Jim AA5BW wrote:
> I'm guessing that the multiple inflections could be
> manifestations of amplitude noise alone (given ~14dB SNR
> and < 7 degrees between inflections)
Yes. To demonstrate...
I mixed a locally (software) generated carrier into the audio
recording of 1st Jan 00:00 to 07:00, placing it at 8823 Hz and
setting the level to match approximately the average amplitude
of W4DEX at 8822 Hz.
Now we can measure the artificial carrier (which has exact
constant phase of -110) against the real background VLF noise.
----W4DEX------ --Simulated----
00:00 to 01:00 11.6 dB -122.1 11.9 dB -110.7
01:00 to 02:00 13.1 dB -119.4 14.1 dB -100.8
02:00 to 03:00 12.6 dB -131.9 14.9 dB -118.7
03:00 to 04:00 14.3 dB -135.0 13.0 dB -104.1
04:00 to 05:00 12.7 dB -111.0 13.8 dB -121.6
05:00 to 06:00 15.2 dB -118.9 14.4 dB -100.2
06:00 to 07:00 13.4 dB -111.7 13.0 dB -88.1
The phase variations due to propagation are probably
similar size...
For example, here's the phase of NAA 24kHz for the same
time period
http://abelian.org/vlf/tmp/naa150101a.gif
The overall increase in path length during the night is apparent
from the reducing phase lead but there is variation at a wide
range of time scales.
I'd guess that as we go lower in frequency, the phase variations
reduce in proportion. So the ~50 degrees range at 24kHz
probably drops to ~20 degrees at 9kHz.
Propagation measurements at low VLF are as rare as hen's teeth.
Radio amateur transmissions could be useful to some researchers.
There is great reliance on propagation models for lower
ionospheric research, lightning measurements and long range
lightning location systems (which have to adjust arrival time
measurements to allow for propagation effects). Data from
amateur signals would enable spot checks on model accuracy.
A challenging experiment would be to measure the actual path
length. This requires transmitting two fairly close frequencies
either simultaneously or alternating with FSK. Accurate phase
measurements allow the path length to be calculated.
--
Paul Nicholson
http://abelian.org/
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