Topband: one-way propagation

[James Wolf]

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 at contesting.com] On Behalf Of k9la at frontier.com
Sent: Monday, December 03, 2012 1:10 PM
To: topband at 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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