This is due to an agreement with Mexico that predates the LPFM service. Nondirectional LPFM stations are limited to 50 watts within 125 km of Mexico. If you in this "strip zone", your antenna height should be at 42 meters height above average terrain to get the full 50 watts to the largest possible service area. Operating at lower antenna heights could limit your service area. Once you exceed 42 meters HAAT, then you will have to reduce power to meet the 5.6 km service contour limitation.
LPFM stations in the "strip zone" at less than 42 meters HAAT can utilize a directional antenna in order to limit radiation to less than 50 watts in directions that are within 125 km to Mexico while maintaining their otherwise maximum ERP in directions that are more than 125 km from the Mexican border. This change will require a construction permit to be filed and granted first. The application must include the directional field values, which can be obtained by the antenna manufactuer and then "rotated" by the appropriate number of degrees that you are intending to point the antenna at. Studies will be necessary to demonstrate the radials (directions) that are within 125 km of Mexico (you must look at one degree increments) and a demonstration that the antenna proposed will not have an ERP of more than 50 watts along those radials. LPFM stations that are using directional antennas solely for an international agreement will not be required to provide a proof of performance or verification of installation by an independent surveyor. LPFM stations will be protected by FM translators based on the directional pattern of the antenna.
All LPFM stations in the border area must also meet specific restrictions of not having a 60 dBu service contour that extends more than 8.7 km along the radials towards Mexico, nor can they have a 34 dBu interfering contour that exceeds 32 km along the radials towards Mexico.
For more information, see LPFM Near Mexico.
