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At least for FM radios, most FM receivers already do transmit, just not very far. That’s why they have a part15 statement on the back.
If you tune one FM receiver fairly low on the band, and tune with another one around a little over 10 Mhz higher, you’ll hear an empty carrier. I’ve never seen a website on it, but it used to be a “parlor trick” to hack into an old table radio to find the spots where you could add a bit of modulation to that carrier. Usually one of the resistors near the tuning capacitor would do reasonably well with any sort of line level signal like the earphone output of an old casette deck.
Basically it was a parlor trick, though. Range of maybe 10 or 20 ft. Handy if you needed to move a signal over a few feet without wires or for practical jokes. Back when I was in high school we used to mess with such things a bit.
(PS via edit) Ok, I’ll take it back about it being a parlor trick. Lee’s bit about using a scanner as a signal generator would be useful enough to be out of the realm of parlor tricks. LOL