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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