Hi Dave, i think part of the problem is that at this time the ionosphere is
still undergoing the chnage to night-time conditions and has not stabilised.
the important time is not ground sunset but sunset at the mid-path point at
an altitude of 100 to 150kms. Geoclock says that Shetland was into the edge
of full shadow at ground level at 2020 BST ...40 mins befor QSO start, But
the shadow edge at 100km would not reach Shetland until 2120 BST. So the
illumination would still not be "dowsedD at the middle of the path until
about 2140 BST at which time the "reflection height"  would still be
changing quite rapidly. The path should start to stabilise to night time
conditions some time soon after 2200BST. Also as the D-layer absorption dies
there may be signals returned from even higher levels which are still
illuminated. This fading is undoubtedly the interaction of two ionospheric
paths, and the groundwave stength is negligable by comparison.

Exactly what the propagation conditions are in this transition phase is open
to conjecture but there is the chance of a tilted region at the reflection
point, focusing, and maybe other effects. A lot of the signals I have logged
at 136 often show deep fades on the rising signal strength at the darkness
edge time. The more interesting data is on the signals once the night-time
phase has settled in over the path, because this should be more reliable.
Conditions are generally very good at present with very little extra
ionosation due to geomagnetic effects. Also the solar wind is relatively
quiet so there are not so many shocks setting up "waves" in the ionospheric
levels.
This may be the last "quiet" winter before the ramp up to the next Solar
Maximum.

Cheers de Alan G3NYK

----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: 14 September 2007 02:50
Subject: Re: LF: 500 QSB


>
> Hi John / LF
>
> That's a very interesting comment.  Not because of the 2-3 minute  period
> that you have observed, but because you think it's unusual.
>
> Having monitored my beacon for a fair time now I've noticed that after
> sunset the typical period for a cycle of QSB seems to be 3 minutes. That's
at a
> range of 150km and 700km (thanks to Dave 'YXM and Hartmut)
>
> I'm intrigued as to the conditions you normally experience with Finbar at
> 22.00 local over a sea path. Is that consistent propagation or do you have
less
> destructive QSB over a shorter or longer period?
>
> I notice in this months Rad Com, which fell through the door today, That
Pat
> Hawker devotes a couple of pages to 500kHz. While suitcase transmitters
are
> thankfully absent, he does quote extensively from a recent QEX article
which
> makes some interesting observations about short vertical vs loop antennas
and
> how 500kHz can produce optimal ground wave in terms of S:N per Watt over
60
> to 200 miles.
>
> Sadly, he also says that as yet, he hasn't heard a single UK station on
500k
> yet.
>
> Still, all publicity is good publicity.
>
> 73
>
> David
>
>
>
> Hello LF Group,
>
> Finbar and I moved our sked forward tonight to  9pm local (20:00UTC) and
> noticed some rather rapid, deep fading. I'm not  sure if this was time
> related due to being closer to our sunset time, or  condition related.
> I'm sure the fading is generally slower and less  catastrophic in the
> dips.
>
> The QSB on Finbar's signal went from RST  579 to NIL and back over a
> period of around 2-3 minutes.
>
> Some  periods were much shorter, some longer, but in the 20 minute QSO
> there must  have been 5 or 6 complete cycles. The QRN made it a bit
> difficult to tell  if the signal was fading or the noise increasing  at
> times!
>
>
>
>
>
>
>
>
>
>