Realizing that this is just an anecdote, I cannot help but remember the
summer of 1965, when I was living in Seoul, Korea and running 100 watts and
a low dipole on 20 meters. Every evening I heard CE3RE 20-30 dB over S9,
with reciprocally good reports, when nothing else in South or North America
was to be heard with signal strengths anything like comparable. It got to
the point where one or the other of us had only to clear his throat between
14150 and 14200, and the other would know he was on the band. Clearly
there was something special going on, and it was not a rarity that summer,
anyway.
73, Pete N4ZR
At 09:57 AM 10/25/2004, KN4LF wrote:
Antipodal focusing is an accepted propagation mode in the MF/HF frequency
SWL and amateur radio communities and many stories abound concerning very
long range reception of relatively weak transmitted signals at high
strength levels. The heterogeneous ionosphere will allow for antipodal
focusing on HF but in practice true antipodal focusing is a rare.
A U.S. Government communications agency propagation study found that true
HF frequency antipodal focusing is indeed rare and partial propagation
path antipodal focusing is a more common mechanism and occurs most
frequently around the Spring and Fall Equinox when the heterogeneous
ionosphere is more uniform. MF frequency true antipodal focusing is
virtually impossible and partial antipodal focusing a distinct rarity. At
MF the propagation mechanisms mistaken for antipodal focusing are chordal
hopping and ionosphere layer ducting.
73,
Thomas F. Giella, KN4LF
Retired Space & Atmospheric Weather Forecaster
Plant City, FL, USA
Grid Square EL87WX
Lat & Long 27 58 33.6397 N 82 09 52.4052 W
kn4lf@arrl.net
Propagation eGroup: http://lists.contesting.com/mailman/listinfo/propagation
PropNET Beacon Program: http://www.propnet.org
KN4LF Daily Solar Space Weather & Geomagnetic Data Archive:
http://www.kn4lf.com/kn4lf5.htm
KN4LF Amateur & SWL Radio History: http://www.kn4lf.com
----- Original Message -----
From: GeoffGrayer@aol.com
To: PSC.Committee List Member
Sent: Monday, October 25, 2004 02:47 AM
Subject: [PSC.Committee] Skip focusing - Further food for thought {03}
Maybe I'm missing something here. My picture seems very simple to me -
I didn't think I needed to go into such detail, but I have attached my
picture of antipodal focusing in a WORD document, as I felt the need to
do some sketches. Please excuse their crudity, but I find WORD difficult
in this respect!
I have only considered focusing in the horizontal plane, because this
is peculiar to antipodal focusing. There is also focusing in the vertical
plane e.g. due to the concave curvature of the ionosphere (while the
convex surface of the earth will have the opposite effect), and other
effects mentioned in previous e-mails, but these effects occur over all
paths, not only the antipodal path, so I haven't considered these here.
I am sorry I cannot produce any estimate of the focusing gain, this
would depend on the width of the bundle of rays focused at the antipodal
point, and this in turn would depend on the homogeneity of the ionosphere
and ground reflection points. I don't have any ray tracing software with
which I could make an estimate, but intuitively it seems that if other
focusing effects are significant, this must be also.
In addition, as I mentioned before, I see no alternative to explain the
consistent propagation from G and near Europe to ZL and nowhere else
along the terminator almost every morning on 40m. At the same time the
terminator would be crossing many Pacific islands (some of them, like
Fiji, populous), and a significant part of Africa. I intend to start
recording this effect (times, signal strengths).
Geoff.
---
Outgoing mail is certified virus free by Grisoft AVG 6.0.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.782 / Virus Database: 528 - Release Date: 10/22/2004
_______________________________________________
Propagation mailing list
Propagation@contesting.com
http://lists.contesting.com/mailman/listinfo/propagation
_______________________________________________
Propagation mailing list
Propagation@contesting.com
http://lists.contesting.com/mailman/listinfo/propagation
|