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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