What is the maximum ERP for an FM translator at a particular height?

Before we take into consideration protections to other facilities...

Fill-in Translators

For Fill-in FM Translators (where the FM translator's 60 dBu contour is entirely within the 60 dBu contour of the full-service FM station that the translator is rebroadcasting), the maximum ERP is 250 watts and limited to assuring that the 60 dBu contour of the translator remains entirely within the 60 dBu contour of the full-service FM station.  In the case of a fill-in station rebroadcasting an AM station, the 60 dBu contour must remain within the greater of either the 2 mV/m daytime groundwave contour or a 25 mile radius of the AM station, whichever is longer in a particular direction.  

Non Fill-in Translators

For Non Fill-in FM Translators (including all commonly-owned LPFM situations), a method called "maximum height above average terrain" (MHAAT) is used.  When evaluating such proposals, we look at the height above average terrain along 12 radials spaced at 30 degrees apart (e.g. 0, 30, 60, 90 degrees, etc.).    

If the translator is proposing a non-directional antenna, we base the station's MHAAT on the largest of those 12 radials (they are not averaged together like we do in other FM broadcast rules) and we apply the maximum ERP using the chart below. 

If the translator is proposing a directional antenna, each radial is considered individually and the ERP cannot exceed the ERP shown in the chart below at that specific radial.

To help determine the MHAAT along the 12 radials, you can use the REC HAAT Tool.  You need to look for the drop-down labeled Number of Radials: and change it from the default 8 radials to "12 radials (used for non fill-in translators)".  The tool will return the HAATs along the 12 radials and the maximum ERPs for both regions, individually for directional antennas and the maximum ERP for nondirectional antennas.

The actual ERP allowed for particular HAATs will be based on the location of the translator as shown below:

Locations east of the Mississippi River, California south of 40 degrees latitude, Puerto Rico and US Virgin Islands

Locations west of the Mississippi River except California south of 40 degrees north latitude, also  in Alaska, Hawaii, Guam and the CNMI

Intermediate Frequency (+/- 10.6 and 10.8 MHz, +/- 53 and 54 channels)

Even if the maximum ERP is 100 watts or more, the ERP of the translator in all directions will be limited to 99 watts if the translator is located within the following distances to any full-service FM station on a channel that is 10.6 or 10.8 MHz (53 or 54 channels) of the translator's proposed frequency/channel:

FM Translators located near Mexico

Regardless of the maximum ERPs shown above, any translator located within 125 km of the Mexican border is limited to 50 watts. If non-directional, the limit is 50 watts.  If a directional antenna is used, the power is limited to 50 watts along all radials that are within 125 km of the Mexican border but can be more than 50 watts along radials that are not within 125 km of the Mexican border.

In addition, for stations within 125 kilometers of the Mexican border, along the radials that are within 125 km of the Mexican border, the proposed translator's 60 dBu service contour (F 50,50) cannot exceed 8.7 kilometers and the 34 dBu interfering contour (F 50,10) cannot exceed 32 kilometers. 

A translator located in excess of 125 kilometers of the Mexican Border may operate with an ERP exceeding 50 watts in the direction of Mexico provided that the 60 dBu service contour (F 50,50) does not exceed 8.7 kilometers measured from the point that is exactly 125 kilometers from the border.