Effectively nothing. I dug into this and found this article (1) from earlier this month on the FCC targeting a couple pirate radio stations in and around NYC. It says they used field agents and FM directional finding equipment to locate the signals to specific property. Probably a big show of force operation on their part.

However, the buck stops exactly there. It becomes the landlords issue, not the tenant who might be operating the radio, where the property owner is liable to be fined if they themselves don't shut the signal off after a certain amount of time. I would guess the FCC lacks the sensitivity to target the signal to a specific unit in a given building in order to issue the fine directly to the involved individual.

I guess the lesson is you can do whatever you like as long as you have the actual radio mast on someone else's property as they will get the fire if the FCC bothers to show up.

Makes me wonder about those old FM radio dongles we had for cars that lacked aux jacks or tape decks if those would technically be pirate radio. You'd have to set them to whatever open channel was locally available and they had actually pretty good range; I remember picking up my friends signal while I was driving behind them.

1. https://www.insideradio.com/free/pirate-radio-crackdown-puts...

> Makes me wonder about those old FM radio dongles we had for cars that lacked aux jacks or tape decks if those would technically be pirate radio.

No. These generally are type-rated and carry a general license to transmit, their power is extremely low and unlikely to be a nuisance to others for that reason.