Crystal sets, POW radios, spy sets & radio related thread

Discussion in 'The Science Forum' started by Electromagnetic, Dec 4, 2017.

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  1. This thread is being started in response to a suggestion by @Joshua Slocum
    Following his posting of picture with a radio set in the "Come and have a go if you think your funny enough" thread, @Fireplace commented "I cannot help but wonder how many know what a cat's whisker is."
    This generated some interest warranting a separate thread.

    Here is a place to discuss your experiences and indulge your interests in such radio related matters.
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  2. When I was a boy two of my neighbours had garages full of radio equipment. They had both been Wireless Operators, one in Bomber Command, the other Coastal. They had practical skills regarding making and maintaining their kit. Skills its hard to master these days with so much being IT-based and of the 'It breaks, send us another' variety.
    I don't know much about the technology other than having made crystal sets as a boy.
    The use of the kit is a potentially huge subject. From SOE operators being captured with terrible results (the Abwehr's 'England Game') to the huge risks run by agents carrying radios. They were not very deniable objects to be found carrying.
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  3. I used to know an old guy who as a civi in WW2 escaped from a Japanese POW camp, he then volunteered to go back into one to get intelligence out again. He was a bit of a radio wizard and made stuff inside bamboo canes and such like.
    We only found out about his past at his funeral.
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  4. MAybe off thread but my first exposure to 'cats whisker' was from the schools programme ' how we used to live.'

    ISTR a family sitting around a kitchen table with the with the headphones for a crystal set insid a mixing bowl listening to a broadcast with a family member playing in a band down a coal mine. All the band members had to wear dinner jackets as you hadn't to be properly dressed to be on the radio.
    Last edited: Dec 4, 2017
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  5. I, as a radio ham, used to build my own receivers and transmitters and worked the world on less than 4 Watts, though it was morse code, not voice (SSB). Both had a small handful of resistors, capacitors and transistors. Used to work quite well. The crystal sets I built were simple affairs and wiring in a variable capacitor gave me quite substantial range. One has to remember that the aerial is a very important component. No bent coat hangers. My present aerial is a 105 foot dipole and even that needs an aerial tuning unit for best results.:cool:
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  6. My Army tech training gave me the skills to fix a radio, and I've done it for my own kit many times. But the radios I'm talking about were either military radios specifically designed to be repairable, or consumer electronics with few or no ICs in them.

    Now everything is moving towards system on-a-chip.

    This little badger:

    [​IMG] the size of a credit card, has a four-core, 64-bit processor at 1.2 GHz, 1GB memory, Ethernet, wi-fi, bluetooth and USB interfaces. It can directly drive a full-HD monitor via HDMI and costs $35. If you don't care about all the interfaces, you can get one of these for $10 that just has wifi and a single core:


    Ten dollars. There's even a non-wifi version for $5. 30 x 65 mm for either.

    If it broke, I would have trouble even seeing the diodes, resistors and capacitors, let alone be able to deduce what had failed and replace it, they are pin-head size. If the SoC had failed, it has 546 pins/pads. Even if it were possible to desolder it and replace it, I would not go through that pain and uncertainty that I'd got the replacement installed right for several hundred dollars, let alone $5/10/35.

    So I've got the skills, but the game's moved on. These tiny computers are every bit as powerful as something like this:

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  7. Just looking at that last photo - I just googled a random representative Pentium 4 computer - it has six hard disks in it. Say they're 10G each for a total of 60G. They'd have cost $100 each. The raspberry pi has a camera card for storage. A 64GB card costs $30 today.

    It's not hard to see why the skills aren't taught any longer. I suppose everything depends on having a replacement available. The supply chain is even more important. If you don't have a spare widget to hand and you can't fix a buggered one because it's unrepairable by design, you're, er, buggered :)
  8. RTL dongles and more sophisticated Software Defined Radios are amazing and comparatively cheap.
    However there seems to be less knowledge of analogue circuits now, so these devices can lack really good filtering.
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  9. You don't even need your own SDR, use someone else's. Of course, you do need an internet connection :)
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  10. Yes, the size of the Surface Mounted Components in even the simpler radios are often too small for home repair. However, larger sizes can be soldered and desoldered by hand. Haven't done that in a long while and need to have a go sometime next year.
  11. Yes, that's a good source of SDRs in various countries. A good place to get familiar with spectrum use, propagation, modes, adjusting bandwidths etc. Handy notch filters too.
  12. On my T3, one of the instructors had a little project for anyone that was interested. It was a design for a small FM transmitter on a bit of Veroboard, IIRC it fitted on a piece a bit over an inch square. It had RF frequency stability comparable to a Labour politician's integrity, but it did work.

    The coil was wound by hand over a pen, and was absolutely critical to get the RF freq in the right range to be able to be received on a normal radio. As it warmed up, the carrier would drift, so you had to retune the receiver.

    Amazing little thing, quite a few of us built one.
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