Message-ID: <5480A155.5020205@abelian.org>
Date: Thu, 04 Dec 2014 18:00:53 +0000
From: Paul Nicholson <vlf0403@abelian.org>
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Subject: Re: VLF: W4DEX in Europe at 8820 Hz
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Status: O


Examining W4DEX at Todmorden overnight on 1st/2nd Dec...

S/N dB in 278uHz and signal phase in 1 hour steps:

    From     dB     phase
    22:30   7.6     80.4
    23:00   8.9     90.9
    23:30   9.1    115.6
    00:00   6.3    106.2
    00:30   7.9     30.6
    01:00   7.9     60.4
    01:30  11.8     88.8
    02:00  10.1     78.6
    02:30  10.8     62.7
    03:00  10.0     77.3

The effect of the terminator is lasting a bit longer than LWPC
predicts and it doesn't really settle down until 01:00.

But the phase doesn't go too wild, just a bit of a wobble
and it could still be usable if the S/N was a bit higher.
 From 01:00 to 04:00 is very good but not long enough for a
worthwhile message.

Some calculation shows that with a rate 1/8 K=25 FEC code a 5
character message could be sent between 23:20 and 04:00 using
30 second symbols with reasonable chance of success if the
S/N was averaging around 10.0 dB during the night.

There's a very steep cliff effect with this coding, an extra
dB would make it almost certainly successful.   We could
probably do that with a bit of extra antenna current and a
low background night.

Background noise for European receivers over four recent nights

  http://abelian.org/vlf/tmp/141204-vlf1.png
  http://abelian.org/vlf/tmp/141204-vlf6.png
  http://abelian.org/vlf/tmp/141204-vlf15.png

Some nights are 3dB better than others, and a good night at
one site is not necessarily good at the others.

At Bielefeld the estimated signal strength is around 30nV.
With antenna capacitance of say 20pF, that's an alternating
charge of amplitude around 6e-19 Coulombs which is about 4
electron charges.

We don't actually have 4 electrons oscillating between ground
and antenna.  Instead there is an oscillating displacement of
the average position of all the free electrons in the antenna
wire with amplitude of around about 15 or so proton diameters.

--
Paul Nicholson
--