40m Pulse Signal
Over-The-Horizon-Radar?

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Since about August 20, 2005 myself and others have been receiving a pulsed signal on the low end of 40m. I've read complaints about the signal from others on various radio related e-mail reflectors. I've heard it from about 7.000 to 7.050 MHz and others have heard it from 7.030 to 7.070 MHz at other times.

• Update: September 7, 2005 - Others have reported the same signal on numerous bands as high as 10m, although I have not confirmed that they are hearing exactly the same signal. However, tonight I did hear the signal at 0200 UTC until 0210 UTC from about 7115 kHz to 7175 kHz. It also has been reported being heard as far away as Australia.

The signal isn't on all of the time but when it is on it is often on for hours at a time. I've heard it well in the afternoon and at night from here in north Texas. Given the pulse modulation and wide bandwidth I suspect that it is some kind of Over The Horizon Radar transmission. During a long recording that I made selective fading is very evident on the signal so it is pretty obvious that it is a skywave signal and not just local RFI.

Here are the technical parameters the best I can tell using a maximum detection bandwidth of 3 kHz:

• Frequency = 7,000-7,050 kHz, 7,115-7,175 kHz (others also report 7,030-7,070 kHz)
• Bandwidth = ~50 kHz
• Pulsewidth = <400 us (my receiver bandwidth limits my ability to measure this)
• PRF (Pulse Repetition Frequency) = 18.15 Hz
• Period = 55 ms
• Unambiguous Range = <5,000 miles (if a radar)

At this time it is just an irritating nuisance much like the old "Woodpecker" of years gone by. Below is a 5 second clip of the signal recorded on 7.043 MHz at 2313 UTC on September 4, 2005:

aa5tb_072.mp3's Spectrum