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Message-ID: <012f01cbea7a$b64695e0$4001a8c0@lark>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <8CDB82718FF0CEC-B4C-BB65@webmail-d038.sysops.aol.com> <4D8B86E7.20602@iup.uni-heidelberg.de> <4427E21AD3434A31B4CF500C93C98101@White>
Date: Thu, 24 Mar 2011 23:12:59 -0000
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Subject: LF: Re: Phase variations and FFT resolution
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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 perio=
d 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 kno=
w is
not consistent :-))

Alan G3NYK

----- Original Message -----=20
From: "Markus Vester" <markusvester@aol.com>
To: <rsgb_lf_group@blacksheep.org>
Cc: "Paul" <paul@abelian.netcom.co.uk>
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-sp=
read
any spectral line especially during the dawn and dusk periods, and
ultimately the signal might be smeared out so much that there is no mo=
re
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 ad=
jacent
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 lo=
ss of
bin magnitude. The effect depends a bit on the probability distributio=
n of
the phase deviation, uniform distribution gives a sin(x)/x behaviour.=
 If all
phases within a 90=C2=B0 interval (ie. +-45=C2=B0) appear equally ofte=
n, the
degradation is only 0.9 dB. With 180=C2=B0 you'd still have -3.9 dB le=
ft. To kill
the signal completely, you'd have to spread the phase across the whole=
 360=C2=B0.

To illustrate this and to coarsely estimate the phase deviation, I hav=
e
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=C2=B0.=
 (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 towa=
rds
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 day=
s 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 sc=
ale
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 lit=
tle
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 Sch=C3=A4fer
Sent: Thursday, March 24, 2011 7:01 PM
To: rsgb_lf_group@blacksheep.org
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 TF=
3HZ.
Anyway i would start with a faster mode and go for a slower, if nothin=
g 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