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
There is no possible combination of refractions or reflections that is not
reciprocal in both directions.
If there is a mechanism at work, it is at an entirely different level than
simple multiple reflections.
After 50 years on 160, I'm still unsure if one way propagation exists. All
of the "one way" I have seen and taken the time to look at involves noise
levels, QRM, or just the capability of the stations.
For example, prior to sunset here my local noise is very low. Even on a
quiet winter night in a quiet direction, noise floor increases 10-15 dB as
it gets darker and conditions improve. I can hear Europeans that I have no
hope of working (on 40 meters this happens all day long). Their noise is
high (even in a quiet location) because it is dark. As it gets darker their
signals come up, and so does my noise and QRM, but eventually they hear me.
The opposite occurs at sunrise. There is the same noise floor drop after
daylight. This means I can hear JA's working west coast stations long after
they stop being able to hear me.
Local storms and local QRM, and the direction of that stuff, is also is a
huge factor in reciprocity.
Sorting it all out is very complicated, and involves far more than observing
"sometimes people at one end don't answer". Sorting it out would involve and
**require** calibrated observations of absolute signal levels and noise
levels at both ends. I think this is why one way prop is, at best, arguable.
73 Tom
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Topband reflector - topband@contesting.com
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