AM Radio Broadcasting Kit

With Radio Shack on the brink of RIP, I thought it would be a good idea to remember them by writing about one of my favorite kits.

I like to build small transmitter circuits for the AM broadcast band, something that started when I was 12 with a Science Fair AM Broadcaster kit, just like the one in this ad.

radiogeekheaven-RS-AM-Broadcaster-kit.jpg
from radiogeekheaven.com

DJs were hot stuff on the radio; I liked listening to them and wanted to do it too! This was a pretty easy transmitter to build, the most important thing was to get the transistor leads into the correct holes, and wrap them on the springs on the underside of the kit's deck, which in its own way was a double sided circuit board.

The transmitter worked well enough, my 'city grade' coverage radius was the next room and down one floor, but it was fringe outside the house, using my pocket radio as field intensity meter. At this rate, I'd never be able to build a fanbase or any quarter hour listening, the signal didn't even reach to the neighbor's house! Ultimately I was able to reach several houses down the street, but that required higher level engineering, like wrapping the regulation green antenna wire that came with the kit around our rotary telephone, a trick I'd learned from building crystal receivers to get them to receive more stations, with the phone and its line acting as a longer antenna.

That kit was a lot of fun, and a great intro to broadcasting for a younger pup, but it suffered from a problem that all of the simple AM transmitter kits did then, poor frequency stability. The very same tuner knob system that let you dial in any open frequency in your area was its biggest shortcoming too, it would drift over onto other stations causing beat tones, and when sound was applied to the microphone, the circuit would produce both AM AND FM, since the audio was affecting the frequency too. Tuning across the signal with an analog dial radio, it was bright, loud and clear on each side, with a 'hole' right in the middle, where the sound was weak and distorted. I didn't understand that my transmitter was 'FMing', and that I was hearing it through 'slope detection', but with the split signal I thought I'd created my own version of AM stereo!

I built other oscillator transmitters like this, including a couple of tube circuits, by re-wiring the guts of old AM table radios, but all of them had problems with drift and FM, and didn't sound like other stations on the dial. I knew that I wanted something better, but didn't know how to get it. Later I found out about quartz crystals from things like walkie-talkies, but didn't know how to apply that idea to the AM broadcast band to get a transmitter to lock on frequency.

Next I want to get into circuits I'm trying today, for both full tuning across the AM band, and high stability at the same time, stay tuned!

Boomer