Bringback thevalve
Very interested, in your project, some more info would be great.
maybe in the project section,
i am in a rural area on a mountain ridge, more people are living up here, so more audio and RF noise.
the noise floor has risen in the last few years on HF and higher.

I haven't used the VLF antenna for many years, had forgotten about it, we did a clean up of one of the sheds a few weeks back and found it.
I was going to use it for the ndb's and a couple of the new amateur radio frequencies on 2200 and 630 meters
135.7 to 137.8 kHz and 472 to 479 kHz and the mw frequencies.

STC830
there are a number of frequencies around the world that transmit to Submarines, around 15 to 30 kHz, there is one transmitter at North west cape in WA, it is on 19.8 kHz and runs around 1Mw of power.
I think the signals transmited are more of an alert to the subs, as the data rate would be very slow at those frequencies
There is free software around that you can listen into these and more, using the soundcard and a wire antenna. lots of sites on the net with info on how to set it up.

Yep, I agree Flukeoneseventyfive,

Sub comms rely on VLF and in extreme cases ELF. ULF would take all day to send a few letters.

Line Monitoring Station - 15th April, 2018
Line Monitoring Station - 19th January, 2017

Documents uploaded to Post 47.

Flukesoneseventyfive

Looking at the 19th April scan, it appears to be a signal every 20 minutes. I suppose if this stopped, the sub would be alerted to the possibility that there may be a crisis to react to. Is this what you mean by an alert?

STC830,

I'm not sure if Flukeoneseventyfive has read your question. I cannot see this thread on General Discussions until I log into the site.
Unless I'm doing something wrong, possibly he is experiencing the same anomaly with the post??

Meanwhile, for your interest, I provide further detail with the graph you mentioned.

The waveform you asked about only lasted for the duration of the scan, which was several hours. It only appeared on one detector, which
tells us it arrived in a narrow corridor (read directional) More on that another day.

I mentioned previously there was no modulation, but considering your question to Fluke175 I can now see it may well be modulated.
With that in mind I retract my non-modulated comment and will go back to the drawing board.

Notice the transients are +VE going. I have seen this twice before. You identified approx. 20 minutes periods, very close, good work,
in fact they are 24 minute periods but slowly speed up to 18 then terminate to 16 minutes.

I cannot think of anything in nature that will do this, so I am more than happy to call it "artificial".

Let me thank you for your interest, happy to discuss further.

Cheers for now,

G.

Flukesoneseventyfive

Thanks, I was speculating on what could be done with a signal that can't carry much information and this was the simplest that I came up with.
Assuming that it is for a sub of course.

G'Day STC830 and Bringback Thevalve.
Have a look at the the wikipedia on the subject
it has a lot of info on the subject and is a good read.

https://en.wikipedia.org/wiki/Communication_with_submarines

Sorry busy fall asleep on the lounge chair Monday evening, didn't get to the forum.