Reporting Interference

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Just like any other radio station, medium wave (AM) stations need listeners. In order to get those listeners, they need to make sure their signal can be received clearly by everybody in their target area.

AM stations are vulnerable to several kinds of interference:

  • Consumer and industrial electronics.
  • Atmospheric noise, which should not be an issue inside a station's target area.
  • Interference by other AM stations, especially at night.
  • Interference by the digital IBOC sidebands of other AM stations.

Contents

Electronics and atmospheric noise

If you have troube listening to an AM radio station, you could try switching off a few electronic devices in your home to figure out exactly which ones are causing trouble. Common electronic devices that can interfere with AM stations are (old) tube TVs and monitors, compact fluorescent lights (CFLs), microwave ovens, computers and switching power supplies.

FCC rules specify that it is legal for such devices to be sold, but illegal for them to be operated when interfering with radio reception. Buyer beware, your own cell phone charger or halogen lighting system could ruin the reception of your favorite sports or talk radio station.

Interference by stronger AM radio stations

The AM regulations in the Americas provide for several classes of radio stations. Class A stations, also called "clear channel" stations, have a high transmission power day and night. They are supposed to be listenable well up to about 400-700 miles away at night, when their signal can bounce off the ionosphere (so-called skywave propagation).

During the day, AM signals only travel along the surface of the earth. Not only does this mean that the Class A stations cannot get their signal out very far, it also opens up space on the band for daytime stations in those areas where the Class A stations cannot be heard. These daytime stations often switch their transmitter off, or go to really low power, at night. If your local daytime station gets overwhelmed by a Class A station at night, tough luck - the Class A station has priority.

Interference by IBOC (digital) signals

Because AM is sensitive to electronic interference, radio stations are slowly switching to digital transmission modes. In North America that mode is IBOC - the rest of the world uses Digital Radio Mondiale (DRM). DRM and all-digital IBOC fit in the normal bandwidth used by an AM station.

However, IBOC also has a so-called "hybrid mode", in which the digital signal is transmitted together with the analog signal. Normally each station on the AM band has a 10 kHz channel, but as you can see the hybrid IBOC signal is 30 kHz wide, covering the two channels adjacant to the station's frequency.

AM IBOC Mask10.gif

Needless to say, the digital noise can be heard when you try to listen to a radio station on a frequency near the station transmitting DRM. In fact, if your radio has wide filters, a station's digital signal could even add noise to the station's own analog signal!

During the daytime, when AM signals do not travel very far, and you can hear few stations on the band, IBOC should not be a big problem - except for stations adding noise to their own signal.

Hybrid IBOC was supposed to be a daytime only transition mode, allowing people to slowly adopt digital radio without stations losing their listeners with older radios. However, since September 14 2007, stations are allowed to keep IBOC on day and night.

At night time, AM radio signals can travel long distances by skywave. Class A stations are supposed to have a clear skywave signal, while medium sized stations are supposed to have clear reception in their local target area. Unfortunately, IBOC sidebands also travel long distances, adding noise to radio stations on adjacant channels and making some stations annoying or impossible to listen to.

How to identify an IBOC signal

Before trying to identify an IBOC signal, you may want to read this web page describing the IBOC spectrum of AM 620 KMKI.

First, remember that IBOC sidebands are symmetrical around a station's main analog signal. If the interference you are hearing is only present on one side of the "suspect" station's analog signal, it is almost certainly not IBOC. If the digital hash is the same on both sides of the "suspect" station's analog signal and gets strongest when you are about 10 kHz away from the "suspect" station, chances are it is IBOC.

If you have a radio with selectable sideband reception (USB/LSB) and are trying to listen to a station 10 kHz away from a station with IBOC sidebands, you may hear that the IBOC interference is weaker on the sideband towards the interfering station and stronger on the other side. The IBOC station is deliberately keeping the stronger digital signals away from its own audio signal.

How to complain about interference

Complaints should go to the station being interfered with, not the station(s) causing the interference. There are a few exceptions, however. If you feel that an IBOC station's own digital emissions have degraded their analog signal quality, you should let them know. Or, if you have reason to believe that their IBOC equipment is malfunctioning, you should contact them, but make sure you're on firm ground before you do so.

Keep in mind that interference complaints are more likely to be taken seriously if they affect reception in or near protected contours. You can find out what your local stations are using V-Soft's zip code radio locator.

The first question to ask yourself is whether the reception is by skywave or groundwave. Only the Class A stations on the "clear" channels have protected skywave service, so complaints about interference to skywave signals will likely fall on deaf ears unless one of those stations is on the receiving end of the interference. If in doubt, you can check a station's class using sites such as fccinfo.com or radio-locator.com. Of course, some of the large market Class A's have little or no interest in skywave service, WOR being a prime example. At the other end of the spectrum are stations such as WSM that still have big skywave audiences, and thus should be more receptive to interference reports.

References



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