FAQ: How come RM-11749 recommends the same distance separations for LP-250 stations as it does for LP-100?
The similar distance separations for LP-250 are in respect to co-channel and first-adjacent channel and only towards full-power FM stations within the United States.
This is possible because when the FCC created LPFM in 2000, they included a "buffer zone" around "full-facility" service contour of the full-power station class.
For example, the maximum facility for a full-power Class A FM station is 6,000 watts at 100 meters height above average terrain. This creates a service contour of 28.3 kilometers from the transmitter site over flat terrain.
In addition to that 28.3 km, the FCC added another 20 km to the service contour. The additional "buffer zone" was intended to allow for full-power FM stations to be allowed to relocate by causing minimal impact to LPFM stations. So in the case of the Class-A station, we now have a hypothetical 48.3 km service contour.
Include the interference contours of an LP-100 facility which is 18.6 km for co-channel and 8.0 km for first-adjacent channel.
For co-channel, if you add 28.3 for the service contour, 20.0 for the buffer-zone and 18.6 for the LPFM interference contour, you get 66.9 which rounds up to 67 km, the minimum distance separation for a class-A. If the FCC never enacted the buffer zone, LPFM stations could be placed as close as 47 km to Class-A stations (but the number of inward interference issues would have skyrocketed).
When the Local Community Radio Act was passed in 2011, section 3(b) of the LCRA set in stone the minimum distance separations in effect at the time of the legislation's passage, which includes the buffer zone.
When the FCC proposed LP-250 in 2012, it proposed the same co-channel and first-adjacent channel distance separations as LP-100. This was an acknolwedgement that the FCC was willing to break into the buffer zone.
Back to the same LPFM to Class A example, at LP-250, the interference contour of the LPFM is now 23.8 km for co-channel and 10.1 km for first-adjacent channel. If we were to include the full 20 km buffer zone, the minimum distance separation for a Class A would increase from 67 to 72 km. Instead, the FCC went the other way by allowing the additional interference contour to be absorbed by the buffer zone.
Therefore, in respect to Class A on co-channel, the buffer zone has been reduced from 20 km to 14.8 km and for first-adjacent channel, 17.9 km.