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Gents
If the signals that are fading at different
frequencies are from the same source, then you can determine the difference in
path length from the null spacing. (assuming you can see signals over a wide
enough bandwidth from the same source to see 2 nulls at the same
time .. I have observed this on DRM OFDM signals on 7MHz, they are
about 10KHz wide
From null to null in the frequency domain implies
that for one frequency the path length difference between 2 paths is n lambda +
1/2 lambda, for the next frequency the path length difference must be n+1
lambda + 1/2 lambda (If we assume only 2 discrete signal paths, and
that one path length is fixed.. groundwave and that the second path is
the same physical length at both frequencies) then we can see that difference in
path length will be equal to the wavelength of the difference frequency. This
principal is often used in commutating line channel combiners at UHF
for a single carrier, it is possible to assess the
rate of change in the difference of the path length. this will give a measure of
ionospheric movement.
if the nulls dip every 1 minute, then at 500KHz the
difference in path length is changing by 1 wavelength per minute, ie 600m /
minute or 10m/s
Working out exactly what is happening to make the
path lengths change, or proving where the reflecting layer is and how it
is moving takes more observations and a bit more work
:<)
Over the years I have been capturing meteor
reflections that have Doppler shift. max Doppler occurs when the meteor enters
vertically perpendicular to the path. From the Doppler you can measure the
'relative speed' of the incoming rock, and its (or at least its trails' )
deceleration.. fascinating stuff, they don't half move fast!
Mark GM4ISM
----- Original Message -----
Sent: Sunday, December 28, 2008 4:21
PM
Subject: RE: LF: 500KHz QSB
Hi
Rik & Graham,
Thanks
for the replies.
All
understood now, the screen shot certainly shows it
up.
I
didn?t relate multi path fading to what I was seeing on the display as I am
normally used to seeing/hearing single narrow signals fading differently
from different sources. So I hope I am correct in saying that with the three
distinct signals from the NDB the additive & subtractive properties of
each multi path signal even though closely related in frequency will react so
noticeably differently at the receiver even from the same
source.
I
missed Grahams report on the UK500KHZ group.
73
Gary
? G4WGT.
I assume its selective fading ,
you can see it at 100 Hz separation at >
add the shot with a few pointers ?
Nb .. check out the 500 band
in those day's , no mal-adjusted cw signals
Sent: Sunday,
December 28, 2008 3:15 PM
Hi
LF,
Just made an
interesting observation regarding QSB at
508KHz.
For my own
benefit I have started running a slow SpecLab showing 500 to 510KHz &
capturing for evaluation.
Occasionally
the NDB ?Z? shows up. The strange thing is that even though the carrier on
508KHz & the two ID offsets at 509 & 507KHz are relatively close they
each display at different depth of fading or an out of phase fade
pattern.
I am unsure why
this should be, any comments please, see capture at link
:-
http://myweb.tiscali.co.uk/wgtaylor/ZNDB.jpg
73,
Gary - G4WGT -
IO83QP.
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