Updated June 23, 2023.
Additional procedures apply for LPFM stations that are physically located near the Mexican border. In accordance with the United States/Mexico Agreement on FM Broadcasting, all proposed broadcast facilities within 320 km of the common border are reported to the other country's administration (FCC for the USA and IFT for Mexico). When reading the US/Mexico Agreement, you may see the term "LPFM". This document pre-dates the creation of the Low Power FM service in 2000 so for the purposes of that document, "LPFM" refers to FM translators, FM boosters, Class D FM stations and since its creation in 2000, the Low Power FM broadcast service.
Stations more than 125 km (77.6 miles) from the Mexican border
Low Power FM broadcast stations that are more than 125 kilometers from Mexico are permitted to operate their maximum allowed power (up to 100 watts, depending on height above average terrain (HAAT)). The only restriction on these stations is that the 60 dBu service contour of the proposed LPFM station may not place a 60 dBu service contour large enough that the contour comes within 116.3 kilometers of the Mexican border. In this case, a directional antenna can be used to limit radiation towards the Mexican border.
Stations within 125 km (77.6 miles) of the Mexican border
The Mexico "Strip Zone" is an area that includes all points that are within 125 kilometers of the Mexican border or any of its outlying islands.
By default, the FCC only authorizes 50 watts maximum for LP-100 stations using nondirectional antennas located within 125 km of the Mexican border.
This is in accordance with the United States/Mexico Agreement on FM Broadcasting that was drafted prior to the creation of the LPFM service. Even though the agreement does address "LPFM" stations, this also addresses translators.
In accordance with the Mexico agreement, all LPFM, FM translator and FM booster stations operating within 125 km of the common border with Mexico will be permitted to radiate no more than 50 watts in the direction of Mexico, which for many LPFM stations can be seen as a reduction in coverage. For LPFM stations located at sites more than 42 meters HAAT, the station will be limited to less than 50 watts like if the station was anywhere in the country and therefore, these stations would be able to enjoy their full coverage subject to some additional restrictions shown below.
In addition to the 50 watt cap, LPFM stations within 125 km of Mexico also have the following limitations:
- The 60 dBu protected service contour may not extend more than 8.7 kilometers in the direction of Mexico.
- The 34 dBu interfering contour may not extend more than 32 kilometers in the direction of Mexico.
Nondirectional antennas
If an LPFM station proposes a nondirectional antenna and is located at a HAAT of less than 42 meters, the LPFM station will be limited to 50 watts in all directions.
Directional antennas
A more complex approach that is now available for LPFM stations located less than 42 meters HAAT is to utilize a directional antenna. The directional antenna must be designed in a manner that it can radiate up to the full available LPFM power in all directions except for any directions where if a straight line is drawn from the transmitter site to the Mexican border and outlying islands in Mexican territory, the measured distance is less than 125 kilometers.
In the diagram above, we placed a hypothetical station well within 125 kilometers of the Mexican border. We then draw a 125 kilometer circle around the transmitter site. From the transmitter site to each point where the circle intersects the border, we plot a straight line from the proposed station to the point where the 125 km circle intersects the border. We then determine the angle of each of those lines (as measured from the proposed LPFM station). Those are our outer boundries of the restricted zone where power is limited to 50 watts. In the example diagram, we plotted two lines from the transmitter site to where the 125 km circle intersects with the border and determined their bearings to be at 127 degrees and 256 degrees. It will mean that in directions between bearings 127 and 256 degrees, the directional antenna must be designed to limit radiation to 50 watts or less. In all other directions (from 256 degrees, all the way around to the north and back to 127 degrees), the directional antenna can radiate up to 100 watts (or the fullest power allowed based on HAAT).
When proposing a directional antenna, the application must include the field values around the entire antenna along at least 36 equally-spaced bearings (every 10 degrees from 0 to 350). These field values are available from your antenna's manufacturer. Even though only 36 is required, it's best to include as many field values as possible (such as every 5 degrees or every 1 degree), especially if the radiated ERP along a particular bearing close or inside the restricted zone is very close to 50 watts.
To determine ERP along a particular bearing based on a field value provided by the antenna manufacturer: take the maximum ERP allowed for this LPFM station, and then multiply by the field value, squared. Example: 100 watts ERP and along a particular bearing, the antenna has a field value of 0.708. 100 * (0.7082) = 50.1264 watts. REC can provide assistance for LPFM stations wanting to propose directional antennas.
For this specific station, we can utilize a Kathrein-Scala FMVMP vertical dipole. We then can use the directional pattern and tabulations (numbers) provided by Scala. For this specific installation, we will "rotate", or point the antenna to 10 degrees:
Based on this chart, we can see that the power along the radials between 130 and 250 degrees are below 50 watts (0.0500 kW). Actually, even the values along the 120 and 260 degree radials are under 50 watts. Since we have to limit radiation to under 50 watts between 127 and 256 degrees, this antenna would work. This antenna is currently $623 at SCMS (as of 4/25/20).
If a directional antenna is being utilized in LPFM strictly for the purpose of an international agreement, such as the Strip Zone application we have depicted here, the FCC will not require a proof of performance nor verification of the installation by a surveyor.
Assistance from a consultant, like REC is highly recommended
When evaluating these limitations, we look at only the bearings where there is any Mexican territory within 125 km along that azimuth. LPFM stations in the strip zone should get professional services such as from REC to assure that the proposed facility meets the treaty requirements. For bearings that are more than 125 km from the border (as well as stations more than 125km from the border), the 60 dBu contour can not exceed 8.7 km from the point that radial reaches 125 km from the border.
Expect delays on application grants
LPFM applicants near Mexico will have a delay as the FCC goes through the Mexican notification process. At a certain point in the process, the FCC will grant the construction permit with a condition that Mexican concurrence is still pending. Construction permits can be tolled (extended) for the length of the overall concurrence process.
So far, no LPFM station from either the 2000/2001 or the 2013 windows has ever been denied as a result of an objection from Mexico.
Reminder about LPFM secondary status
In addition to being secondary to FM broadcast stations in the United States, LPFM is also secondary to foreign FM full power stations. Just like there is a notification process when the US places a station within the border area (within 320km of the border area), Mexico has to do the same. Although we are not aware of it happening yet, a change in Mexican facilities may cause displacement or additional interference.
