re: the 20', there is the optical horizon, the radio horizon and the geometric horizon.
TV signals are line of sight, maybe a little longer.
If you don't have a lot of test equip., look at what antennas other people have and the pic quality they get on each station, pull the antenna specs, and figure out what antenna might get you what stations to what level of quality.
Since TV stations have an interest in your seeing their pic., write to the station engineers for recommendations.
I thought everyone in FL was rich.