Message-ID: <BFF6723B062BFF40B4B7A6CF670160B4041262C5@argos2.clb.tcfr.thales>
From: Jean-Louis.RAULT@fr.thalesgroup.com
To: rsgb_lf_group@blacksheep.org
Date: Thu, 8 Jul 2010 13:12:47 +0200 
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Subject: LF: RE: Re: RE: Re: QRN
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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: owner-rsgb_lf_group@blacksheep.org 
> [mailto:owner-rsgb_lf_group@blacksheep.org] 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: <Jean-Louis.RAULT@fr.thalesgroup.com>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: Thursday, July 08, 2010 7:57 AM
> Subject: LF: RE: Re: QRN
> 
> 
> >
> >
> > > -----Original Message-----
> > > From: owner-rsgb_lf_group@blacksheep.org
> > > [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of 
> Alan Melia
> > > Sent: Thursday, July 08, 2010 12:27 AM
> > > To: rsgb_lf_group@blacksheep.org
> > > 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
> >
> 
>