When we look at FM translators, there are really two types of FM translators, each with their own limitaitons.
Fill-in Stations
FM translators can be used to "fill-in" a service area of an AM or FM station. In these cases, the 60 dBu protected service contour of the FM translator must remain entirely inside a designated area:
- For translators providing fill-in service for FM stations, the contour of the translator must remain within the protected service contour (60, 57 or 54 dBu) of the primary FM station.
- For translators providing fill-in service for AM stations, the contour of the translator must remain within the 2 millivolt per meter (mV/m) daytime contour of the AM station. In directions where the 2 mV/m contour does not extend 25 miles from the AM station, the 60 dBu contour of the FM translator can extend to 25 miles from the AM station. (This rule was implemented in March, 2017.)
Information on a station's contour is available at REC's fccdata.org.
Translators operating as a fill-in service are permitted to operate up to 250 watts ERP at any height above average terrain (HAAT) as long as the service contour of the translator remains within the service contour of the primary station as defined above and that the translator properly protects other broadcast stations, FM translators and LPFM stations.
Translators to Extend Coverage
These are translators that have a protected service cotnour which goes outside of the protected service contour of the primary FM station being rebroadcast. Translators can not be used to extend AM stations.
These translators are normally owned by non-commercial broadcasters, speculators and municipal governments. Non fill-in translators for commercial FM stations can not be commonly owned by the owner of the primary station and there are restrictions on the financial relationship between the owner of the primary commercial FM station and the translator owner.
These types of translators are used primarily by non-commercial educational broadcasters such as statewide public radio networks and chain religious broadcasters such as Educational Media Foundation (K-Love/Air1) and Calvary Chapel of Twin Falls.
Translators used to rebroadcast LPFM stations (regardless of the cross-ownership status) would fall under this type of translator.
ERP for these translators are limited based on maximum height above average terrain, which is a different formula than that used for LPFM. FM translators located east of the Mississippi and in California south of 40 degrees latitude are pretty much limited to a maximum of 250 watts at 32 meters HAAT. FM translators in other areas of the west are limited to 250 watts at 107 meters HAAT. The actual power specification is described in two tables in §74.1235(b) of the rules:
Translators east of the Mississippi or in CA south of 40 degrees latitude:
| Radial HAAT (meters) | Maximum ERP (MERP in watts) |
|---|---|
| Less than or equal to 32 | 250 |
| 33 to 39 | 170 |
| 40 to 47 | 120 |
| 48 to 57 | 80 |
| 58 to 68 | 55 |
| 69 to 82 | 38 |
| 83 to 96 | 27 |
| 97 to 115 | 19 |
| 116 to 140 | 13 |
| Greater than or equal to 141 | 10 |
Translators west of the Mississippi except in CA south of 40 degrees latitude:
| Radial HAAT (meters) | Maximum ERP (MERP in watts) |
|---|---|
| Less than or equal to 107 | 250 |
| 108 to 118 | 205 |
| 119 to 130 | 170 |
| 131 to 144 | 140 |
| 145 to 157 | 115 |
| 158 to 173 | 92 |
| 174 to 192 | 75 |
| 193 to 212 | 62 |
| 213 to 235 | 50 |
| 236 to 260 | 41 |
| 261 to 285 | 34 |
| 286 to 310 | 28 |
| 311 to 345 | 23 |
| 346 to 380 | 19 |
| 381 to 425 | 15.5 |
| 426 to 480 | 13 |
| 481 to 540 | 11 |
| Greater than or equal to 541 | 10 |
How to compute Maximum HAAT
To find the maximum HAAT, you can use the FCC's HAAT Tool but you will need to change the number of radials to 12. Ignore the overall height above average terrain, but instead look at the HAATs for the indvidual radials.
For a translator with a non-directional antenna, take the highest HAAT of the 12 radials and compare it to the chart above. That is your maximum ERP in all directions. Translators with directional antennas may use all 12 radials in their pattern design but in no case shall the ERP along any of the 12 radials exceed the designated ERP for the HAAT shown along that radial.
Here's an example
This is for a location in the foothills of the San Fernando Valley in Southern California. The elevation is 342 meters above ground level but because of the mountain range north and west of the proposed site, the 8-radial HAAT (which we would use for LPFM) is -66 (negative 66) meters HAAT. If this was an LPFM station going in there, it would be allowed to operate a full 100 watts. For this example, we will put the radiation center of the antenna 10 meters above ground level.
Here are the HAAT measurements for each of the 12 radials and their maximum ERPs in each direction:
| Radial | HAAT | Max. ERP |
| 0º | -231 meters | 250 watts |
| 30º | -197 meters | 250 watts |
| 60º | -128 meters | 250 watts |
| 90º | 42 meters | 120 watts |
| 120º | 101 meters | 19 watts |
| 150º | 113 meters | 19 watts |
| 180º | 55 meters | 80 watts |
| 210º | 64 meters | 55 watts |
| 240º | -75 meters | 250 watts |
| 270º | -10 meters | 250 watts |
| 300º | -214 meters | 250 watts |
| 330º | -289 meters | 250 watts |
As we mentioned, there are mountain ranges to the north and west which explains the very low HAATs and 250 watt maximums in those directions. However, because the site overlooks the San Fernando Valley to the east and south, you will see that the location has much higher HAATs in those directions.
When we look at this site, the 150 degree radial has the highest HAAT calculation. That is our maximum HAAT.
Even though a non-directional LPFM of 100 watts (or potential 250 watts) can be placed at this site, A non-directional non-fill-in FM translator at this site would only be limited to 19 watts ERP. A directional antenna can be used to operate 250 watts ERP along all radials between 240 and 60 degrees, but it would make no sense booming 250 watts into a mountain.
How is this different from LPFM?
LPFM uses the 8-radial average calculation for HAAT. This is similar to those used in commercial rules to determine ERP for stations located higher than the maximum HAAT for a particular station class.
At this same location, let's look at the 8 radial HAAT:
0 degrees: -231.4m
45 degrees: -263.3m
90 degrees: 41.6m
135 degrees: 118.5m
180 degrees: 55.2m
225 degrees: 33.9m
270 degrees: -10.1m
315 degrees: -268.7m
To determine the HAAT, take the sum of all 8 radials and divide by 8:
-231.4 - 263.3 + 41.6 + 118.5 + 55.2 + 33.9 - 10.1 - 268.7 = -524.3
-524.3 / 8 = -65.5375
rounds down to -66 meters HAAT.
In this way, LPFM has an advantage over translators.
