I asked myself that question a dozen times up front, and always got the
answer that since diffraction is a function of the radius of curvature
of the diffracting surface, if the diffracting surface is asymmetric
(different radius of curvature for the incident wave as it travels in
either direction) the effect of the diffraction cannot be completely
reciprocal. Whether it is valid to say that diffraction itself is
non-reciprocal is a semantics issue on which I'm no expert.
An incident wave does not act like an infinitely thin ray .. it acts
like a wave, and it is influenced by its surroundings within a distance
determined by its wavelength. A wave that passes close by (in terms of
wavelength) a surface will suffer dispersion and loss that is a function
of the radius of curvature of the surface ... an edge with a small
radius disperses the wave (some of which bends around the surface) more
so than a surface with a larger radius. You can either accept that or
not ... it isn't my job to convince you.
My original postulation was that since terrain features are sometimes
not symmetric, it seemed that an outgoing wave might be diffracted
differently than a return wave attempting to follow the same path. I
later discovered that N6BV, the author of HFTA, had said pretty much
that same thing in the ARRL Antenna Book. If I get the chance I'd like
to try to demonstrate that experimentally ... glad to have your blessing.
A few additional comments.
First, the favorable bending (diffraction) of a signal over a terrain
feature is mostly noticeable at very low angles. If the path between
two stations is such that low angles aren't required, or propagation is
such that low angles are useful but higher angles are significantly
involved as well, then whatever effect that may be caused by asymmetric
terrain is not likely to be noticed. I think the very great majority of
actual QSOs would not be affected.
Secondly, the actual elevation profile of the signals coming from either
direction is not a simple function of diffraction. It's a function of
the combination of the reflections and diffractions that occur because
of the terrain. So far in this discussion I've only mentioned
diffraction because it seemed to me that we if established the
asymmetric effect of diffraction on the path that would be sufficient to
suggest a mechanism for non-reciprocal transmission of signals. But in
reality, it is almost impossible to assume that all those combinations
of reflection and diffraction can occur identically from either side of
an asymmetric terrain feature, making the case (in my opinion) even
stronger.
Thirdly, a wave leaving Station A with horizontal polarization will
interact with its foreground in a manner calculated by HFTA.
Conventional knowledge says that after passing through the ionosphere
the polarization of a signal becomes mostly elliptical. Since
diffraction (I think) and reflection (I know) are a function of
polarization, it would seem that the reverse direction would behave
somewhat different based upon that alone, although probably not by very
many db. Still, it might be relevant when signals are in the mud.
Finally, I'm not searching for all the possible mechanisms that might
possibly create non-reciprocal signal readability. I'm not even trying
to prove that non-reciprocal communication exists ... only that IF it
exists, asymmetric terrain at one end of the path may be a possible cause.
73,
Dave AB7E
Ethan Miller K8GU wrote:
> Given what
> we know about reciprocal networks, ask yourself, "Why wouldn't
> diffraction be reciprocal?"
>
>
>
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