MyLPFM Super Power and HD Hijack support

To give users of MyLPFM who are considering a channel change, as well as getting prepared for another LPFM filing window in the future, REC has made two enhancements to the results in myLPFM v.2.0

Super Power Stations

When the FCC rules for LPFM stations were written, each full-service station class was given minimum distance separations on co, first and second-adjacent channels.  Those distances are based on the maximum service contour size for the respective service class.  There are some stations where the service contour size exceeds that of the class maximum.  The FCC addresses this for stations in the reserved (88.9 to 91.9 MHz) portion of the band where LPFM stations would protect full-service stations to a service class level appropriate to their actual service contour size but no more than Class C.  This type of overprotection does not exist for the commercial (92.1 to 107.9 MHz) portion of the band.  A part of this may be due to Commission opinion made in a 1962 Report and Order that would define the modern FM station classes.    As a result, LPFM station protections, such as to Class B, are based on the maximum service class parameters.  In 1962, the FCC recognized that there were a considerable amount of stations at the time (which comprises about 2% today) of stations where the service contour size exceeds the class maximum.  Some good examples are the Mount Wilson FM stations in Los Angeles.  

Even though an LPFM station may meet the §73.807 minimum separations on a certain channel, there may be a station that meets the minimum distance separation but because it is operating at a "super power", then it is much more likely to cause interference than other channels that may be available in a given area.  myLPFM will look at all facilities within 150 km of the study location to determine if there are any "super power" stations (all full-power classes) and if there is one on co-channel for first-adjacent channel and if that channel is available in accordance with §73.807, it will warn of the super powered station's call letters and percentage that the service contour exceeds the class maximum.  

HD Hijack

With the expansion of hybrid HD Radio on FM including the recent power increase, some LPFM stations are experiencing a phonenomena where their station's signal is being completely replaced by a distant full-service station, even though the receiver is fairly close to the LPFM station.  REC's investigation into the issue is suggesting that it is something we call "HD Hijack".  In addition to the normal subcarriers that an analog FM station carries, an FM station running hybrid HD Radio will have subcarriers that are past the range that had been used in the past by traditional FM radio.  These digital subcarriers appear +/- 100 to 200 kHz from the center frequency of the HD full power station.  Under this theory, those HD subcarriers from the weaker full-service station are being detected by the HD Radio receiver which in turns triggers the radio into HD mode and then plays the audio coming from the HD stream instead of the analog program from the much stronger LPFM station. 

The myLPFM support will evaluate channels that have a co-channel incumbent station that is either one that meets §73.807 minimum distance separation but does not meet the §73.807 guidelines for not receiving interference or that that the station is super powered.  In addition, the HD station must have the FCC digital indicator in order to trigger this message. 

The presence of the message is not a definite indication of a HD Hijack, but it allows you to evaluate the potential of a hijack if this channel is chosen.  LPFM applicants should conduct a "drive test" using an HD receiver to determine if the radio is detecting digital in the LPFM station's desired service area.

105.1 286 Reduced interference showing required. Possible Inward Interference. Channel may experience HD Hijack from WZFT. This channel has a co-channel station (WZFT) that is operating 126% of the maximum for its class. This channel has a first-adjacent channel station (WPRS-FM) that is operating 126% of the maximum for its class. Waiver required

New Super Power message.

New HD Hijack message.



Friday, April 26, 2019
Systems affected: 
myLPFM v2
Issue type: