FAQ: The LPFM station is currently on a second-adjacent waiver but otherwise qualifies for LP-250. What happens?
A second adjacent channel waiver where the interfering contour of the LPFM station is inside the protected service contour of the short-spaced station (a majority of the waivers) uses a formula based on how strong the short-spaced station is at the LPFM transmitter site to determine the field strength of the LPFM interfering contour encompassing the area where no populated structures can exist. We also call this an "overlap area". As an LP-250 station has a higher power, the size of that overlap area will be larger and there's a possibility that the larger contour may include occupied structures. As the LCRA only permits waiver of the second-adjacent channel where there is no interference to "any radio service", there can't be any population in the upgraded interfering contour.
How this will be handled by the FCC has yet to be seen. This could go one of two ways:
On the upgrade application, the FCC may require a new second adjacent channel waiver. Because this is a change in class of service and the overlap area gets larger, the applicant will need to demonstrate that the LPFM station will not put the specified interfering contour (at the higher ERP) into any occupied areas.
On the other hand, the FCC may choose to make a policy where no new waiver request is necessary (although we don't see them doing this). In that case, you will be permitted to increase power, however, in the event of an interference complaint by the short-spaced station, the LPFM station will be forced to either shut down or revert back to LP-100 until the interference can be corrected.
Any time you construct on a second adjacent channel waiver, you do run the risk of receiving an interference complaint from the short-spaced station, even if the complaint ends up having no merit. This is why it is important to assure that the station is constructed with the same antenna that was used in the presentation of the waiver request showing how the proposed LPFM (or higher station class) will not cause interference to listeners of the short-spaced station.