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Impending Interference .. when to change frequency

R

RF101

A discussion and question for readers.

I know of an LPFM station that eventually will experience interference from a co-channel commercial station when that distant station launches in 2024. The LPFM will experience desirable-to-undersirable signal margins < 20dB. There will be no "official" impact from the LPFM to the new commercial station so the LPFM can stay on its long-appointed channel.

I did find an example of an LPFM changing frequencies to improve signal margins. See KOUV-LP and its latest filing for this LPFM in Vancouver, Washington. Paperwork here:


Fundamental Question: If the LPFM knows of the impending interference does it have to wait until the new interfering station signs on or can it proactively file right now for a major frequency change (> 600 kHz)?
 
Yes it can. Whether they do it now or later, they will have to demonstrate that there will be a reduction in interference. This reduction is normally regarded as "incoming" interference (full power to LPFM). What the FCC would like to see is a comparison of the arriving interfering contours of the full-power (and translator) co- and first-adjacent channel stations at the LPFM transmitter on the current channel, compared with those contours on the proposed channel. If the contour study can show that the interfering contours arriving on the new channel are lower (lower dBu numbers) and that there are fewer interferers or in some cases, it reduces the amount of population inside the LPFM service contour receiving an interfering signal (40 dBu for co-channel or 54 dBu for first-adjacent), then reduced interference can be demonstrated.

For example, on the current channel, there is a co-channel FM station that places a 46.3 dBu interfering contour at the LPFM tower. Another channel, that meets minimum distance separation has a 42.3 dBu interfering contour at the LPFM tower. Since 42.3 dBu is weaker than 46.3 dBu, then it can be demonstrated that the station will experience less interference on the channel.

For outgoing interference from LPFM stations, §73.809 has the qualifications where a new or modified full-power station can file a complaint to shut down an LPFM station for interference. Interference takes place when:
(1) The interfering contour (40 dBu for co-channel or 54 dBu for first adjacent channel) of the LPFM station overlaps the 70 dBu contour of a full-service FM station; or
(2) The interfering contour of the LPFM station reaches the city limits of the community of license of the full-power station.

LPFM stations that have a client relationship with REC Networks (min. $350 lifetime spend) have access to a tool on the myLPFM portal that can do a quick check to see if a different channel could possibly be used for a reduced interference study.

REC has done many of these in the past. The station is always invited to inquire with REC to check on their current situation and what can be done.
[email protected]
 
Many Thanks! Yes, the Vancouver study used the interfering contour's intersection (crossing point) with the LPFM's transmitter site as the measurement point to demonstrate interference reduction. The LPFM was able to reduce interference by 5.1 dB at their transmitter site simply changing channels. KOUV also had to demonstrate (again) a second-adjacent channel interference waiver.
 
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