When reviewing the minimum distance spacing required between stations, the FCC looks at both outward (LPFM interfering with full-power) and inward (full-power interfering with LPFM). The distances are based on the interfering contour of the interfering station and the service contour of the protected station.
Let's use a domestic Class A FM station as an example. Under §73.807(a) of the rules, an LP-100 station must protect a Class-A station:
- 67 km on co-channel,
- 56 km on first-adjacent channel and
- 29 km on second-adjacent channel (and third-adjacent radio reading services).
The Class A station full facilities are based on 6,000 watts at 100 meters height above average terrain (HAAT). This means the following:
- The 60 dBu service/protected contour is 28.3 km,
- The 40 dBu interfering contour (for co-channel) is 86.7 km,
- The 54 dBu interfering contour (for first-adjacent channel) is 43.7 km and
- The 100 dBu intefering contour (for second and third-djacent channel) is 2.8 km.
Likewise for an LP-100 station:
- The 60 dBu service contour is 5.6 km,
- The 40 dBu intefering contour is 18.6 km,
- The 54 dBu intefering contour is 8.0 km and
- The 100 dBu interfering contour is 0.7 km.
For LP-250:
- The 60 dBu service contour is 7.1 km,
- The 40 dBu interfering contour is 23.8 km,
- The 54 dBu interfering contour is 10.1 km and
- The 100 dBu interfering contour is 1.1 km.
So, to determine second-adjacent channel spacing, we need to add a 60 dBu service contour and a 100 dBu intefering contour to determine the spacing.
Let's first look at the full-power interfering with the LPFM. If we add interfering contour of the Class A of 2.8 km with the service contour of the LP-100 station at 5.6 km, we get 8.4 km. But the issue here is not protecting the LPFM from second adjacent channel inteference from a full-power but instead, the interference that an LPFM can cause to the service contour of a full-power station.
Therefore, we must look the other way. If we add the service/protected contour of the Class A (28.3 km) with the interfering contour of the LP-100 (0.7 km), we get 29.0 km, which is the §73.807 minimum distance.
For LP-250, the LPFM interference contour will be slightly larger (1.1 km instead of 0.7 km). Therefore, we will add 1.1 to the 28.3 km service contour which comes out to 29.4. Normally when determining minimum separation, it is fairly standard to always round up. Therefore, 29.4 would become 30 km, the minimum distance that REC proposed in RM-11749 and what the FCC proposed in 2011.
