Could a radio detector be built with 12th century tech?

They could be carried.

Regarding building the piezoelectric earpiece for your crystal radio receiver:

If you could get your hands on a piezoelectric crystal (like a small, thin piece of quartz), would you actually need to turn it into an earphone? I’d think it’d be kind of tough to attach the crystal to a diaphragm, and then enclose that diaphragm in a tiny chamber shaped to go into your ear. Could you tell if you were/weren’t getting a signal by just putting your finger against the piezoelectric crystal and FEELING its vibrations?

Or are the vibrations going to be too weak for a normal finger to be able to feel them?

(EDIT: Experimentally, when I put my finger on a speaker, I can feel bass notes but not high treble notes. The signal would have to have components to its waveform that were slow enough that you could feel them as vibrations, which is a much lower “highest pitch” threshold than the human ear has.)

Just say that you’re building a witch detector.

It’s a simple question of weight ratios. A five ounce bird could not carry a one pound coconut.

Supposing two swallows carried it together?

My understanding is that it’s a short three hour tour for coconuts.

Oh, and something else to keep in mind:

You know what else they didn’t have in the 12th century?
Soldering irons.

(And actual solder, for that matter.)

Solder is just lead and a soldering iron is a hunk of metal heated in a fire.

I thought lead wasn’t conductive enough on its own, and you had to add other stuff like tin or zinc to make it work right. Maybe you could use pewter?

Commercial solder used to be 60 percent tin and 40 percent lead. Tin has been around since the bronze age and lead has been around longer than that. Ancient Romans confused the two for some reason and called tin “bright lead” and called lead “dark lead”.

Not that it matters. The first radio that I built didn’t use solder. You can twist the metal connections together. I used a diode instead of a razor blade (because 8 year old kids shouldn’t be playing with razor blades) and the magnet was a commercial round magnet, No solder required.

I wouldn’t say that it worked well, but it worked. The “speaker” was just wire wrapped around the magnet. It was very difficult to hear. Adding a paper cone to the wire probably would have helped tremendously.

Lead is not great but will do the job. If tin is available it would be help a lot. Adding silver would greatly increase electrical conductivity but would also increase melting temperature. A little antimony would help also. But the job can be done with just lead, the conductive surface should be a very thin layer of lead. Any acidic substance could be used as a flux but assuming copper conductors scrubbing down to bare metal should be sufficient by itself. Any kind of flux used might contribute more to corrosion over time.

And as @engineer_comp_geek points out, solder is hardly necessary.

How do you think stained glass windows were made?
How do you think all that lead pipe that drove the Romans crazy was joined?
Hell, the ancient Greeks had soldering irons, based on the kind of solder joints they made.

I’d always assumed they used Duck tape.
It holds the universe together, after all.

The simplest method of soldering is to pour molten metal on the contact area. I’m sure that’s how they made lead pipes in Rome. They also joined bronze and copper together that way. I don’t know for sure if anyone was soldering iron and steel with lead long ago, it’s not a strong bond like welding, but it works for light duty uses, and when combined with rivets makes a strong sealed joint.

And gold can just be squished together. If it starts clean enough, you’ll end up with a better bond than solder.

For what it’s worth, the Romans mostly didn’t get lead poisoning from their plumbing. They mostly got it from artificial sweeteners. When you store wine in lead bottles, you get lead acetate, which is very sweet. So of course once someone discovered that, everyone did it, because there’s nothing modern about people liking overly-sweet beverages.

How are you paying for these supplies? What did you have on your person when you fell into the vortex?

I never go near a time vortex without a belt loaded with Krugerrands, a Magnum Research Desert Eagle, a Rolex Submariner, and a Duncan Butterfly YoYo.

It’s the YoYo that will get you drawn, quartered, then burned.

I know where all the gold and diamond mines will be discovered in the centuries to come.
Will that mean they’ll be depleted and never discovered, so that I shouldn’t have known about them any more, thus creating a grandfather paradox?
Not if you subscribe to the Many Worlds model of time travel!