The Height Above Average Terrain (HAAT) is typically calculated based on the average elevations at 50 evenly-spaced points along 8 radials in 45-degree increments (0 degrees, 45 degrees, 90 degrees, and so on). These 50 elevation points along each radial are first averaged, and then these radial averages are further averaged and compared to the antenna’s radiation center’s height above sea level to determine the HAAT.
The Height Above Average Terrain (HAAT) is typically calculated based on the average elevations at 50 evenly-spaced points along 8 radials in 45-degree increments (0 degrees, 45 degrees, 90 degrees, and so on). These 50 elevation points along each radial are first averaged, and then these radial averages are further averaged and compared to the antenna’s radiation center’s height above sea level to determine the HAAT.
If a station is situated near a large body of water, such as an ocean or one of the Great Lakes, or near the borders of Mexico or Canada, there’s a possibility to adjust the HAAT through a special technical process.
Here’s how to approach this adjustment for each of the 8 individual radials:
- Determine the 34 dBu interfering contour around your site for each radial. If there is any U.S. land area along a radial, even if it’s more than 16 km away, such as an island (even if it’s uninhabited) or deviations in the international border (like along the Rio Grande in Texas), that land must be considered.
- If a specific radial lacks any U.S. land area between 3 and 16 km, you can exclude that radial from consideration. When calculating the average of the remaining radials, divide the sum of the averages by the number of remaining radials. For example, if you’re originally summing up the averages of 8 radials, but you exclude one radial, divide by 7 for the overall average.
- If a radial does contain U.S. land area between 3 and 16 km, determine the elevations at points within the U.S. land area, disregarding water or foreign territory. In this case, you don’t need to calculate elevations at 50 evenly-spaced points along the shortened radial. Instead, look at elevations in 260-meter increments between 3 km and the coastline or border. When calculating the overall average, divide by the number of radials used.
For instance, consider KXRN-LP in Laguna Beach, CA, which is located near the coast without any offshore islands within the 34 dBu interfering contour. Initially, the HAAT along the 8 radials appears as follows:
- Azimuth 0: -119.33 meters
- Azimuth 45: -101.8 meters
- Azimuth 90: -87.41 meters
- Azimuth 135: -14.18 meters
- Azimuth 180: 28 meters
- Azimuth 225: 28 meters
- Azimuth 270: 28 meters
- Azimuth 315: -53.78 meters
The average is -36.56 meters HAAT. However, considering the U.S. land area along certain radials, you can exclude the 180, 225, and 270-degree radials. The 315-degree radial doesn’t qualify for any adjustment as it has a land area between 3 and 16 km along its entire length.
After excluding these radials, you add up the remaining 5 radials and divide by 5, resulting in a new average HAAT of -75.30 meters.
This method, allowed by the rules, can reduce the HAAT, which can be advantageous when the original HAAT exceeds 30 meters, allowing for increased effective radiated power in those cases.
For LPFM stations near the Mexican border, there is still a limit of 50 watts ERP along any radial that includes Mexican land area (including offshore islands) within 125 kilometers of the LPFM station. However, the adjusted ERP can be considered for radials not within 125 km of Mexico when a directional antenna is used.
Refer to FCC Rules §73.313(d)(2) for further details.