Total posts : 45366
The Cassini probe around Saturn has less power than an average cell phone, so power out is not the most important thing.
I am using a FM transmitter at approx 6 watts. With a bad antenna design and located in my apartment only about 10 feet above ground, my signal gets out maybe 3/4 of a mile.
With a bad antenna design, on the roof -approx 30 feet above ground- I got out maybe 4 miles. With a tuned antenna the signal went even further and less spurious emissions.
Antenna height is the key. A good antenna and lots of height makes up for very little power.