Hi-Fi Stereo FM Transmitter

[MrForbes]

Originally Posted by Dave Flowerday

And, most importantly, you cannot interfere with any licensed broadcaster in the band. If you cause interference for any of your neighbors who are trying to listen to a licensed station, you are violating federal law. The FCC has been known to crack down on offenders too, so don't just assume that no one will notice and you'll get away with it.

This is the crux of the matter...and whether or not you'll be able to do this depends on how far away your neighbors are. I'm lucky, we live on 4 acres, and the nearest houses are about 500 ft away, so we could probably make an FM transmitter system for our house that would meet the interference requirements, even though it might exceed the allowable field strength. If you're in an apartment or high density housing tract, it might be impossible.

[DonRotolo]

The distance you'll be able to go with an unmodified kit depends on what is between you and the antenna. A bunch of metal, not so far. Nothing but air, a lot further. Antenna in the basement, not so far. In the attic, or on the ground floor (in the linen closet?) better.

Build it, try it, and only if it's not OK do you go further. 200 feet on a few microwatts is easy.

500 mW or 1 Watt is WAY in excess of legal limits. A typical NAL (Notice of Apparent Liability = a fine) for unlicensed FM broadcast exceeds $10,000. Oh, and the FCC can find such a transmitter in perhaps 10 minutes, if it goes looking.