This myth culminates from a general misunderstanding of the characteristics of the LPFM service class and the role that height above average terrain (HAAT) plays into determining the size of the service contour of an LPFM station.
ยง73.811 defines the maximum facility of an LPFM station as 100 watts effective radiated power (ERP) at 30 meters (100 feet) HAAT. It further states that when the HAAT exceeds 30 meters, the ERP must be adjusted to what would result in a 60 dBu service contour of 5.6 meters (3.5 miles). Finally, it states that no LPFM facility will be authorized at a HAAT that exceeds 450 meters.
HAAT along a certain direction (radial) is calculated by looking at the elevation at 50 points along the radial between 3 and 16 km (2 and 10 miles) and taking the average of each of those elevations and comparing that with the elevation above mean sea level (AMSL) of the radiation center of the LPFM antenna. For example, along a radial, if the average elevation at the 50 points is 40 meters and the radiation center of the LPFM antenna is at 60 meters AMSL, then the HAAT along that radial is 20 meters.
To calculate the HAAT for a proposed facility, the HAATs along the 0, 45, 90, 135, 180, 225, 270 and 315 degree radials are averaged and then compared with the AMSL height of the radiation center of the antenna. If the overall HAAT exceeds 30 meters, then the FCC or REC Curves Calculator is used to determine the maximum ERP for the station based on a service contour size of 5.6 kilometers.
If the overall HAAT at ground level is less than 0 meters, the station can have a radiation center higher than 30 meters above ground level and still have 100 watts. For example, if the HAAT at ground level is -10 meters HAAT and the radiation center is 40 meters above ground level, the LPFM would still be able to operate at the full 100 watts. If the HAAT at ground level is 0 meters, the 40 meter tower would exceed 30 meters HAAT and power would need to be reduced. In a case where the radiation center is at 40 meters HAAT, then the LPFM would be limited to 56 watts ERP.
Antenna heights may be restricted by the Federal Aviation Administration and in those cases, it would require a study to be conducted to assure that there is no hazard to navigation and based on the results, would require antenna structure registration with the FCC. In areas more than 8 kilometers (5 miles) from any airport runway, FAA notification and structure registration is required to any tower structure that exceeds 60.1 meters (200 feet). In areas within 8 kilometers of an airport runway, a glide slope method is used to determine the maximum height of a tower structure where notification and antenna structure registration is not required.
LPFM applicants should also be mindful of local ordinances that may restrict the construction of towers at certain heights or may require a permitting process or a hearing before a zoning administration.
FACT: LPFM tower structures are not limited to 100 feet above ground level or lower. Additional restrictions to tower height may apply based on proximity to airports and local ordinances.
