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A VLF Capture

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Chris Russell

Mon May 01 2006, 09:00PM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

Hello all,

For you radio junkies out there, especially hams, here is a segment of a recording I took of the VLF bands last week. It reads from bottom to top; the first signals received are at the bottom, and the latest are at the top. The file itself is a 1.9MB download, it's a 1154 x 6246 jpg. Here are the system specs:

Receiver: Laptop soundcard, sampling at 48kHz. Using the microphone input, with the 20dB boost option enabled.
Antenna: 60 turns of 18AWG stranded copper wire, around a 19" diameter coil form. (actually, one of these

)
Software: Spectrum Lab 2.7, set to auto-capture every 30 seconds. Lots of little tweaks to make the weak signals more visible.

Signals visible in the image:

24kHz: You can't miss it. Four dark bands at the rightmost edge of the image. For a while, I thought this was some artifact created near the upper edge of the sampling rate of the soundcard. Nope. This is a signal from the US Navy's transmitter in Cutler, Maine. They transmit two megawatts into an antenna atop 26 towers, each 800-1000 feet tall. This explains why it's off the chart! They're just 2-3 hours from here by car.

22.8kHz: Unknown. A very strong signal, which means it is probably somewhere in the United States. However, this frequency is not listed as being used by anyone. This frequency used to belong to a US Navy transmitter in Northwest Cape, Australia. It's possible they've started using it again, but the signal strength makes me doubt it.

22.1kHz: A very faint signal is visible here. Source not known.

21.75kHz: This frequency is listed as belonging to a French Navy transmitter in Le Blanc, France. The faintness of the signal would seem to confirm this.

21.4kHz: Another US Navy transmitter. This one is located in Lualualei, Hawaii, on the island of Oahu. Great signal strength, considering how far away it is.

~20.5kHz: Near the bottom of the image, you can see a huge pulse with a lot of power behind it. The pulse lasts almost 30 seconds, and seems to be rich in harmonics. This is probably caused by some sort of huge inductive load switching on the power grid, but I am not sure. Similar, pulses have been recorded in Europe by VLF listeners.

~ 20kHz: Some sort of local interference that I haven't been able to identify. The pulses are regular, but drift a bit when compared with a time standard, roughly a few seconds each hour. Once in a while, it shuts off, usually when my upstairs neighbor has just arrived home, and stays off for a few hours. No idea what it could be.

17.8kHz: A transmitter fades into life, suddenly stops, then starts transmitting a wider signal that drifts a bit. Unknown source. This particular transmitter has been recorded by many people, and has many strange operating modes.

15.735kHz: If you look through the harmonics and interference, you'll see one line that's darker than all the others, on this frequency. This signal is generated by the flyback transformers in NTSC standard TV sets. Hard to say what the source is, probably the combined signal of every set within several blocks.

Other miscellany: Some of the horizontal lines you see are generated by lightning all over the globe. Some are from local sources, like a light being switched on or off. Still others could be noise from switching actions on the power grid. Most of the harmonics you see at the lower edge of the band are actually high order harmonics radiated from the power grid (notice that they are spaced 60hz apart).

Enjoy! Comments and questions are welcome!

Alex

Mon May 01 2006, 09:11PM

Geometrically Frustrated
Registered Member #6 Joined: Thu Feb 02 2006, 04:18AM
Location: Bowdoin, Maine
Posts: 373

That's neat! Much better than the results I got. I ought to get my antenna outside or something. The noise in my room is terrible, but it worked enough to be interesting when I tried it a few weeks ago.

Simon

Tue May 02 2006, 12:16AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Hey, that's nice! It's cool how all this stuff (flybacks, lightning, power system switching) is chirping away all the time and we don't "hear" it. It's like you're bringing out an invisible side of the world. Geeky, but cool when you see what's actually there.

...

Tue May 02 2006, 04:32AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Interesting, I always wondered if sound cards had the ability to take rf signals directly in.

Have you figured out what that large band of activity near the beginning was?

