[solomonb]

To my knowledge there is no legal way to do unlicensed carrier current transmission. Do you have information otherwise? I've always wanted to try it..

The Part15 regulations for AM and FM are more subtle then what you present here. On FM it is based on field strength readings, the exact values of which escape me, but yielding roughly the range you describe.

For AM the rules are more interesting. You can have up to a 3m antenna length and 100mW of DC power input to the final stage of amplification. The optimal setup is a class E amplifier with ~95-99% efficiency into a properly grounded 3m base loaded vertical antenna. The antenna will be grossly undersized but you try to compensate with a huge loading coil. In ideal conditions this setup can get you about 0.5km range.

LPFM is a much more significant undertaking and it is not trivial to get an LPFM license. I know because I have one :)

[dredmorbius]

Quoting the FCC link above:

"Carrier Current stations and Campus Radio stations do not require a license to set up and operate."

(Emphasis added.)

<https://www.fcc.gov/media/radio/low-power-radio-general-info...>

[solomonb]

Interesting! The rules for AM carrier current appear to be more similar to the FM rules, that is they are based on field strength readings that result in a roughly 200ft range.

There is probably a bunch of subtlety about where you measure from as your antenna could be quite large.