I was watching a special on Colditz last night, WWII British prisoners locked in a castle MacGyvered their own radio to listen to the BBC.
And it left me wondering.
What are the fewest basic materials needed for a working radio.
Any crude point-contact rectifier will pick up a strong AM broadcast and cause a simple diaphragm to vibrate, acting as a mono earphone. You can add an adjustable coil of wire if you want to be able to tune through the airwaves to search for specific broadcast frequencies.
Some people even claim to be able to hear radio broadcasts in their heads. I believe that current theory on this is that a dental filling is acting as the rectifying junction and the resulting sound wave is transmitted up the jawbone to the inner ear.
Headphone, crystal, coil, antenna.
If you’re improvising a crystal radio, wouldn’t the headphone be the hardest part to make?
Not if you had an old telephone you could cannibalize.
antenna, radio detector (crystal), sound reproducer
you will heard all radio signals that your antenna receives. if you had a single strong station signal then this might work. in a practical world a tuning circuit (coil and capacitor) to provide tuning besides what the antenna provides for tuning is needed.
Is it possible to use a piezoelectric material for the sounder? That would be simpler (in construction, if nothing else) than a coil speaker
yes a crystal earphone is used on crystal radios. a crystal radio is low power and doesn’t work well with a magnetic reproducer in most cases.
I made one or two crystal radio sets when I was a kid. IIRC, for sound I used one of the old monaural, single-earphones that were typical of the time. I thought it was pretty cool to be able to tune in stations with a few wires, the crystal, and a window screen–and no power supply!
And AM radio wasn’t nearly so bad in those days.
Not quite true. Try it and see. The resonator is not just a bandpass filter. Without the tuning coil, your antenna needs to be 1/4 to 1/2 wavelength long. (BBC was at what freq in WWII?)
If your antenna is well shorter than 1/4 wavelength, or if you have no antenna, then a resonator is essential. It produces a large “effective aperture” which intercepts far more EM wave energy than its size would suggest. I never see this point covered in discussions of xtal radios. Maybe because it’s usually buried in math. It’s part of antenna theory courses.
Cool. Since we’re talking simplicity in this thread, I wonder if the sounder can be constructed using a fairly crude piece of naturally-occurring mineral?
I’ve heard of sets made using a tin cup and a lump of coal - presumably containing pyrites - for the rectifier. What’s the crudest form of earphone we could make?
What’s the simplest FM receiver?
An AM receiver tuned to the edge of an FM signal’s frequency band will decode the FM signal. The FM signal varies in frequency (that’s why it’s called frequency modulation, duh) so as the frequency gets close to what the AM receiver is tuned to, the amplitude of the received signal gets bigger. As the FM signal’s frequency shifts away from the AM receiver’s frequency, the rejection of the AM receiver’s tuned circuit causes the amplitude to get smaller, thus converting the frequency into an amplitude, which is what the AM receiver decodes naturally.
This is generally called an FM “slope detector”. You can google that term for more details.
engineer_comp_geek: Thanks.
On a practical note, is tuning an AM receiver to an FM station slope-detector-style more difficult than tuning an AM receiver to an AM station? (I know I’m not getting the noise resistance a real FM receiver would have, I’m just wondering if there’s some real deal-breaker of a bug I’m not seeing here.)
You have to do a little more fine tuning than you do with an AM signal, but other than that, no, it’s not really any more difficult than tuning an AM signal. If you tune it to the middle of where you get the strongest signal it sounds a little weird. You just tune it a bit to one side or the other and the sound clears up.
The AM lack of noise immunity, as you posted, is the only real down side to it. On the old tube type radios I used to try this on the sound quality wasn’t that great, but that probably had a lot to do with the fact that it was a 30+ year old tube style radio and no matter what you did it was never going to be hi-fi.
When I was a kid I made one out of a coil of wire and a metal shoe polish can. It didn’t work very well, but it did work. You had to put your ear just about right on top of the thing to hear anything though.
As mentioned up thread, the earpiece from an old style Ma Bell telephone works pretty well. They were high impedance and fairly sensitive.
Any high impedance headphones worked pretty well, but they are becoming quite hard to come by.
I think a coil-based earphone works well if your detector crystal is connected as a tap on the resonator and not directly across it. In other words, the magnetic earphone needs high current at low voltage, so it requires a step-down transformer. Or, just use a small tube-audio stepdown transformer. Crystal earphones need high volts at low current, so in that case the detector/earphone section can be connected right across the resonator.
But I think this applies to catswhisker, where the diode drop doesn’t confuse the issue as much.
What’s the theory around the razor blade detector? I made a foxhole radio as a kid with one, but could never get it to work at all. My theory is it was the wrong kind of single-edged razor blade – are they all the same for this purpose?