Hi Jean Louis ....it is good to have some guys who REALLY know what they are
talking about to keep me in check :-)) I have found it so difficult to get
information in some of these areas that I often lean towards making
"contentious statements" in the hope that someone will gently say....."Its
not quite like that Alan!" that is how I have learned the most in this
area!! My last big jump in understanding (or thinking I do, a little) came
with the help of Dennis Gallagher who you may know in the Yahoo VLF group.
I do remember the beginnings of Omega because I shared an office with some
of the chaps responsible for phase stabilising GBR in the early experiments
(Post Office Research in those days)
Kerguelen.....mmmm sounds like a great place to work, but I suppose there
were downsides :-))
Best Wishes
Alan G3NYK
----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Thursday, July 08, 2010 12:12 PM
Subject: LF: RE: Re: RE: Re: QRN
> Understood and fully agreed
>
> When I saw your message about the dips, I thaught you were talking about
the sharp Vs
>
> Sorry for that
>
> Kind regards
>
> Jean-Louis F6AGR
>
> > -----Original Message-----
> > From: [email protected]
> > [mailto:[email protected]] On Behalf Of Alan Melia
> > Sent: Thursday, July 08, 2010 1:00 PM
> > To: RSGB LF Gp
> > Subject: LF: Re: RE: Re: QRN
> >
> > Hi Jean Louis that is a common belief but it would seem to be
> > the wrong
> > "shape" for interference. Interference dips are caused by
> > near equal signals
> > strengths on two paths, whose relative phases vary due to
> > path length. In
> > most cases of this kind of fading the shape of the amplitude
> > plot is a sharp
> > "V",
> > whereas the morning dip is a broad "trough". If you record
> > the dip minimum
> > amplitude it is very consistant from day to day which seems
> > unlikely for a
> > such a variable mechanism as a fade.
> >
> > I have also seen (at 136kHz again) interference induced on
> > the "morning
> > dip" by an early morning flare during that period. Again if it was
> > interference it would vary from location to location. This
> > does happen when
> > the distance to the transmitter is short
> > (500km or so) when the ground wave is stronger than the
> > signal returned from
> > the daytime ionosphere. The "dip" becomes a "hump" and the
> > maxima correlates
> > well to expected ground wave strength. The during the day the
> > amplitude of
> > the received signal decreases from a peak at dawn in a "dish" shape.
> >
> > See a QST article by the late Bob Brown NM6M around 2000 (I
> > can't find the
> > reference) which describes the illumination at the dawn edge of the
> > ionosphere from underneath which creates an intensely
> > absorbing condition.
> > As the intensity increases with solar elevation and the
> > illumination is from
> > above the attenuation actually decreases. At 136kHz the
> > morning dip strength
> > correlates well to the expected (calculated) groundwave
> > signal. This is
> > obviously a difficult area to investigate but I think the
> > explanation is
> > supported by papers from Stanford who are world leaders in
> > this area.. The
> > ionoshereic absorbtion actually decreases during the day to a
> > minimum at
> > noon at mid-path. However the shape of the amplitude curve
> > you might plot
> > against time can be a "dish" or a "dome" depending upon the
> > relative phases
> > at the receiver.
> >
> > Fascinating stuff !
> > Best Wishes
> > Alan G3NYK
> >
> >
> >
> >
> > ----- Original Message -----
> > From: <[email protected]>
> > To: <[email protected]>
> > Sent: Thursday, July 08, 2010 7:57 AM
> > Subject: LF: RE: Re: QRN
> >
> >
> > >
> > >
> > > > -----Original Message-----
> > > > From: [email protected]
> > > > [mailto:[email protected]] On Behalf Of
> > Alan Melia
> > > > Sent: Thursday, July 08, 2010 12:27 AM
> > > > To: [email protected]
> > > > Subject: LF: Re: QRN
> > > <snip
> > > at 20kHz
> > > > the morning an evening dips on the stations monitored by
> > SID detectors
> > are
> > >
> > > > due to increased aborption.
> > >
> > >
> > > Well, I would say that the differences of amplitude of VLF
> > signals between
> > > day and night are due the presence or absence of the D
> > layer and that the
> > > deep morning and evening dips are due to interferences
> > between the ground
> > > and the sky waves.
> > >
> > > Jean-Louis F6AGR
> > >
> >
> >
>
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