Hi Markus I didnt look at the dates but if you look at the X-ray flux
variations over those days, you may see something that might correlate with
a slight difference in the "reflection height". During the quiet period a
couple of years ago I saw sililar amplitude variations on Lakihegy as
received here, but was not able to montor phase which anyway as we know is
not consistent :-))
Alan G3NYK
----- Original Message -----
From: "Markus Vester" <[email protected]>
To: <[email protected]>
Cc: "Paul" <[email protected]>
Sent: Thursday, March 24, 2011 10:43 PM
Subject: VLF: Phase variations and FFT resolution
Hi Stefan and Paul,
thank you for sharing the most interesting NAA phase plot.
Yes I understand that variations of ionospheric height will Doppler-spread
any spectral line especially during the dawn and dusk periods, and
ultimately the signal might be smeared out so much that there is no more
benefit in higher resolution. But I do not think that a 47 uHz FFT is beyond
the limit. Though the signal is integrated over about 6 hours, due to the
usual windowing the tails are less heavily weighted, reducing the
"effective" duration to about 4 hours.
A perfectly linear phase slope would simply place the maximum in an adjacent
bin. So the trace will wobble a bit, with a lower frequency while the
terminator moves across the path during the evening, and higher in the
morning period. Only the deviation from a linear fit will lead to a loss of
bin magnitude. The effect depends a bit on the probability distribution of
the phase deviation, uniform distribution gives a sin(x)/x behaviour. If all
phases within a 90° interval (ie. +-45°) appear equally often, the
degradation is only 0.9 dB. With 180° you'd still have -3.9 dB left. To kill
the signal completely, you'd have to spread the phase across the whole 360°.
To illustrate this and to coarsely estimate the phase deviation, I have
sketched slant coloured boxes across the slopes in Paul's graph. The slopes
have a duration of about 6 hours (morning, +60 uHz) and 4 h (evening, -40
uHz), and the vertical width of the boxes is on the order of 90°. (It looks
as if the sign of the phase was opposite to the usual convention, as the
sunlit path (12 to 18 UT) should be shorter, with phase advancing towards
more positive (up) rather than negative values).
Assuming that NAA is perfectly GPS synchronized, the systematic shift of the
daytime phase is astounding. A possible simple explanation is that days are
becoming longer during the spring season, and midday solar elevation is
increasing from day to day.
With a given absolute path length variation, phase deviation should scale
more or less proportional to frequency, so the adverse effect on 9 kHz
should be 2.5 times less than on 24 kHz. In essence, I expect only little
phase spreading loss across an intercontinental path at 9 kHz and 47 uHz
resolution.
On the other hand, a very much longer integration across day and night (eg.
12 uHz) would not seem to make much sense, not only due to the phase effects
but also due to the variations in signal level and background noise.
Best 73,
MArkus (DF6NM)
From: Stefan Schäfer
Sent: Thursday, March 24, 2011 7:01 PM
To: [email protected]
Subject: Re: LF: Wasilla Alaska VLF - offline
Hi Laurence, Markus, VLF,
Hmmmm, about the 60000 windows, Paul Nicholson means.... (see a part of our
emails below)
Am 19.03.2011 09:41, schrieb Paul:
Hi Stefan,
> Well, you see people are running slower and slower windows,
> such as "DFCW-60000"
There is a limit set by the variability of the path length.
For example, see
http://abelian.org/vlf/tmp/110319a.gif
This shows the absolute phase of NAA at 24kHz over a great
circle distance of 4672km. The graph spans 5 and a bit days.
During the day the phase advances because the D-layer height
falls from about 90km to 70km. The pattern repeats each day
with the night-time phase (path length) pretty much the same
each night, but the daytime path varies.
The path change between day and night is almost a complete
cycle at 24kHz, so we might expect 100 to 120 degrees at 9kHz.
The value of a long coherent integration will be significantly
reduced if the phase changes by more than 30 or 40 deg.
If the phase changes by more than 90 deg, a long integration
will be worse than a shorter one.
Maybe "60000" is better anyway, as seen between DJ8WX or OE5ODL and TF3HZ.
Anyway i would start with a faster mode and go for a slower, if nothing is
visible. It's your dicision, Laurence :-)
73, Stefan/DK7FC
Am 24.03.2011 11:40, schrieb Markus Vester:
Hi Laurence,
if I had only one to choose from, I'd definitely opt for the slowest
"60000" window at 47 uHz. Depending on antenna orientation, this would
definitely give Scott a chance, or perhaps one of us over here. Would an
onnidirectional E-field work as well for you as the RX loops?
Running the second FFT for the Alphas nearly doubles the CPU load, so you
may want to skip that.
Thanks for the great work!
Best 73,
Markus
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