Normally, Height Above Average Terrain (HAAT) is calculated based on the average elevations at 50 evenly spaced points along 8 radials in 45-degree increments (0 degrees, 45 degrees, 90 degrees, etc.). These elevations are averaged, and the resulting averages are then compared with the radiation center of the antenna above sea level to determine the HAAT.
Normally, Height Above Average Terrain (HAAT) is calculated based on the average elevations at 50 evenly spaced points along 8 radials in 45-degree increments (0 degrees, 45 degrees, 90 degrees, etc.). These elevations are averaged, and the resulting averages are then compared with the radiation center of the antenna above sea level to determine the HAAT.
However, if a station is in close proximity to a large body of water (such as an ocean or one of the Great Lakes) or near the border of Mexico or Canada, there is a special technical adjustment that may be applied to potentially lower the HAAT and increase the Effective Radiated Power (ERP).
To make this adjustment, you need to evaluate each of the individual 8 radials:
Determine the 34 dBu interfering contour around your site.
If there is any U.S. land area along a radial, even if it’s more than 16 km away, such as an island or deviations in the international border (e.g., along the Rio Grande in Texas), that land must be considered.
If a radial does not have 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 by the number of remaining radials.
If a radial has U.S. land area within 3 to 16 km, determine the elevations only within that U.S. land area, disregarding water or foreign territory. Do not calculate elevations at 50 points along the shortened radials. Instead, look at elevations in 260-meter increments between 3 km and the coast or border. When calculating the overall average, divide it by the number of radials used.
Here’s an example:
By default, their HAAT along the 8 radials looks like this:
0 degrees: -119.33 meters
45 degrees: -101.8 meters
90 degrees: -87.41 meters
135 degrees: -14.18 meters
180 degrees: 28 meters
225 degrees: 28 meters
270 degrees: 28 meters
315 degrees: -53.78 meters
The average of these values is -36.56 meters HAAT. However, when considering land areas along the radials, certain radials may be excluded, and calculations are adjusted accordingly. In this example, some radials are excluded, resulting in an adjusted HAAT of -75.30 meters.
This adjusted HAAT can impact ERP calculations, allowing stations to specify operation at the lower HAAT and potentially increase their effective radiated power, although this may not benefit all stations.
LPFM stations near the Mexican border still have a maximum of 50 watts ERP along radials that include any Mexican land area within 125 kilometers of the station. However, the adjusted ERP can be applied to radials not within 125 km of Mexico if a directional antenna is used, potentially allowing for greater ERP.
For more details, refer to FCC Rules §73.313(d)(2).