Don't forget your trusty friend, the Crystal Radio.
Remember that whole "wireless power" thing Tesla was on about?
SPOILER: It's still real, so with some wire, a crystal, and a high-impedance earbud, you have a radio that draws it's operating power directly from the transmitted signal.
Very crude versions of these were supposedly used in the first world war by soldiers who didn't have much to work with. A rod or wand of a good crystal, a length of wire, and an earbud. Then it's just ground the crystal, wire the earbud inline into the wire, throw the long end of the wire up a pole or tree or something, and fray the short end, then touch it around on the crystal to tune the thing. This is all from memory so some details may be wrong here, but it's something like that.
>https://en.wikipedia.org/wiki/Crystal_radiotl;dr passive radio receivers that don't need batteries are a thing.
