FCC Gives Digital Radio Stations More Power Flexibility With New Sideband Rules

Image credit: Generated for Power JDM The Beat.

The Federal Communications Commission has made it easier for many terrestrial FM stations using HD Radio to manage their digital signal power.

Under rules adopted in September 2024 and made effective in May 2025, eligible FM digital stations can operate with different power levels on their upper and lower digital sidebands. The change is designed to improve digital coverage and building penetration while helping stations protect nearby analog FM signals from interference.

The rule applies to terrestrial broadcasters: not online radio streams. For listeners, the most visible impact may be more reliable HD Radio reception in areas where digital signals have previously been weaker.

What changed under the FCC rules?

FM HD Radio stations transmit a traditional analog signal along with digital information placed on digital sidebands above and below the main carrier. Those digital sidebands carry the information used for HD1, HD2, HD3, HD4, and other digital multicast channels.

Before the rule change, stations generally had to use matching power levels on both sidebands. This was known as symmetric sideband operation. If one neighboring station created a greater interference risk than the other, the digital station still had to set both sidebands according to the more restrictive condition.

The FCC now allows asymmetric sideband operation. This means a station can use different power levels on the upper and lower digital sidebands.

For example, a station could reduce power on the sideband closest to a nearby analog station while using more power on the opposite side, where there may be no nearby station or a more distant adjacent-channel signal. The goal is to place digital power where it can improve coverage without creating additional interference.

The FCC’s First Report and Order in “Modifying Rules for FM Terrestrial Digital Audio Broadcasting Systems” was adopted on September 24, 2024. The amendments were later published in the Federal Register and became effective in May 2025.

Previously, a station seeking asymmetric operation had to request experimental authorization and renew that authorization annually. Now, most eligible stations can begin the process by filing FCC Form 2100, Schedule 335-FM.

A station using asymmetric operation must certify that its digital power levels comply with the FCC’s limits. The filing must identify the power used on the upper and lower sidebands, along with the total digital effective radiated power.

Professional FM broadcast transmitter rack with digital signal reflections

Image credit: Generated for Power JDM The Beat.

Who pushed for the change?

The effort grew from a petition filed in 2019 by HD Radio developer Xperi, the National Public Radio, and the National Association of Broadcasters.

The petitioners argued that the existing rules limited digital FM coverage because stations had to manage both sidebands as if the interference conditions were identical on both sides. In many markets, that is not the case.

The proposal received support from major broadcasters and radio groups, including iHeartMedia, Audacy, and Cumulus. Supporters said the rule would provide stations with more engineering flexibility and could make HD Radio more useful, particularly for digital multicast channels.

The FCC also considered comments from public broadcasters, engineers, technology companies, and other radio organizations. In its order, the Commission said asymmetric operation could promote digital radio adoption without increasing the risk of interference to existing analog broadcasts.

The FCC pointed to previous real-world experience with stations that had operated under experimental authorizations. Some broadcasters reported years of asymmetric operation without interference problems. The Commission concluded that a notification process would reduce the regulatory burden while still allowing it to track which stations were using the technology.

Why aviation groups raised concerns

The FCC did not apply the new authorization equally across the entire FM band.

Stations operating between 88.1 and 106.9 MHz can use asymmetric sidebands under the new rules, subject to the required filing and technical limits. Stations operating between 107.1 and 107.9 MHz remain excluded from the standard authorization and must continue seeking experimental authorization for asymmetric operation.

The reason is the proximity of the upper FM band to the Aeronautical Radio Navigation Spectrum, which begins at 108.0 MHz. Aviation groups, including the Air Line Pilots Association International, raised concerns that higher digital power near the top of the FM band could potentially affect navigation systems.

The FCC’s order noted that aviation organizations, Xperi, and the NAB were continuing to study the issue. Because additional testing was needed, the Commission kept the upper five FM channels under the experimental authorization process.

