HFTA clearly shows that signals can be refracted over sharp terrain
features to bend closer to the horizon (sometimes significantly so) than
would otherwise occur based upon the actual antenna height above
ground. Some portion of the transmitted signal might end up with an
effective takeoff angle of, for example, six degrees that wouldn't be
there without a fortuitously located hill. Since terrain features are
almost never symmetrical, though, there doesn't seem to be any guarantee
that a return signal from the DX end would necessarily bend back down to
the local antenna, at least not in exactly the same way. I would think
that the sharper side of a hill would defract a signal differently than
a gently rounded side.
I wonder if this could possibly explain some of the non-symmetrical
"propagation" that most of us think we may have occasionally
experienced. I would think even a large building of the right shape and
composition could act like a terrain feature ... especially on the
higher frequency bands.
Dave AB7E
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