- Stu WØSTU
Maximum Antenna Structure Height (G1B01)
The 2019-2023 General License question pool asks about the maximum allowable height of antenna structures without requiring special registration or notifications:
G1B01: What is the maximum height above ground to which an antenna structure may be erected without requiring notification to the FAA and registration with the FCC, provided it is not at or near a public use airport?
A. 50 feet
B. 100 feet
C. 200 feet
D. 300 feet
Flying into a radio tower unexpectedly usually makes for a bad day for a pilot and passengers. Amateur radio antennas must not be a hazard to aircraft, so the FCC and the Federal Aviation Administration (FAA) limit the height of an antenna structure to a maximum value. Towers may exceed this maximum value only with formal notification to the FAA and registration with the FCC. With the notification and registration the tower information can be placed on aviation navigational charts and minimum safe altitude values for aircraft may be adjusted accordingly. It is a win-win for the pilots and the antenna owner.
We will come to the specific maximum allowable height in a moment. First, let’s consider a special case to which this question alludes that is detailed not in FCC Part 97 where the Amateur Radio Service rules and regulations reside, but rather in the FAA Code of Federal Regulations, Title 14, Part 77.9(b). You’ve read that one, right?
In distilled form, this FAA regulation says that if your radio station is within 20,000 feet (3.79 miles) of a public use or military airport you must not erect an antenna structure higher than a calculated value that is based upon the distance of the antenna from the runway. This makes some sense generally – if you are really close to a runway you want to keep the antenna quite low. If you’re a couple of miles away from the runway a bit higher antenna is probably OK, but how high can you safely and legally go? The height limit is defined as a sloped imaginary surface up from the runway. (With shorter runways the distance for consideration is 10,000 feet, as noted below.)
Here’s what the FAA part actually states, keeping in mind an antenna tower is “a construction or alteration:”
Any construction or alteration that exceeds an imaginary surface extending outward and upward at any of the following slopes:
(1) 100 to 1 for a horizontal distance of 20,000 ft. from the nearest point of the nearest runway of each airport described in paragraph (d) of this section with its longest runway more than 3,200 ft. in actual length, excluding heliports.
(2) 50 to 1 for a horizontal distance of 10,000 ft. from the nearest point of the nearest runway of each airport described in paragraph (d) of this section with its longest runway no more than 3,200 ft. in actual length, excluding heliports.
(3) 25 to 1 for a horizontal distance of 5,000 ft. from the nearest point of the nearest landing and takeoff area of each heliport described in paragraph (d) of this section.
The graphic below illustrates an example of a station one mile from a longer runway (>3200 feet) and a station one mile from a shorter runway (<3200 feet). Essentially, the antenna tower must rise no more than 1/100 the distance from the tower to the nearest edge of the runway if the runway is greater than 3200 feet long. If the runway is 3200 feet or less, the tower height may extend up to 1/50 the distance to the nearest edge of the runway.
Of course, the reason for the 20,000 feet (3.79 miles) horizontal range within which this calculation must be considered for a longer runway is due to the general location maximum allowable height of an amateur radio antenna structure without notifying the FAA and registering with the FCC… our question of interest.
When you are 20,000 feet from the nearest runway edge your tower may rise to 20,000 x 0.01 = 200 feet. Similarly, for the shorter runway case, if you are 10,000 feet from the runway the tower may rise 10,000 x 0.02 = 200 feet.
With a heliport, the slope defining the maximum antenna height is 25:1, or the antenna may be 1/25 of the distance to the nearest edge of the heliport. At exactly the 5000 feet distance within which this must be considered, the antenna may again rise up to 200 feet high.
So, if you’re not near the airport, 200 feet is your limit without getting entangled in government red tape. If you are near the airport, get out your measuring tape and calculator.
The answer to General Class question G1B01, “What is the maximum height above ground to which an antenna structure may be erected without requiring notification to the FAA and registration with the FCC, provided it is not at or near a public use airport?” is “C. 200 feet.”
-- Stu WØSTU