There are two different types of translators, fill-in and non fill-in.
A fill-in translator is one that is located entirely within the service contour of the primary station that it is rebroadcasting. Historically, these fill-in translators were intended to provide additional (fill-in) coverage for large full-service broadcast stations in areas that are inside in the service contour but are otherwise blocked due to terrain. In the modern day, fill-in translators are also used to allow a full-service FM station with HD Radio to provide an analog version of one of their HD multicast streams (such as HD2). They are also used by AM broadcast stations in order to provide an FM version of their station's audio. For AM stations, the translator's 60 dBu contour must remain within the AM station's 2 mV/m daytime groundwave contour or within a 25 mile radius from the AM station, whichever is longer in each direction.
The maximum power levels of translators that do not meet that criteria (including most, if not all translators for LPFM stations) are based on heights above average terrain in 12 different directions where directional antennas cannot exceed the power allowed in that particular direction or in the case of nondirectional antennas, the lowest allowed for the 12 different directions. To determine these maximum power levels, use the REC HAAT Tool and select "12 radials" to see the maximum power levels allowed each of the 12 directions and the maximum power allowed for nondirectional antennas.
In all cases, the FM translator's maximum power level in a particular direction may be further restricted to assure that the interfering contour of the translator does not overlap the service contour of the protected station.
