i would do everthing i could to avoid mounting a dish on siding....is the roof or ground not an option?....if you MUST do it then i would mount a piece of plywood to the wall and give it a few coats of paint to protect it from rotting....then mount the dish to the plywood....
of course this all depends on how big of a dish we are talking about....im assuming you are talking about a smaller ku-band dish....
