I've scoured the internet looking for this question, but Google has returned no results. So, wouldn't having a radio tower at the poles with the biggest dish cover the entire solar system? (except for a blind "area" near Kerbin, starting just slightly beyond Minmus' orbit, assuming a tower that's 20m high) A taller "tower" (airships anyone?) can reach 20,000m; having a blind spot that's 2.3 million meters high, close enough that geostationary orbits are covered.