73.870: Difference between revisions

==== Changing to a "major" channel  ====

In the case of interference either received by or caused by the LPFM station and none of the "minor" change channels are available or otherwise suitable, the LPFM station can change to any of the 100 FM channels (88.1~107.9).  To change to a channel that does not meet the minor change requirements, a contour study must be conducted to demonstrate reduced interference.  This means that an LPFM must compare the potential interference on the current channel with the potential interference on the proposed channel.  This is different than the rule for FM translators which only requires a showing of interference on the current channel but no demonstration of improvement on the proposed channel.

The most common form of LPFM interference is where a change by another facility increases the "noise floor" in a certain direction from the LPFM station.  In some cases, this interference will cause an LPFM station to be completely obliterated in a significant part of their service area.  When reviewing these cases, we look at the interfering contours of other stations on co-channel and the first-adjacent channels.  We look at where the 40 dBu interfering contour (for co-channel interfering stations) or the 54 dBu interfering contour (for first-adjacent channel interfering stations) appear in respect to the LPFM station's 60 dBu service contour.  We also look at the field strength of the interfering contour of those stations at the LPFM transmitter site.  We do the same thing for the proposed channel.  if, by looking at the 40 or 54 dBu interfering contours, we can show that on the new channel, there is more area of the 60 dBu LPFM service contour that is not overlapped by an interfering contour, then we have a showing that interference would be reduced.  If interfering contours on both the current and proposed LPFM channel continue to overlap the LPFM 60 dBu interfering contour, we look at the field strength of the interfering contours at the LPFM site.  If the field strength of those contours are weaker (a lower dB number), then we can make a showing of reduced interference.  

Like with incoming interference, outgoing interference is caused by application activity by another facility.  This would mean that the interfering contours of the LPFM station are overlapping the service contours of the other facility as set forth:

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