The other day I remembered that as a kid I wanted to build a radio transmitter. No, not to be a pirate station, but simply to stream music to whereever I was in the house.
I was inspired by my eldest brother's demonstration in a school lab of a radio transmitter based on valves. In those days, only valves could handle the power levels required. It was an amplitude modulation (AM) transmitter as frequency modulation (FM) transmission and radios had not appeared in Malaysia yet. It is also the easier modulation scheme, as in a typical circuit, the output of an audio power stage modulated the anode voltage of a radio frequency power stage. In FM you vary the frequency of an oscillator, which is more complex.
I think my brother's circuit came from a Hong Kong hobby electronics magazine, or a Chinese book of circuits. Many articles were translated from other magazines, but sometimes their own designs appeared. Often substitutions were made, such as Japanese transistors types for European or US types. I even remember the schemes in the 3 systems. This makes me feel ancient.
Anyway I never built that valve transmitter. Part of the difficulty was obtaining a suitable transformer for the modulation. The impedances on both sides were the same order of magnitude. Nearly all transformers commonly available converted between valve impedances (in the order of kilo-ohms) to speaker impedances (in the order of ohms).
Years later my brother sent me a present from Japan, where he was studying, of an electronics beginners kit. This was an ingenious kit in which components like transistors, resistors, capacitors, connectors, and so forth, were enclosed in small hollow plastic cubes with connections on the walls to adjacent cubes. The top of the cube bore the symbol of the component. There was an arena provided by a plastic well, which also supplied the power from a battery. By arranging the cubes in various configurations within the well you could build various circuits. I wish I could find a photo of this ingenious teaching aid. Of course, it never occurred to me at the time to take a photo of it for posterity. One of the circuits was a low power radio transmitter and this worked well over a short distance, say within a room.
Part of the frisson was because private radio transmissions were illegal, though there was little chance of detection given the short range of home built devices, unless you managed to annoy a neighbour enough to report an anomaly.
Fast forward 50 years and now there are short range FM transmitters for older cars that have only radios. And of course everybody can buy bluetooth transmitters and speakers.
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A digital clock I once made, nothing to do with transmitters
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