Two paths, not one
The downlink (repeater to user) is strong: high power and a good antenna. The uplink (user to repeater) is limited by the user, often a handheld with 5 W or less and a poor antenna. So the real coverage is the area where the weak handheld can still be heard, not where the repeater can be heard. A tall site with a powerful repeater and a deaf receiver gives users who hear it and cannot answer.
Choosing the site
A high place with a clear view over the area you want to cover is best: remember the radio horizon of 41 km at 100 m and 71 km at 300 m. Look at the terrain around the site, because a ridge close to the antenna shadows a whole sector. Keep a good distance from other transmitters and use a duplexer and cavity filters to separate transmit and receive; the frequency offset between them depends on the band and on your region (for example 600 kHz on 2 m in many countries).
Check before you build
Place the repeater on the coverage map with its real height above ground, choose the receiver of a handheld (low height, low gain) and read where the signal stays green. Repeat with a mobile antenna on a car. Compare VHF and UHF from the same site: the map shows how the hills change the shadows. Read also how terrain affects coverage.
