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.
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