>> It seems to me that a good portion of Station B's signal arriving at the
>> hill could overshoot Station A because it wasn't diffracted enough. The
>> path taken by the signal from B to A would be identical to the path
>> taken by the signal from A to B EXCEPT for the portion between Station A
>> and the hill. Think directional coupler.
Uhh, nope.. even though it sort of seems that way at first glance..
Imagine that the top of the hill is a big lens or prism that bends the
ray (which is what diffraction is, in one sense)..
Launch a ray from A towards B and it follows a certain path, apparently
bending over the top of the ridge. Some distance after the hill, stop
the ray and send it back exactly as it came. When it gets to the
prism/lens it will bend it just like it did on the outbound trip and
wind up at A.
As long as there no nonlinearity in the system, it works.
Where there IS non-reciprocal propagation you need something else. here
are some examples:
a) propagation through the ionosphere, which is anisotropic. The
polarization is rotated by the Faraday effect, but the same rotation
regardless of direction, so, if you have polarized transmit and receive
antennas, oriented, say, 45 degrees apart, and the rotation is 45
degrees, A signal from A to B winds up perfectly lined up, but a signal
from B to A is cross polarized.
b) the ordinary and extraordinary ray (polarization dependent) follow
different paths, so when they combine the net effect is different
depending on the direction (actually another case of the phenomenon in A)
c) sound propagating in a medium with a velocity gradient. This causes
sound to bend down when going downwind, and bend up when going upwind.
d) various and sundry light propagation through various crystal
phenomena (e.g. polarizers, birefringent materials, etc.)
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