73.809
Part 73, Subpart G, §73.809 is a regulation that addresses the procedures that would apply when a full-power FM station applies for a facility that would result in the LPFM station causing interference to the new or modified full-service FM facility. This is also known as "The Encroachment Rule". This rule also covers interference caused by an LPFM station to broadcast or non-broadcast services because of a faulty transmitter.
Interference due to a new or modified full-power FM facility
Secondary nature of LPFM service
Low power FM stations and FM translators are considered "secondary" services. Full-service (full-power) FM stations are considered as "primary". This means that a primary FM facility can request a facility without the need to protect any of the secondary facilities. Depending on the full-power station's new location, power and terrain, the LPFM station may experience no difference, a diminished service area in one portion of town or very substantial interference rendering the LPFM station useless. In addition, the operation of the LPFM station may create harmful interference to the new or modified full-power FM operation. The latter is where this rule comes into place.
How interference from the LPFM station is determined
Using broadcast planning software, the representative of the full power station will create a map showing the interfering contours of the LPFM station and comparing them with the service contours of the protected full-power FM facility.
Criteria for Interference
For full-power application activity, the FCC requires one of the following situations before it can force an LPFM station to shut down:
Community coverage contour interference
Actionable interference is deemed to take place if the 70 dBu community coverage contour of the full-service FM station (also known as the "city grade" contour) is overlapped by the 40 dBu interfering contour of the LPFM station operating on the same channel or the LPFM 54 dBu interfering contour on a first-adjacent channel.
Interference within the community of license
Actionable interference is deemed to take place if the LPFM's co-channel 40 dBu interfering contour overlaps into a portion of the community of license of the affected full-service FM station. If the LPFM is on a first adjacent channel, then it is the LPFM's 54 dBu interfering contour.
Contour overlap in the community community of license
Actionable interference is deemed to take place if the LPFM's co-channel 40 dBu interfering contour overlaps into the 60 dBu protected contour within the community of license of the affected full-service station. If the LPFM is on a first-adjacent channel, then its the LPFM's 54 dBu interfering contour.
Interference remediation process
If the contour study demonstrates that the LPFM station would be causing prohibited interference, the full-power station can file a complaint with the FCC Media Bureau and must serve a copy of the complaint to the LPFM station. Once the LPFM station receives the complaint (either from the FCC or from the affected full-power station), they are to cease operations (shut down) within 24 hours. The LPFM station must take measures to eliminate the interference. This could include changing to a different channel if another channel is available to move to.
The full-service FM broadcast station is required to cooperate with the LPFM station during the remediation process. If the FCC determines that the complaining station was not cooperative, then the FCC can absolve the LPFM station of the complaint.
Spurious Emissions
A properly designed and certified LPFM transmitter in good working order, will keep the signal on the LPFM station's assigned channel/frequency. Sometimes, due to either an issue with the transmitter or improper installation, the transmitter may also broadcast additional signals on undesired frequencies. This is known as spurious emissions (or "spurs"). Transmissions from spurs are not just limited to the FM broadcast band and over the years, there has been numerous reports that a defective transmitter will place a signal on a frequency in the spectrum adjacent to the FM band on frequencies that are used for aeronautical navigation or communications.
If the FCC notifies the LPFM station that their station is causing spurious emissions, it is very urgent that the LPFM transmitter be shut off immediately. There is no 24 hour waiting period as the transmissions may cause danger to the safety of life. During the time the station is suspended, it may make short test transmissions to test the repairs to the transmitter.
Post suspension report
If an LPFM station operation is resumed at the authority of the FCC for any interference reason, the LPFM needs to follow up with a report stating details of the interference, the source of the interference and what remedial actions took place.