|
Hi Mal, this is a fairly common case in most parts of the
cycle I have looked at so far. I can only surmise what I think causes
the effect. I believe that as the daytime absorption dies away and
the D-layer electron density drop the "reflection heaght" for long distance
signals moves up from 50 to 60 km to around 100km. This means a greater distance
can be covered in one hop and the received signals increase in strength. As the
night progresses I think it may be that higher angle signals are propagated so
around the 0200 to 0330z period you will see quite deep fading on long distance
signals as the differnt paths cancel each other out (or sometimes reinforce each
other) I notice that the centre part of the night does not normally show lower
levels but it does seem to have more dips which carve the QRSS signals up. Why
this should change again towards morning is a mystery still, but over all I
agree with you that the best times are around an hour to 90 minutes before
dawn.
The effect oftent mentioed of signals continuing until after
dawn is fairly common at "quiet sun" times, and particularly in the
morning. In Europe the noise is mainly from the east (the land mass). The
sunrise sweeps over eastern europe up to a couple of hours before the dawn
reaches the UK. This can mean that there is a quieter period in the morning
just before UK dawn, this is good for looking to the west but not so good for
the Russian stations. The important cut-off is the time that the darkness shadow
reaches the region of the "reflection" at the height of the "reflecting
layer" ( I keep putting reflection in quotes to save myself hassle
from the purists who say it is not a mirror-like reflection but a gradual
bending or refraction). Tis reveals some interesting ideas, although
geometrically the maximu, rage on LF for a one-hop signal is about 2200 to
2500km, my suspicion is that the first signals to reach the NA coast from the
UK may do so in just one hop (!!) They are quite weak but at this time half
the Atlantic at 100km altitude is in sunlight. It is possible that there is some
"daytime" propagation but there are strong reasons related to the "tea-time dip"
that suggest this is not the case. Daytime signals were often visible from CFH,
usually in winter when the solar radiation was weaker and probably the
attenuation was less. These often peaked at about 1500z when the mid-atlantic
point was getting the highest dose of UV. I did copy Joe at daytime in 2004 (I
think) around 1300 in summer, but I think the reason for that was a very high
concentration of electrons in the D-layer due to a geomagnetic storm. Normally
the summer radiation levels with wipe out daytime propagation at that
distance.
I do have ARGO signal traces from Joe John Dex and Warren
(when active) through most of the last week or so. The suffer fading in the
middle hours and come up quite strong before dawn. It is interesting to see
the way that the fade at different times due the phase differences on the
different path lengths. The same effect occurs at the receive end,
different station will see the fades at different times and one station may
report excellent signals (beacuse of addative effects) whilst another will
moan of poor signals because the fading at his site as not been favouable. The
distance between such widely varying reception can be as little as
50 km.
Its fascinating stuff (to me who is well adicted anyway !) and
more effects are appearing during these quiet years which may help to
harden up on some of my "flights of fancy".
Cheers de Alan G3NYK
----- Original Message -----
Sent: 26 January 2006 09:12
Subject: LF: TR
Hi All
Stns observed during the night. VO1NA, XKO and
XES.
XKO strongest sig last nite and vy good
around 0800z, XES not as strong as usual and VO1NA only visible at times
but a good signal when he popped up. There were a couple of others that I
could not make out *MJ
My observations indicate that the best time for
TR to my QTH is late morning 0700 - 0900z other time is 0001 - 0200z
approx and little or no propagation the rest of the time during
night time. ie my sunrise seems best and
sunset at the USA end. This is the RX situation at my end, TX could be
different from here. I am surprised that there is no propagation during the
majority of the dark hours at both ends.
What does Alan think about that.
73 de Mal/G3KEV/IO94SH/Scarborough
| 