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From: Paul Nicholson <vlf0403@abelian.org>
Message-ID: <582C19D6.5080808@abelian.org>
Date: Wed, 16 Nov 2016 08:33:26 +0000
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Subject: Re: VLF: EbNaut transmissions on lower frequencies?, pre-tests:
 6.47kHz
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Stefan wrote:
 > What is the most distant location from where a stream
 > is available?

Texas vlf25 (8491km) but first you must reach Virginia
vlf35 (6819km). Of course I already checked vlf35 summing
five days of 'TEST' but not a trace.

 > What's the result of 15 th and 11th to 15 th on your side?

Message 'TEST' 8K19A 60 second symbols 11th to 15th at Todmorden.
All decoded with constant reference phase.

Straight summing, no amplitude or phase adjustment:

               Eb/N0   S/N  Phase   relative noise RMS
  11th Nov      3.9   -62.5 -141.2     2.56
  12th Nov      4.9   -59.7 -134.3     2.01
  11th + 12th   6.6   -58.1 -137.5
  13th Nov      8.5   -55.6 -138.5     1.07
  11th to 13th  8.6   -55.2 -137.8
  14th          3.5   -61.3 -136.8     2.48
  11th to 14th  9.9   -54.3 -137.6
  15th          1.1   -64.0 -139.9     3.59
  11th to 15th  8.5   -54.4 -138.0

15th was noisy and adding it to the sum has reduced the S/N.

Summing after each day normalised to relative noise RMS 1.0:

               Eb/N0   S/N  Phase
  11th to 12th  7.5   -57.9 -137.1
  11th to 13th  9.6   -53.8 -137.8
  11th to 14th 10.4   -53.1 -137.6
  11th to 15th 10.7   -52.9 -137.8

By normalising each day, a bad day may not add much, but it
is less likely to subtract from the total.

 From now on when coherently summing, I will routinely normalise
each contribution by dividing by its overall RMS value.

There is a diminishing return here: With ideal stable conditions,
doubling the number of transmissions adds 3dB.  After many days,
each additional day only adds a tiny increment.

Gain over a single day is up to log(days)/log(2) * 3dB

For example 7dB for 5 days, which is very close to what we are
seeing with the normalised summing.

Trying to predict day to day VLF propagation is on a par with
predicting the weather!  LWPC is at best only as good as the
ionospheric model we give it to use and we can only expect it
to provide a rough idea of received signal level and day/night
phase change.

Long distances from Europe are a real challenge.  T/A east to
west is fighting against anisotropy - a strong effect at VLF.
Going further east is overland therefore much more attenuation.
N/S paths are hard work too, long overland path.  Very few
sferics rxed here from South Africa but lots from South America.

Later I'll fire up LWPC and plot some examples of long distance
paths.

--
Paul Nicholson
--