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Alan,
based on Loran-C measurements, I would agree
on the ballpark figure of -6 dB per hop. Not sure though if land reflections are
really so much worse than seawater, as for low angle incidence the
reflection coefficient may still be close to unity.
> A 16dB fade suggests 20dB or so difference in
amplitude.
No. -20 dB = 0.1 x amplitude for the weaker
signal. But 1+-0.1 = 1.1...0.9,
which would be a very flat fade (1.74 dB max to min). To
produce deep fades, the signals would need to be rather similar in
magnitude.
73,
Markus (DF6NM)
Sent: Monday, November 04, 2013 12:40 AM
Subject: LF: Re: RE: Hartmut's Grabber On Last
Night...
Hi Jim yes I think it is a phase effect, the
reinforcement/cancellation is probably between 2 hop and 3 hop paths. A 16dB
fade suggests 20dB or so difference in amplitude. I used to reckon on about a
12dB per ionospheric hop and less, maybe 6dB, for a sea "bounce" based on signal
from Joe VO1NA and the Naval station at Newport Corner near Halifax NS. Each hop
has a maximum range of about 2000km. There are several effects around the sunset
shadow area that could add to the effect too. Higher angles at the
ionosphere may penetrate futher before returning giving higher attenuation to
the higher hop modes.
A thumbnail calculation suggests hop3 ampltude of
about -23dB below hop 2 would give a 17dB fade as it goes in and out of
phase.
Very much a "hand waving" estimate but a useful
empirical guide.
Alan
G3NYK
----- Original Message -----
Sent: Sunday, November 03, 2013 8:56
PM
Subject: LF: RE: Hartmut's Grabber On
Last Night...
Bob,
This
is the third time that I?ve noticed Dex?s signal peaking substantially
(>15dB?) starting around 0500, at approximately the same time that your
signal begins a substantial fade. I?ve seen this twice on captures from
Hartmut?s grabber, and once on a capture from Mike?s grabber.
An
interesting example is 21 October 2013, when captures by Henny, Mike and
Hartmut all showed a fade of roughly 10-15dB in your signal from 0500-0530,
with Mike?s grabber (which showed Dex?s signal clearly around that time)
showing a rise in Dex?s signal of roughly 10-15dB.
I
wonder if this could be attributable to modal phase reinforcement/cancellation
near the terminator? The relative change between your signal and Dex?s seems
rather high for that mechanism, but I suppose not out of the question. AGARD
CP 305 includes a paper (Morfitt) with some interesting plots in this context,
and a number of other papers with interesting propagation studies at
frequencies near 74kHz; is there a good place for me to post info of this
type?
73,
Jim
AA5BW
Good
shots of Me and Dex-Bob WG2XRS/4 NY [XRS4] and [XRS5]...
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