At what point does transmitting FM or AM signals become illegal? If my wattage was low enough, is it possible to broadcast in such a way that the signal is really only available on campus.

If it is legal to the point that I need it, what hardware would I need and for which band?

This page has some FM transmitters for sale. To be sure what's legal, you'll need to find out from the FCC the rules and regs, Good luck searching the site at http://www.fcc.gov .I tried briefly and it's summaries are terrible.

As I understand it, it is illegal to broadcast on FM or AM frequencies without a license anymore. There is a newish Low Power FM license for which you would probably qualify.

Otherwise, you'd have to transmit in the 900MHz or 2.4GHz range.

You know what -- I take it back. There are some unlicensed low power rules left.

I had the same problem with the FCC site... The link is awesome... thanks....

Licensed LPFM is hard to get if you're in a city, it turns out. I blame the NAB, who basically conspired to stack the deck in such a way that the adjacent channel protections which apply to a low power station are more stringent than a full power station.

Full disclosure: I'm on the board of an organization which recently was not granted an LPFM license when licenses for PA were granted.

I recommend pursuing part 15 broadcasting.