Total posts : 45366
“Can I legally construct a ground plane under the transmitter, as long as it is not connected to it? And if so, does it matter how I ground that?”
Short answer: Sure. If you have enough metal laid out on the roof, it might help. Dunno about grounding it, but I’d think that would be OK, too.
The only other caveat is, as already mentioned, the non-interference rule. If the signal is getting into someone’s audio system or partially blocking an adjacent station, and they complain to where the FCC shows up …. you’re done, at least on that frequency.
I’ve forgotten what your goals are, if you said, couldn’t find same. What do you want to do with your Part 15 station? Feel free to write me off forum if you wish.