To expand a little on Carl's explanation, the ionosphere is not as isotropy as
we commonly imagine - in that it is not a nice smooth balloon shaped surface to
bounce a signal off of. Instead, there are varying degrees of irregularity,
not unlike a sandy desert where the wind moves the landscape around and creates
moving peaks and valleys. So imagine the ionosphere in this condition when a
signal from one direction has a nice reflection down to the receiver, but in
the other direction, it is reflected at a different angle and lands somewhere
else.
Jim, KR9U
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of
k9la@frontier.com
Sent: Monday, December 03, 2012 1:10 PM
To: topband@contesting.com
Subject: Topband: one-way propagation
Jim K9YC asked about other possible mechanisms besides atmospheric noise to
account for one-way propagation on 160-Meters. For the record, I also believe
atmospheric noise (and even man-made noise as experienced by the PT0S ops) is a
big player in these observations. By the way, I also would like to extend a big
THANK YOU to the PT0S ops, especially for their topband effort.
One-way proapgation was an interesting topic in the AM broadcast industry in
the 1970s (I do not know if it still is). The issue was tied to allocation of
frequencies and the difference in loss depending on whether it was an
east-to-west path or a west-to-east path. John C. Wang of the FCC measured
signal strengths of 18 MF broadcasting stations (540 – 1630 KHz) at 4 receiving
sites throughout the continental US to compare the measurements to the CCIR
model.
Subsequent analysis of this data by Douglass D. Crombie of the Institute for
Telecommunication Sciences in Boulder found that for paths between 200 km and
3000 km, the east-to-west path loss is some 9 dB greater than in the
west-to-east direction. Further analysis shows this is highest for frequencies
above 830 KHz. The data did not show any such difference on north-to-south
paths and south-to-north paths.
The second-to-last sentence in the previous paragraph suggests that the
ionosphere, being immersed in the Earth's magnetic field, is anisotropic – in
other words, the ionosphere looks different for RF going east-to-west versus
west-to-east. But if you run ray traces with Proplab Pro (it includes the
effects of the magnetic field and electron-neutral collisions) for both
directions, you will see no significant difference – at least I have not for my
runs). We have to watch it here, though – the model of the ionosphere in
Proplab Pro is a monthly median model (as it is in all of our propagation
prediction programs) – so it does not capture the day-to-day variability, which
may be involved in these observations.
So I believe there is a possibility that there are other mechanisms at play to
give us one-way propagation. Unfortunately, as far as I am aware, we do not
understand them.
Carl K9LA
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Topband reflector - topband@contesting.com
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Topband reflector - topband@contesting.com
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