[TangerineSDR] Effect of VLF Receiver Overloading During Local Thunderstorms

[Jonathan]

I also wanted to add these time series plots of sferics that overload the
VLF receiver.

You can see at ~15s in, there is quite a large duration sferic that has
been clipped. I thought it was interesting that it was definitely clipped:
[image: Screen Shot 2023-04-24 at 8.45.52 PM.png]

Here is that sferic in greater time resolution, showing the clipping and
the amplitude content:
[image: Screen Shot 2023-04-24 at 8.47.19 PM.png]

The duration of that signal is ~1s long! I've noticed that local sferics
tend to be much longer in duration.

Jonathan
KC3EEY

On Sun, Apr 23, 2023 at 10:42 PM Jonathan <emuman100 at gmail.com> wrote:

> 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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