that it runs from the 12V car battery.

In short, the whole idea of this project is allow any line-level audio signal to modulate an RF carrier in the AM broadcast band, so that it can be played through a nearby conventional AM radio.

The carrier frequency of the transmitter can be tuned to virtually anywhere in the lower half of the broadcast band – ie, from 550kHz to about 980kHz. This allows you to choose a frequency that’s away from any of the broadcasting stations operating in your area, to ensure interference-free reception.

The audio quality from the transmitter’s signal is very close to that of the program material you feed into it, because it uses a special balanced modulator IC. There’s also a modulation level control, so you can easily adjust the transmitter for the best balance between audio volume and minimum distortion.

But the best part is that the whole transmitter uses just a handful of parts (including two transistors and the modulator IC) and fits inside a standard UB3 sized plastic jiffy box. It’s also low in cost and easy to build, as all the parts fit on a small PC board. And it’s run from either a plugpack power supply or a 12V battery, so safety isn’t a problem, even for beginners.