What would happen if you plugged an FM transmitter into a radio's headphone jack and set the radio to the transmitter's transmitting frequency?

You would get a positive feedback loop, probably resulting in an audible tone. The amplitude would be limited by the transmitter’s modulation limiter, so all you’d have is a very ordinary oscillator.

Uh, what?

I don’t know but I am going to try it :)

Rex nailed it. Think of rock stars on stage who hold their guitars up to the mics or amps and it starts squealing until they touch the strings again. Same principle.

Except the tone probably wouldn’t be “audible” because the headphone jack would be in use.

However, if you tuned another radio into that frequency, you’d probably hear it.

Awe, shoot. I was hoping for some sort of explosion…

I think he means you use the headphones as a mic for the FM transmitter so that what happens is noise is picked up by the radio which generates audio picked up by the headphones(or mic) that the FM transmitter transmits to the radio, which makes audio of it….ect, ect.

@z28proximo: No, no microphones involved. One of these plugged into one of these.

That’s the setup.

@dverhey, nope. There would be no damage of any kind, either to the radio or the modulator. Probably would not even hurt your ears.