The restriction does not mean that stations on 107.1–107.9 MHz can never use asymmetric sidebands. It means they must receive specific authorization while testing and technical review continue.

What the power numbers mean

The FCC’s rules use dBc, or decibels relative to the analog carrier, to describe digital power.

A digital signal operating at –10 dBc is transmitting at a level 10 decibels below the main analog carrier. The FCC’s rules continue to prohibit total digital effective radiated power above –10 dBc.

The new process does not give stations permission to exceed the total digital power that would have applied under symmetric operation. Instead, it allows stations to distribute that permitted power differently between the two sidebands.

The FCC created a per-sideband table based on the digital station’s field strength at the protected contour of a first-adjacent analog station. Depending on the engineering conditions, the maximum permissible power for an individual sideband can range from –17 dBc to –13 dBc.

That approach gives engineers a way to reduce power where interference risk is greatest and increase it where conditions are more favorable.

Xperi, the NAB, and NPR studied approximately 10,875 FM stations. Their analysis estimated that about 3,496 stations could increase one sideband to –10 dBc under asymmetric operation. Another 532 stations could increase one sideband to a level between –14 and –10 dBc.

Those figures are study estimates, not a guarantee that every station will qualify. Each broadcaster still has to complete the required engineering analysis and comply with the FCC’s technical rules.

Spectrum analyzer showing a central FM carrier with two digital sidebands at different power levels

Image credit: Generated for Power JDM The Beat.

What this means for broadcasters

For eligible terrestrial FM broadcasters, the rule creates several practical changes.

More control over coverage: A station may be able to strengthen its digital signal in a direction where there is less interference risk.

Better building penetration: A stronger digital signal can improve reception inside buildings, where concrete, steel, and other materials can weaken radio signals.

Protection for adjacent stations: A broadcaster can lower the sideband closest to a vulnerable neighboring analog station while maintaining more power on the other side.

Less paperwork: Stations no longer need to request and renew annual experimental authorizations for most asymmetric operations. A compliant station can notify the FCC through Schedule 335-FM.

Continued engineering responsibility: The rule does not remove the need for technical calculations. Stations must certify that their upper and lower sideband power levels meet the FCC’s limits and that total digital power does not exceed the applicable symmetric limit.

The FCC also moved certain digital power requests above –14 dBc into the Schedule 335-FM process. Stations seeking to increase total digital ERP above that level must submit the required certification and supporting engineering information. Unlike ordinary asymmetric notification, that higher-power operation requires FCC approval before it begins.

What listeners should know

The rule affects terrestrial FM and HD Radio broadcasters. It does not change how online radio streams work.

An FM HD Radio signal is transmitted from a physical broadcast facility over licensed spectrum. An online radio stream is delivered through internet connections, servers, mobile apps, websites, or connected devices. Internet streams are not transmitted through FM digital sidebands and are not covered by these particular FCC rules.

For listeners who use an HD Radio receiver, the rule could eventually mean stronger or more consistent reception from stations that adopt asymmetric sideband operation. The actual improvement will depend on the station’s frequency, location, antenna system, surrounding stations, terrain, receiver, and building environment.

Listeners should also expect the change to happen on a station-by-station basis. The FCC has not required every station to change its digital transmission system.

Radio-friendly talking points

  • The FCC now lets most HD Radio stations use different power levels on their upper and lower digital sidebands.
  • The change can help a station protect a nearby analog signal while strengthening digital coverage in a safer direction.
  • Stations between 88.1 and 106.9 MHz can use the new notification process.
  • Stations on 107.1–107.9 MHz still need experimental authorization because of aviation navigation concerns near 108 MHz.
  • Xperi’s study estimated that thousands of FM stations could benefit from asymmetric digital power.
  • The rule affects terrestrial FM and HD Radio: not online radio streams.
  • The total digital power limit still applies, so asymmetric operation is not a free pass to exceed FCC limits.

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