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In quite a lot of places below 7000kHz I see a repeated noise figure. A hum, then about 12kHz above it a "flapping" with beeping noise, and then about 50kHz from that whistling with three clearly-defined spectral spikes.

Here's a link that shows all three: https://www.youtube.com/watch?v=X5Yl2q5-2tc

And another link that just shows the "flapping" and beeping: https://www.youtube.com/watch?v=Cnbqs3xmdlo

I've seen this set of three things all the way down the low bands and am wondering what sort of thing may be producing it.

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    $\begingroup$ I tried turning off the main breaker and all UPSs and running my rig on a battery and still see it. $\endgroup$ Oct 8, 2021 at 17:06
  • $\begingroup$ I'm glad you tried to turn everything off, especially anything with a switching power supply. Perhaps this could be noise from the power lines, maybe even power-line carrier. $\endgroup$
    – Jack0220
    Oct 8, 2021 at 19:54

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Combination of a neighborhood power outage (which unrelated to this reduced the noise floor from S-5 to S-1...sigh) and a portable AM radio I scrounged up solved it. It turns out it was a UniFi USW-8-60W network switch (it was on a UPS I had forgotten about which is why I missed it when I did my "main breaker off" test). Oddly, I have three of the exact same model switches and only one of them puts out that "signal".

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