[dredmorbius]

There are various kinds of crystal sets.

I don't know the theory, though I did make one as a child.

It was the non-tuning type, and would pull in only the strongest-broadcasting local AM channel.

It's also possible to create a tuner which can pull in a specific station (or more accurately: frequency). These can also operate without a battery, though battery-powered radio is of course more versatile.