Author Topic: Odd Signal On VLF, Need Help Identifying Source  (Read 187 times)

Offline Tetris

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Odd Signal On VLF, Need Help Identifying Source
« on: June 18, 2022, 05:17:11 AM »
Hi! IDK if anyone remembers me, but I was TeslaCoilGirl on the old 4HV forums.
Anyway, I decided to redo an old project I did from back then, a VLF antenna since that's simple enough to build.
My build is about 2m in diagonal with 100 turns of 29awg wire, fed into a preamp with a high pass filter. I don't know much of the details as my professor gave it to me, but I have the schematics if necessary.
Signal is being fed directly into SpecLab, and set to filter for Sferics and Tweeks, as it seems that it filters out mains hum VERY well.


There are two questions I'd like answered about this signal snippet I picked up. The first one is what is that hum at 8/15.7/16.2kHz? It seems to be exacerbated when I turn on my AC, but it's present even when it's off, and occasionally disappears and comes back independent of whether my AC unit is on or off (granted it might just be the OTHER AC unit turning on or off, but I am unsure as to why those frequencies exist for the AC).

The second question is what in the absolute hell is this weird trill I picked up? I cannot for the life of me figure out what could've caused it. I see all the lightning peaks that I so wanted to measure all those years ago, but I didn't expect to hear singing ghosts in the VLF spectrum. Can someone bust my ghost? What is the rational explanation for this odd sound I picked up? Has anyone else picked up a weird sound like this in VLF?

Please help, I don't really know where else to ask something like this.

Thanks!

--
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Offline davekni

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Re: Odd Signal On VLF, Need Help Identifying Source
« Reply #1 on: June 18, 2022, 09:30:46 PM »
Quote
There are two questions I'd like answered about this signal snippet I picked up. The first one is what is that hum at 8/15.7/16.2kHz? It seems to be exacerbated when I turn on my AC, but it's present even when it's off, and occasionally disappears and comes back independent of whether my AC unit is on or off (granted it might just be the OTHER AC unit turning on or off, but I am unsure as to why those frequencies exist for the AC).
I have almost no VLF experience, just a failed attempt to detect WWVL (60kHz) as a kid decades ago.  However, this does remind me of a somewhat different project back then, audio intercom using ~160kHz carrier sent over line power wiring.  My brother and I could talk from 4 houses down the block, but only when a 240V AC unit was running.  Appears we were plugged into 120V circuits on opposite phases.  The AC unit provided coupling between the line phases.  So, perhaps the AC unit is coupling (or shunting) the signal rather than generating it.

Quote
it seems that it filters out mains hum VERY well.
Does it filter out all mains frequency harmonics?  Light dimmers and such can create large noise spikes with harmonics extending far above line frequency.  Such noise was what thwarted my attempt to detect WWVL (with simple home-built analog receiver).

Quote
8/15.7/16.2kHz
Frequency seems low for most electronic power supplies etc.  However, newer high-efficiency appliances are using electronic drive with variable-reluctance (or other) motors.  Such motor drives might switch in those frequency ranges.
On second thought, newer SMPSs often drop into "chirp" mode at low load for improved efficiency.  Even small wall-warts (phone charger etc.) use this technique to meet efficiency regulations.  I haven't looked at the spectrum generated by such supplies operating at low load, but could imagine frequency content in the above range.

Quote
The second question is what in the absolute hell is this weird trill I picked up? I cannot for the life of me figure out what could've caused it.
I don't hear any audio with the video.  Is this "trill" something to be seen on the screen?  Since I know nothing about this program or VLF radio in general, I have no idea what the screen is showing.  Hopefully you will find someone who knows something about this field.
David Knierim

Offline Tetris

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Re: Odd Signal On VLF, Need Help Identifying Source
« Reply #2 on: June 21, 2022, 04:39:23 AM »
From what I can tell, it does filter out the large majority of mains frequency harmonics. I found an even better setting that does an even better job at filtering, and I've since added a better cabling that offers a sort of pseudoshielding to the input of my preamp filter device.
https://i.imgur.com/lojzzJr.png

Theoretically what I had heard should be picked up on here as well, but I've listened for several hours and haven't been able to hear anything like it again. If it were from inside my house, theoretically it should happen again, right? There's no way that I picked up some weird rare signal on day 1 of me using a VLF antenna.

But yeah, especially now it's virtually noiseless, picking up only sferics and anything other than hums.

The old 4HV IRC chat, someone said it could be a PSU but I was thinking it seems to be an odd frequency range for a PSU, as I always assumed those would be either very high frequency (the 20kHz hum ended up being my 3D printer PSU) and a hum, or 60hz harmonics.

Also the audio is very low, i should turn up the gain on OBS next time I record but that could lead to ear rape with any hiss that occurs. You have to turn your audio all the way up.

And yeah it's just a frequency band. Theoretically this antenna should be able to pick up frequencies into the MHz range, but of course it's being fed directly into my sound card so it only detects the audible frequency range, which is good enough for sferics, which is the purpose of this device.

I would assume it'd be weirder if I never heard that trill again, as that means it's not from inside my house and was some random signal from wherever I picked up.

Edit: I ran some tests and turns out it is my blender. I never realized a blender was electromagnetically noisy enough to be picked up from the other side of the house. So mystery solved :)
« Last Edit: June 22, 2022, 01:47:39 AM by Tetris »

Offline johnf

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Re: Odd Signal On VLF, Need Help Identifying Source
« Reply #3 on: June 28, 2022, 01:19:41 AM »
Odd indeed
Maybe ultraslow submarine comms
also if you are seeing lightning
https://vlfstanford.ku.edu.tr/research_topic_inlin/introduction-whistler-waves-magnetosphere/
is a possibilty

maybe ET's trying to communicate??

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Re: Odd Signal On VLF, Need Help Identifying Source
« Reply #3 on: June 28, 2022, 01:19:41 AM »

 


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