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Observations of Over-the-Horizon Radar Interference in Ham Bands Top All Others


The volume of reports of over-the-horizon (OTH) radar interference from observers working with the International Amateur Radio Union (IARU) Region 1 Monitoring System (IARUMS) dwarfs that of all other interference sources, the November IARUMS newsletter reports. Nearly 800 OTH radars were observed during November alone, and the total number of reports for the year is more than 4,500 — although that number likely includes multiple reports of the same OTH radar systems. While the intruder watcher is focused on IARU Region 1, much of the major interference it recounts can affect the amateur bands in the world’s other two IARU regions. IARUMS has tracked most OTH radar signals to sites in Russia and China, but a recent report indicated that India has an OTH radar system in the works. IARUMS is calling the system “of concern,” although it’s not known what frequencies it may use.

During November 17 – 21, IARUMS reported a “strange, somewhat washed-out signal” on the order of 7 kHz wide, that “drifted slowly back and forth” in 40 meters and below.

“The first evaluation suggested a double-sideband signal,” the newsletter said. “Wolf, DK2OM, could determine the symbol rate at 2400 bps. It was probably a signal from the family of single-tone 1800 Hz [8-PSK] modulated modems.”

Some IARUMS stalwarts made multiple time-difference-of-arrival (TDoA) observations that pinned the signal’s source to east-southeast of Toulouse, France, not far from a French Naval radio station. The signal disappeared on November 21.

IARUMS said it’s been observing wideband radars — likely of Chinese origin — with bandwidths of up to 160 kHz at 10 sweeps per second. In November, however, reports began to accumulate of radars — or perhaps a combination of OTH radar signals — with bandwidths of up to 360 kHz. “It was also often observed that the [OTH radar] shifted its frequency back and forth, which is also sometimes the case with the [Chinese] ‘Foghorn’ systems,” IARUMS said. In the Far East, IARUMS speculated, such a signal could blanket the entire 40-meter band.

Ongoing is the “radio war” between Russia and Ukraine, which IARUMS said has been “a constant nuisance for years.” The broadcasts are heard on 7050 and 7055 kHz, as well as on other frequencies.