6.1 Meter Rule
From The RadioReference Wiki
The 6.1 meter control station rule is found in 47CFR90.119(b). It allows any number of control stations, fixed stations that transmit on repeater input frequencies like mobile stations, using antennas no higher than 6.1 meters (20 feet) above gound or an existing non-antenna structure, to be located anywhere within an area of operations without needing their locations to be listed separately on the station license.