At 02:02 AM 11/3/2007, David Gilbert wrote:
>HFTA
>uses ray tracing calculations for edge diffractions, surface
>reflections, and combinations thereof to compile a net elevation profile
>of the energy passing over the terrain after leaving the antenna. To
>say that a single ray coming from the far end of the path and "seeing"
>an entirely different terrain profile from the return side (assuming
>asymmetric features) would follow the same path to the antenna just
>doesn't make sense.
The assumption that an individual ray will follow a different path
when moving left-to-right than right-to-left is what shouldn't make sense.
As you shoot individual rays outward from the antenna and the ray
encounters an obstacle, that single ray diffracts into multiple
diffracted rays travelling in a range of directions with a
well-defined distribution of amplitude with direction.
All reciprocity says is that for any one of the multiple diffracted
rays, the amplitude of that diffracted ray depends only on the
included angle between the incident ray and the diffracted ray and
not on the direction of the incident ray.
>Even HFTA says (per the very simple test profiles I
>generated) that an asymmetrical terrain feature results in asymmetrical
>diffraction when viewed from both sides. Unless you want to challenge
>the algorithms and calculations built into HFTA, I don't see how how you
>and N6RK can challenge that result.
No one is challenging that result. That's because N6BV's comments in
the HFTA documentation and your tests both address a point that is
quite different from simple reciprocity.
HFTA evaluates the outward travelling signal at infinity (more
precisely as a plane wave after the last refraction/diffraction) by
shooting rays from the antenna through the obstacle to infinity.
Placing the antenna on the opposite side of the obstacle and
re-running the HFTA ray trace is not the same as reversing the
direction of the rays in the original configuration and tracing from
infinity through the obstacle to the antenna.
73,
Mike K1MK
Michael Keane K1MK
k1mk@alum.mit.edu
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