You pretty much can't hide a transmitter. If you're broadcasting with illegal power levels, it's pretty trivial to walk around with a directional antenna and triangulate the source transmitter. If you're curious, look up "radio foxhunting". Some people do it as a hobby.
Your only hope is to set up an unattended transmitter, connect to it through the internet, and spend your effort obscuring that connection. Creating an untraceable connection through the internet is a lot simpler.
Aside: you could actually obscure the location of your transmitter by building multiple of them and doing some tricks with phase and frequency. Ultimately you're still trying to hide a guy in a crowd screaming into a megaphone. They'll find you eventually, you can only make it harder for them.