"In standard atmospheric conditions, for an observer with eye level above sea level by 1.70 metres (5 ft 7 in), the horizon is at a distance of about 5 kilometres." I imagine that for 20km you'd have to stick that on a very tall building, or pole. Maybe less tall if on both sides. But still, 20km wireless? My home router can barely go through a wall!

Ubiquity gear can say things about all the above. Longest we managed to connect to an AP was 11,7 KM, on a sea shore region. Not the best connection, but one can have internet and normal stuff.

Silva:

"In standard atmospheric conditions, for an observer with eye level above sea level by 1.70 metres (5 ft 7 in), the horizon is at a distance of about 5 kilometres." I imagine that for 20km you'd have to stick that on a very tall building, or pole. Maybe less tall if on both sides. But still, 20km wireless? My home router can barely go through a wall!

Not so the earth is flat no need for a pole

Ripcord:

Not so the earth is flat no need for a pole

Not even cool enough for a joke mate.