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Comment by defrost | Hacker News Reader

In an extreme emergancy (eg: captured in a POW camp) you might just have to build your own:

Queensland Prisoners secret radio revealed

>> Two Queensland brothers, Ernest and Charles Hildebrandt, built a secret wireless radio receiver which they operated in the camp near Bandoeng in Java which was overran by Japanese soldiers in 1942.

>> Constructed of parts scrounged from the internment camp where they were held prisoner in Java during the war, the transmitter was built into a Dutch gas-mask container and hidden under a square of concrete measuring 12 by 7 inches.

[*] https://blog.qm.qld.gov.au/2019/11/12/on-this-day-queensland...

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Scanning that article I was blown away to see it said "the transmitter..." - then I realized the author is play fast and loose with the terms transmitter and receiver (surprise! They aren't interchangeable!). It looks like a tube-based regen receiver.

In Europe, Allied POWs would build "foxhole radios" which had no selectivity and you'd hear the strongest signals best - and during WW2, that was the BBC.

> which had no selectivity

All of the foxhole radio designs I've seen actually do have an (albeit very crude) way to select a frequency. Maybe all the designs I've seen aren't faithful to the ww2 versions?

I mean, if you could get your hands on a variable capacitor, you're in business. That was unlikely to be an item you could scrounge up though. The guys in the article managed to find a tube and other parts, so that's quite impressive, but you can still get more minimalist.

The most basic is a length of wire wound around a coil form (toilet paper tube, plastic bottle, etc), the "detector" was a blued steel surface (DE safety razor blade), a safety pin to act as a "cats whisker" (bonus if you can use graphite pencil lead attached to it), some high impedance headphones (WW2 tanker/air crew headsets usually fit the bill), a good ground and as much wire for an aerial/antenna as you could get away with.

Using the length of the coil, and where you connected the antenna, you could kinda-sorta tune for better efficiency at different parts of the band (be it SW or AM). (Longer coil = lower freq)

I made a few of them years ago, and posted the results on my now-defunct blog. The DB is still intact with my files and findings, but my code base is hosed (need to remake it).

Coverage of earth by earth sensing satellites in visible spectrum is so high, that you can literally write different message into sand / earth every hour. No radio construction necessary XD

Someone needs to be looking, though.

And someone less desirable could also be looking.

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