6.1 Meter Rule

From The RadioReference Wiki

47CFR90.119 (b) is commonly referred to as the 6.1 meter control station rule. 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 ground or an existing non-antenna structure, to be located anywhere within an area of operations specified on the station license without needing their locations to be listed separately on the station license.