American UHF Woodpecker

For two years, ham radio operators of UHF repeater systems in the Colorado Front Range have been hearing a regular pulsing or clicking noise on weak signal repeater users.

The interfering signal has a rhythmic clicking sound, similar to a slow motor vehicle ignition spark or as some hams have nicknamed it, "The woodpecker". Almost every UHF repeater system in the Denver area has experienced this noise.

In November of 2002, several hams volunteered to help the Colorado Council of Amateur Radio Clubs find the source of the noise via radio direction-finding techniques. The offending transmitter was surprisingly found to be located on Buckley Air Force Base, just east of Denver.

At first, the hams assumed the system was likely to be a
malfunctioning transmitter, and various attempts at contacting the proper authorities were made. For a short time, the system was even switched off from time to time, presumably when not in use.

As more information was gathered, hams learned from various sources including the Internet, that the system causing the harmful interference is a military radio system known as the Enhanced Position Location Reporting System, or EPLRS.

EPLRS and its airborne counterpart, AEPLRS are spread spectrum systems designed such that they have multiple UHF channels that overlap. These channels are numerous enough that they cover virtually the entire weak-signal and repeater input portions of the UHF Amateur band in Colorado, from approximately 426 MHz to 450 MHz. Spectrum analysis data captured by local hams and the FCC confirms this.

How does this affect you?

Recently, Colorado hams have been hearing from our neighbors in Albuquerque, New Mexico that they are hearing similar interference on UHF repeaters in the Sandia Peak area, and at least one source indicates that hams may also have reported similar interference to the FCC in Sacramento, California. Furthermore, other research indicates that it is likely that more than 25 EPLRS systems are slated for
deployment or are already deployed in the Continental United States, Alaska and Hawaii.

Hams who enjoy UHF weak signal and FM repeater operation near military installations should be alert to the possibility of such interference. It is hoped that more widespread knowledge of this very broadband UHF interference source may help hams in other areas who have been hearing it.

For further information, a public e-mail discussion group has been set up that anyone can join by sending an e-mail to
uhfnoise@googlegroups.com.

Nate Duehr, WY0X, Amateur Radio Newsline

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