LPFM stations are subject to power limitations set by the Federal Communications Commission (FCC) to ensure the efficient use of available spectrum and prevent interference with other stations. The specific power allocation for an LPFM station, including the 50-watt limitation in the mentioned areas, is determined by various technical and regulatory factors, including the availability of frequencies, the proximity to other stations, and the FCC's licensing rules. These power limits are established to maintain a balance between maximizing spectrum utilization and minimizing interference. If you have further questions about your station's power allocation, you may consider reaching out to the FCC or consulting with a broadcasting expert for more detailed information specific to your case.
This limitation on LPFM station power to 50 watts in specific areas like San Diego, Tucson, El Paso, Brownsville, and Yuma is a result of a longstanding agreement with Mexico that predates the LPFM service. In these regions, non-directional LPFM stations are restricted to 50 watts within 125 kilometers of the Mexican border. To maximize the service area while maintaining this power limit, LPFM stations operating in this "strip zone" are advised to have an antenna height of 42 meters above average terrain (HAAT). Operating at lower antenna heights may reduce the station's coverage area. However, exceeding 42 meters HAAT requires reducing power to meet the 5.6-kilometer service contour limitation.
For LPFM stations within the "strip zone" and below 42 meters HAAT, there is an option to utilize directional antennas. This allows them to limit radiation to less than 50 watts in directions within 125 kilometers of Mexico while maintaining maximum Effective Radiated Power (ERP) in directions farther from the Mexican border. To make this change, LPFM stations need to file and obtain a construction permit, including the directional field values, which can be obtained from the antenna manufacturer and adjusted to the appropriate number of degrees for the intended antenna direction. Detailed studies are necessary to demonstrate the directions within 125 kilometers of Mexico and to ensure that the proposed antenna will not exceed 50 watts ERP along those directions. Stations using directional antennas solely for international agreement compliance are not required to provide proof of performance or independent surveyor verification. Additionally, FM translators will protect LPFM stations based on the directional pattern of the antenna.
In the border area, all LPFM stations must also adhere to specific restrictions, including not having a 60 dBu service contour extending more than 8.7 kilometers along the directions toward Mexico and ensuring that the 34 dBu interfering contour does not exceed 32 kilometers along the directions toward Mexico.