[TowerTalk] Takeoff Angles and Non-Reciprocal Propagation

[Terry Conboy]

At 02:24 PM 2007-11-01, David Gilbert AB7E wrote:
>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.

As I understand it, in the absence of non-linearity and anisotropy, 
reciprocity still applies to cases of multiple refraction.

I suspect most cases of apparent "one-way" propagation are due to 
differing signal-to-noise ratios caused by local noise sources.  This 
can be compounded by inefficient transmit antennas at one end.

73, Terry N6RY