[TowerTalk] Choke on feed point of dipole

[Paul Christensen]

>"However, in the years since, I've become convinced that ray tracing that considers only a single azimuth angle has serious accuracy limitations that preclude its use in all but the simplest terrain."

I raised this issue back in 2019.  Some reply comments:

http://lists.contesting.com/archives//html/Towertalk/2019-01/msg00140.html

Paul, W9AC

Ground reflection and diffraction at any azimuth angle can wind up at the angle of interest. Imagine what happens when radiation intersects the slope of a hill off your target angle. Since even a directional HF antenna has a broad forward lobe, it illuminates lots of ground away from where it's aimed. Some of this power can come back to haunt you.

I've cautioned HFTA users with complex terrain about the limitations of single-azimuth ray tracing. The response is invariably, "I know it's accurate." When asked how they know, the answer is never satisfactory. I think HFTA and TA blind users to their shortcomings by offering fascinating and easily digestible results.

Ray tracing involves calculating power not only for direct reflection and diffraction, but for reflection from reflection, diffraction from reflection, reflection from diffraction, and diffraction from diffraction. Then do it again for higher-order cascades. This must be repeated over a dense elevation angle set to capture everything relevant. The power of 1990s computers limited the speed of TA. I wrote the time-consuming code in assembler to provide results in a reasonable amount of time. Today's computers are much faster, have multiple CPUs, and come with powerful vector instructions that can do eight floating-point calculations simultaneously. Ray tracing over all azimuths should be feasible today in a reasonable amount of time.

I've thought of writing a 2D (or is it 3D?) terrain analysis program.
But there's a showstopper: there's no empirical data to test it against.
Because the calculations are so complex, there's no way to ensure they are correct without checking results against measured data for complex terrain. As far as I know, none exists. I've searched for it and come up empty.

I've thought about what it might take to make radiation pattern measurements over complex terrain with a drone. But it's a complicated problem with many hidden sources of error. When I was considering this, each day I'd wake up with a new source of error that hadn't occurred to me the day before. I think it would be easy to get in over your head without ever knowing it. A computer program validated with fishy data is not worth anyone's attention.

Brian

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