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Topband: one-way propagation

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Subject: Topband: one-way propagation
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Date: Mon, 3 Dec 2012 10:09:45 -0800 (PST)
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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 
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
Topband reflector -
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