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Line of Sight Range
Low-frequency radio signals such as those from AM radio stations bounce against the Earth̵7;s ionosphere; this effect lets these broadcasts travel distances up to thousands of miles. High-frequency television and FM radio signals, however, get no help from the ionosphere; they pass through it. Radio engineers call these broadcasts line-of-sight -- if you can see the antenna, you can receive the broadcast. The signals travel in a straight line; at a certain point the curvature of the Earth prevents them from going any farther. These high-frequency signals travel as far as the horizon.
Ideal Versus Real-World Range
If the Earth were a perfectly smooth sphere, figuring the broadcast distance for radio waves would be easy. You calculate the distance to the horizon using the Earth̵7;s radius and the antenna̵7;s height above the ground. Broadcast companies, however, must frequently situate antennas near mountains, valleys, buildings and other variations in the landscape. Though valleys improve an antenna̵7;s range, large objects of more than 3 kilometers reduce transmission distance. As a consequence, radio and TV stations locate antennas as high as possible relative to other surrounding tall structures.
Calculating AAT
To calculate the height above average terrain, get a map of the local area that includes elevations, and draw a circle centered on the antenna. The circle should have a radius of 16 kilometers. Draw eight or more evenly spaced radial lines starting with true north and work your way around the circle. Read the elevations of 50 evenly spaced points along each radial line between 3 and 16 kilometers and find the average. Take the average of the results for all the radial lines.
AAT Regulations
The Federal Communications Commission regulates an antenna̵7;s effective radiated power, or ERP, and its height above average terrain. Title 47 of the Code of Federal Regulations classifies radio stations according to their broadcast signal power. Generally, the minimum AAT height for an antenna increases along with a station̵7;s power class. Low-power stations can get by with a shorter antenna, though the FCC puts strict requirements on antenna heights for all station classes.