HAAT is normally measued based on the average elevations at 50 evenly-spaced points along 8 radials in 45 degree increments (0 degrees, 45 degrees, 90 degrees, etc.). Along each radial, the 50 elevation points are then averaged and then those averages are averaged again and compared with the radiation center of the antenna above sea level to calculate the HAAT.
If a station is close to a large body of water (such as the ocean or one of the Great Lakes) or close to Mexico or Canada, then you may be able to adjust your HAAT with a special technical showing.
You need to look at each of the individual 8 radials.
First thing you need to do is determine the 34 dBu interfering contour around your site. If along the radial there is any US land area along that radial (even more than 16 km away) such as an island (even if it is uninhabited) or deviations in the international border (such as along the Rio Grande in Texas), that land must also be considered.
If a certain radial does not have any US land area between 3 and 16 km, you can completely exclude that radial from consideration. If you remove that radial, then when determining the average of the remaining radials, you must divide the averages of the remaining radials by the number of remaining radials. For example, you normally would add up the averages of the 8 radials and then divide by 8 to determine the overall average. If you are only needing to add up 7 radials, then you need to divide by 7 to determine the overall average.
If a certain radial does have US land area over a portion of the area between 3 and 16 km, then you only need to determine the elevations at points within the US land area and disregard the water or foreign territory. You do not need to determine elevations at 50 points along the shortened radials. Instead, look at the elevations in 260 meter increments between 3 km and the coast or border. When you reach the point of calculating the overall average, you will still divide it by the number of radials used. For example, if the 0 degree radial only has qualified land area between 3 and 9 km, then you will take the recalcuated average along that radial and then add the averages of the 7 other "full" radials and still divide by 8 (as opposed to 7.5).
Let's take an example, KXRN-LP in Laguna Beach, CA. This station is located right near the coast. There are no offshore islands within the 34 dBu interfering contour. By default, their HAAT along the 8 radials looks like this:
| Azimuth | HAAT |
|---|---|
| 0 | -119.33 |
| 45 | -101.8 |
| 90 | -87.41 |
| 135 | -14.18 |
| 180 | 28 |
| 225 | 28 |
| 270 | 28 |
| 315 | -53.78 |
This would come out to an average of -36.56 meters HAAT. However, from that site:
- Land area along the 180 degree radial only extends to 0.2 kilometers from the transmitter site.
- Land area along the 225 degree radial only extends to 0.15 kilometers from the transmitter site.
- Land area along the 270 degree radial only extends to 0.2 kilometers from the transmitter site.
- Land area along the 315 degree radial extends to 1.1 kilometers from the transmitter site, goes over the ocean, but because of the shape of the coastline, intersects the coastline again at 2.2 kilometers from the transmitter site and remains over land area out to 16 kilometers from the transmitter site.
In this case, we can totally exclude the 180, 225 and 270 degree radials. The 315 degree radial is not eligible for any adjustment (as there is land area between the entirety of 3 to 16 kilometers).
| Azimuth | HAAT |
|---|---|
| 0 | -119.33 |
| 45 | -101.8 |
| 90 | -87.41 |
| 135 | -14.18 |
| 315 | -53.78 |
Therefore, we will add the remaining 5 radials and divide by 5.
(-119.33 + -101.8 + -87.41 + -14.18 + -53.78) = -376.5
(-376.5 / 5) = -75.3
Therefore, in this case, while the HAAT tools at REC and the FCC may show KXRN-LP's HAAT at -35.56 meters, using this method allowed by the rules, we are able to reduce their HAAT to -75.30 meters. In this specific example, it provides no benefit to KXRN-LP as all HAAT below 30 meters is considered 30 meters, but in other cases where the originally calculated HAAT is over 30 meters and performing this additional calculation will result in a lower HAAT; this will allow the station to specify operation at the lower HAAT, thus resulting in an increased effective radiated power.
LPFM stations located near the Mexican border are still limited to a maximum of 50 watts ERP along any radial (taking into consideration the full 360 radials) that include any Mexican land area (including offshore islands) within 125 kilometers of the LPFM station. However, the adjusted ERP can be used to consider a maximum ERP greater than 50 watts along radials not within 125 km of Mexico if a directional antenna is used.
For more information, see FCC Rules ยง73.313(d)(2).
