A Linux-related radio interference problem / 2015-05-21 - Koos van den Hout

2015-05-21 1 year ago
Recently I noticed serious interference very visible on 14.070 MHz. Where it annoyed me a lot since that is the regular frequency for 20 meter PSK31 where I make most of my contacts at the moment. I also saw some interference at 28.120 MHz, the PSK31 frequency in the 10 meter band.

Noise on 14.070 MHz
The noise in the fldigi waterfall, click for the full fldigi screen.
The interference showed as steady carriers about 58 Hz apart and sounded like a serious buzzing noise. The first conclusion was that one of the over the horizon radars decided to drop on that part of the 20 meter band.

But later on the interference stayed at exact the same frequency. And active over the horizon radar is also quite visible on the Utwente websdr as I noted before when I noticed interesting interference in the 10 meter amateur band. This specific interference wasn't visible there.

So I kept searching locally for the source. I was afraid a nearby neighbour was using a new interfering CFL or power supply which would make it hard to diagnose and fix.

My own equipment was also a serious suspect. One piece of hardware that recently changed near the antenna was the monitor for the server. The monitor changed from an old CRT to a less-old LCD screen. The power supply wasn't the culprit, but searching further it turned out the video signal to the monitor was! And then I saw what else changed: after upgrading the homeserver to Ubuntu 10.04 and upgrading the homeserver to Ubuntu 12.04 it started coming up in framebuffer mode. Which has a different video frequency from the 80x50 textmode I used earlier.

The problem goes away completely when I unplug the video cable from the computer. I had an extension cable without ferrite cores because an earlier monitor needed the extra length. Removing the extension cable makes the level of interference drop a lot but it's not away completely. So the solution is to unplug the monitor cable from the computer.

Then things are back to the 'normal' noise level on 20 meters and 10 meters. It's still a city environment.
Listen to audio attachment:
MP3 media: Noise on 14.070 MHz (rightclick, select save-as to download)

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