47 CFR 95.408 - (CB Rule 8) How high may I put my antenna?
(a)Antenna means the radiating system (for transmitting, receiving or both) and the structure holding it up (tower, pole or mast). It also means everything else attached to the radiating system and the structure.
(b) If your antenna is mounted on a hand-held portable unit, none of the following limitations apply.
(c) If your antenna is installed at a fixed location, it (whether receiving, transmitting or both) must comply with either one of the following:
(1) The highest point must not be more than 6.10 meters (20 feet) higher than the highest point of the building or tree on which it is mounted; or
(2) The highest point must not be more than 18.3 meters (60 feet) above the ground.
(d) If your CB station is located near an airport, and if your antenna structure is more than 6.1 meters (20 feet) high, you may have to obey additional restrictions. The highest point of your antenna must not exceed one meter above the airport elevation for every hundred meters of distance from the nearest point of the nearest airport runway. Differences in ground elevation between your antenna and the airport runway may complicate this formula. If your CB station is near an airport, see http://wireless2.fcc.gov/UlsApp/AsrSearch/towairSearch.jsp to help you figure the maximum allowable height of your antenna. Consult part 17 of this chapter for more information.