[staytrue]

And...receiving a signal requires only wire, a headphone, and a crystal diode.

really :

I thought you needed a wire (antennae), a simple lrc (inductor, resistor, capacitor) circuit with a variable resistor as the r, a battery or ac power with a ac to dc transformer, and an amplifier tube or crystal diode.

[elpinta]

Right indeed: http://makearadio.com/crystal/index.php

I am old, I built them things and used them just for fun.
As long as there is an AM station in relative proximity, they work fine.

[bitterohiogunclinger]

Actually, a wire, a rusty razor blade, a piece of pencil lead and an earphone will do the trick quite nicely. Google “foxhole radio”.

[Rodamala]

Once upon a time, making a crystal radio was a rite of passage for every Boy Scout... but that was before the Faggotry and Twerking merit badges were created.