The FCC will allow FM stations to operate their upper and lower HD Radio sidebands at two different power levels without special permission — though for now, stations on 107.1 to 107.9 will have to operate under the previous rules.
The move to allow “asymmetric” sidebands more widely will give broadcasters greater flexibility to maximize their coverage areas while minimizing the risk of harmful interference to other stations, advocates say.
The commission adopted a proposal that grew from a petition filed in 2019 by HD Radio developer Xperi, National Public Radio and the National Association of Broadcasters. Broadcasters like iHeartMedia, Audacy and Cumulus Media gave full support to the FCC’s proposal.
Xperi believes the changes will expand signal coverage and building penetration for many digital stations. Its studies found that many more digital FM stations could increase power on at least one sideband above the current limit of –14 dBc.
Out of 10,875 stations studied (basically all the FM stations in the country, not all of which currently broadcast in digital), Xperi says 3,496 stations could increase one sideband to –10 dBc, with another 532 being able to increase one sideband’s power to between –14 and –10 dBc, according to the FCC report and order.
The change drew opposition from some aviation industry groups. The Air Line Pilots Association International (ALPA) and others worried about possible interference to navigation systems used by pilots from 108.0 to 117.975 MHz.
ALPA, which represents the safety interests of 75,000 pilots flying for 43 airlines in the United States and Canada, identified several locations in the United States where licensed FM stations operate near the edge of the 108 MHz Aeronautical Radio Navigation Service (ARNS) band near airports with active aviation navigation aids.
Seeking to advance the rule change while taking those concerns into account, the FCC originally planned to exempt stations on 107.9 MHz provisionally until testing could prove it safe. But ALPA late last week expanded its concerns to include the five FM channels at the top of the band. Xperi then filed comments saying it supported the request to exclude those channels temporarily while testing is ongoing.
The commission has not acted on a separate, related proposal to alter the formulas by which radio stations calculate their overall maximum digital power levels. Xperi and the aviation groups have been meeting and continue to test the potential impact on the ARNS of all of these changes.
Until now, digital FM stations have had to operate with matching sideband power levels; a station that wished to employ asymmetric sidebands had to apply for experimental authorization and renew it annually.
Now broadcasters on 88.1 to 106.9 MHz no longer need to use the same power levels on both upper and lower digital sidebands. For example, a digital FM station can protect an analog FM station on a lower first-adjacent channel while increasing digital power on the upper sideband where there is no adjacent analog FM or a more distant adjacent station.
“The commission observes that adopting rules simplifying the process of initiating asymmetric sideband operation is likely to reduce inter-station interference rather than exacerbate it,” the FCC stated. It also believes that its existing interference mitigation and remediation procedures “are sufficient” to deal with interference issues that may arise.
Once the rule change takes effect, broadcasters will use a modified Schedule 335-FM to notify the FCC of asymmetric operation; they’ll need to indicate the digital ERP of the upper and lower digital sidebands as well as the total digital ERP. “We believe that requiring these data to be included in Schedule 335-FM should suffice to advise broadcasters and the public, through LMS [its Licensing and Management System], about digital operations,” the commission stated.
The FCC is also making an administrative change to the procedures used by licensees seeking to increase digital power above –14 dBc. “Those requests, previously submitted by informal request, will be submitted using Schedule 335-FM. This minor administrative change will help streamline processing of these requests and will further the commission’s efforts to transition all broadcast filings to the bureau’s LMS.”