Chris Russell

Tue May 02 2006, 05:33AM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

Not sure. In Europe, people often pick up what's called "twenty second pulses," which last about 20 seconds, and take place between 17-18kHz. They are supposedly caused by "heating burners switching on and off." Given the similarity of the two, I'd guess that I'm picking up the US equivalent of the twenty second pulses. They last a bit longer, and are at a somewhat higher frequency.

I should note that I recorded about a dozen pulses like that one on that night, at irregular intervals. I don't see them every night, though.

Penguin7471

Tue May 02 2006, 07:15AM

Registered Member #71 Joined: Thu Feb 09 2006, 08:23AM
Location:
Posts: 63

Chris, that is an clever coil form for an antenna.

I'm not sure if this would be relevant but..

In Grimeton, Sweden, there is a 'dinosaur' of a CW transmitter, delivering 200kW of CW signal at ~17.2kHz. It uses an old alexanderson alternator for power, and is the oldest surviving long wave station based on this type of alternator. Its transmission antenna system consists of six towers, each 127m high.

I dont know how this popped into my head, but its interesting nonetheless.

Bjørn

Wed May 03 2006, 12:48AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

How do we syncronise several recordings accross the globe so we can triangulate the signals and find the ones that truly are global? A microcontroller and a GPS that injects a clock pulse into the recorded signal?

Chris Russell

Thu May 04 2006, 03:37AM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

That is probably the best way to determine which horizontal bands are caused by lightning, and which were caused by your neighbors down the street turning on the A/C. I am fairly certain there are already programs in place that allow you to use a GPS to sync your PC clock.

In the image above, I mark every 30 seconds. I can just as easily mark every second, and scroll the waterfall faster. Two identical recordings could then be placed side-by-side and compared.

For constant signals, like the one above from Cutler, the usual method for triangulation is to use two loop antennas at 90 degrees to one another. The software I use can then calculate a direction, based on differences in the signal measured by the two antennas.

I should note that the transmitters on 24kHz, 22.8kHz, 22.1kHz, 21.75kHz, and 21.4kHz are all transmitting encrypted data, modulated by MSK. If one could figure out how to reliably demodulate the data, you could just treat it the same as the pseudo-random code generated by GPS satellites. You could synchronize two recordings based on those strings. Obviously the resolution here isn't high enough to do that.

Edit: Almost forgot about Grimeton! They do still fire it up every so often, but I haven't caught it yet. There's not often much notice given, sadly.

Bjørn

Thu May 04 2006, 10:28AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I made an antenna out of enamel wire and a bucket but most of the many signals I got were weakest when the coil was at a 90 degree angle to my computer so I would need to place it at significant distance from it. A battery operated amplifier would not hurt either to keep the noise down.

Here is one of the peculiar events I got. I suspect it is my computer that does something strange. I had just zoomed in on 12 kHz when it went quite mad in exactly that area.

Carbon_Rod

Thu May 04 2006, 11:09AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Well, I think perhaps filtering the sound cardâ€™s noise sources first may be logical.

1.) Laptop soundcards and built-in motherboard are notoriously noisy. Often clock ticks, fan noise, legacy dos system timer, and even the peripheralâ€™s own circuitry can add noise.

2.) Some PCI sound cards do make a nice LFO wave below the perceivable level (Creative and ESS cards do this and its below 20 Hz.) If the house wiring is bad one may even hear the 60/50 Hz hum even when nothing is plugged into it. The noise can become really noticeable if you have lamp dimmers running near by etc.

3.) There is a rare feedback condition where the circuit will self oscillate. Notably your PC speakers may form a weak feedback loop with your pickup coil and generate all sorts of noise.

4.) Some sound cards canâ€™t actually shut off the speaker driver thatâ€™s right next to the preamp. Others multiplex A to D unit for left and right input for line, mic, and CDROM line level input. Badly shielded drives and network i/o can generate all sorts of noise too.

Still it may be interesting to see if its possible to guess your location on the planet based on raw background noise alone. No 6 foot helical antenna required.

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