[TangerineSDR] Effect of VLF Receiver Overloading During Local Thunderstorms

[Jonathan]

I observed an interesting effect that I wanted to share. I noticed that
when I listened to my VLF audio stream that during local thunderstorms with
local lightning, power line hum levels would be much louder. I thought this
was an effect of the mains tracking filter under overloading by local
sferics, but it turns out, it's the VLF receiver.

I'm guessing that when the VLF receiver is overloaded (whether the front
end, line driver, isolation transformer, or all three), harmonic levels
increase drastically. This can be seen in an amplitude plot of the 180 Hz
harmonic, showing that drastic increase exactly at the time of local
lightning:
[image: Screen Shot 2023-04-23 at 3.15.56 PM.png]

Here is a closer look:
[image: Screen Shot 2023-04-23 at 10.17.10 PM.png]

This coincides with increased sferic activity shown in the noise level of
the sferics band, 5-7 kHz:
[image: Screen Shot 2023-04-23 at 3.17.57 PM.png]

Here, shown in increased time resolution:
[image: Screen Shot 2023-04-23 at 10.20.56 PM.png]

I wanted to note too, that the data going into vtsid to create these plots
is *before* the mains tracking filter, so this effect is definitely due to
the VLF receiver. In my conversations with Paul, the author of vlfrx-tools,
he mentioned strong signals can overload the isolation transformers and
create lots of harmonics, so I'm guessing this is what is happening here,
increasing the levels of mains harmonics (so much that I can hear them
clearly, too). Anyone else have any ideas?

In any case, I thought this was pretty interesting to report.

Jonathan
KC3EEY
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