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I was listening to the WebSDR of Twente University, just outside the amateur band, and saw these signals. The x axis is frequency in kilohertz, the y axis is time, where the whole axis shown is about 15s.

enter image description here

It occurred to me that the two signals are alternately left and right of the line on 13,971kHz. Would that be a coincidence, or is this some kind of transmission technique? If the latter, how is it called and how does it work?

The line seems to be unrelated, it's still there although the signals are gone.

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  • \$\begingroup\$ Is that a single-sideband FM transmission, perhaps? With the other sideband being used for some other transmission at the same time? \$\endgroup\$
    – Anindo Ghosh
    Commented Jun 23, 2013 at 13:22
  • \$\begingroup\$ In the UK, 13.870-14.000 MHz is government use for aeronauticl, mobile and land mobile - maybe it is a two way comms with send and receive at different frequencies. What country are you in? Info from OFCOM. \$\endgroup\$
    – Andy aka
    Commented Jun 23, 2013 at 13:37
  • \$\begingroup\$ @Andyaka the receiver is located in Twente, The Netherlands. \$\endgroup\$
    – user17592
    Commented Jun 23, 2013 at 13:43
  • \$\begingroup\$ @Andyaka: It's kHz, not MHz. (Although I don't understand Camil's comment about "just outside the amateur band".) \$\endgroup\$
    – Dave Tweed
    Commented Jun 23, 2013 at 14:34
  • \$\begingroup\$ @DaveTweed when you look at websdr.ewi.utwente.nl:8901, you can see that the specific frequency is just outside the 20m band as it's indicated by the green line. That's what I meant :) \$\endgroup\$
    – user17592
    Commented Jun 23, 2013 at 14:37

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That looks to me like two unrelated USB signals, with a carrier that happens to be in the middle. It could be coincidence, or perhaps this is a pilot carrier to make tuning easier. Were you able to listen to what was being said? That could be a big clue.

Here's another USB waterfall sample:

USB waterfall

(many more good samples at source)

The asymmetrical shape, and the concentration of power at the low end, corresponding to the \$f_1\$ formants of human speech, gives it away.

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    \$\begingroup\$ Maybe two people are talking to each other on adjacent channels. \$\endgroup\$
    – travisbartley
    Commented Jun 24, 2013 at 1:24
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This is really just a wild guess, but it might be a WSPR signal. Low-power FSK data.

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    \$\begingroup\$ definately not WSPR. WSPR is 4 tones at about 1.5 Hz spacing. It looks like a carrier unless you look very closely, and it transmits only intermittently. \$\endgroup\$
    – Phil Frost
    Commented Jun 23, 2013 at 22:57

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