Let’s say you fell into a time vortex, and ended up stuck in 12th century Europe. You need to find another time vortex to get back. You know, from your extensive knowledge of time vortex physics, that these vortices emit a lot of radio noise at a high frequency (say, 10 or 12 MHz).
Could you build a circuit that could detect these radio waves, using only items that you could obtain in the 1100s?
(You may be able to enlist the aid of a craftsman from the era, such as a coppersmith or a jeweler. We can assume you know how to speak Medieval English/French/etc…)
True, a crystal radio should work – you can make the induction coil by wrapping wire around a nail or a small drum, and make the tuning capacitor with two metal tubes (one inside the other).
But I’d be worried about the actual radio detection. The signal we’re looking for isn’t an AM radio station. It won’t be amplitude-modulated. It’s just radio noise at a frequency far above human hearing. What would you use to register the presence or absence of such a signal? Would you even hear anything, if you rectified the current and put it through an earphone like in a regular crystal radio?
Hmmm … since there are no other radio stations broadcasting in the 12th century, could you get away with omitting the tuning portion of the circuit entirely? I.e., if you didn’t have to worry about engineering an inductor and a capacitor of known millihenries/microfarads, could you get away with just rectifying ALL of the incoming radiofrequency signals, and assuming that anything you picked up was your source of interest?
It doesn’t take much of a microphone, traditionally a piezoelectric element is used. A piece of quartz might do, better if you can find a quartz crystal. It might take a while to find a mineral which perform as a diode. Without instruments you’ll be testing your equipment with the things like iron filings to see if you can produce a electrical field. And you need some kind of capacitor also. You won’t have aluminum foil but gold foil would work. You don’t need that much, just a coating on the paper should work.
Then you have to figure out if you’ve found the signal. I don’t know if you can just pick that up out of all the static. You have no reference to turn your antenna coil with. It’s all quite a project in that time period but if you want to get back to the future it’s your only choice. I don’t think you’ll be burned as a witch, at most people will laugh at your construction which likely does nothing at all.
Well, the amount of noise inherent in your receiver will be unknown. If the vortex emitted wideband white noise you would have no way to distinguish it from background noise. If bandlimited noise, you would need to be able to tune to listen to the noisy frequency band and compare it to a quiet band.
If the vortex somehow modulated the noise like a spark transmitter it would be easy…
Some wire, a razor blade, and a magnet are all you need to build a super-duper-el-cheapo radio receiver. Use the razor blade and a chunk of wire to make a point contact diode. Use some of the wire to make a coil (inductor) and some more wire to make an earpiece of sorts using the magnet. You’ll need a few odd bits to hold it all together but it’s all easily obtainable using only 12th century tech.
In fact, one of the first things they teach you in EE school is how easy it is to make a radio receiver. Then you spend the rest of your career trying NOT to make accidental radio receivers everywhere in your circuits.
ETA: Poking around quickly on Google it looks like folks started drawing metals through dies to make wire around 2,000 BC, so that’s your limit to how far back you can go and easily make a radio receiver.
It doesn’t need to use carbon particles, a conventional magnetic speaker could be made. It would require the construction on a very small electromagnet. A piezoelectric element makes it much easier to make a speaker, the piezo element can be directly attached to some sort of stiff material that will audibly vibrate. The output of a crystal set is very weak, you need dead silence to hear one with an unamplified speaker. I’ve heard it’s pretty quiet at night in the 12th century.
10-12MHz is about the 30 metre band. Directional antenna can be done, but unwieldy. Elements of circa 15 metres length. A basic direction finding capability isn’t too hard. A 3 or 4 element Yagi could be lashed up.
Something to indicate presence of energy is harder. The output of your typical crystal set is quite high impedance, so you need a high impedance device. For an earphone this translates to lot of very fine wire. That is going to be a limitation. Experimenting with various piezoelectric crystals might get you a suitable device as well. But you don’t need to hear anything, just get an indication of the presence of energy. It could be viable to make a very sensitive moving spot galvanometer. A coil suspended on a very fine thread inside a magnetic field with a mirror (tiny bit of polished metal foil). Shine a light from a slit lamp on the little mirror and watch the spot as it illuminates a surface some distance away. Even very minute current from the set’s detector could be detected.
Magnet could be loadstone, or an electromagnet. You are going to have to make a basic battery probably, again, quite doable.
Coating the coil wire will take lots of experimentation.
Detector of the set could be the razor and wire, or if you can find galena, or pyrite (aka fool’s gold) crystal, or a range of other effective primitive semiconductor mechanisms. Even copper oxide on a copper surface.
Mica may not be, but quartz is. How do you get thin quartz sheets without something advanced like a diamond saw? Just smash rocks until you get an appropriate flake. Eventually you’ll get something that cleaved nicely.
If you’re lucky, you might even find some quartz with gold inclusions, solving the problem of wiring the thing up.