A keyboard activates wiper contacts which pick up pulses from the drum for transmission by radio or telephone/telegraph wires. The decoding of the signal is done by striking paper tape against an inked spiral screw with a solenoid coil controlled by the pulses. The rotating screw causes a vertical scanning motion, while the tape being drawn through the printer results in a horizontal scan, and the pulse train is transformed back into an image of the transmitted character bitmap. I think of it as a facsimile system with fixed images sent from 'ROM'.
Everything you could want to know and more is described better than I ever could on the excellent website of Frank Dörenberg: https://www.nonstopsystems.com/radio/hellschreiber.htm
So onto my own involvement...
I learnt about hellschreiber many years ago when I received a book about amateur radio 'data modes' for Christmas. It sounded interesting and I found some computer software to send and receive it, but it seemed like a 'dead mode'. Not something you ever heard 'on the air' - I wasn't aware of groups like the Feld Hell Club back then.
One of my other hobbies is Meccano modelling (some of you might remember my Meccano televisor that I brought to the convention many years ago) and a couple of years ago the memory of hellschreiber popped into my head when I was trying to decide on an idea for my next model.
I'll not bore you with all the dead ends and failed experiments on the Meccano side of things, but the result was the Meccano Hell Feldfernschreiber Machine
It's a fully functional and compatible implementation of the mechanical parts of a feld hell machine.
The character drum was coded by applying kapton tape to 40 polished Meccano wheels, then using a craft knife to remove patches in the appropriate pattern for each symbol using a template and a spreadsheet of the pattern of 'bits' on an original drum from Frank's website.
The printer which has gone through a few iterations is now surprisingly reliable and produces easily readable text. Even the print screw itself is derived from a piece of Meccano plastic 'gun barrel' which I cut the appropriate thread into using a kind of single purpose toolpost-milling-lathe contraption also build up from Meccano parts!
