[TowerTalk] a fancier hfta

[jimlux]

For those interested in a more sophisticated approach, one starting
point is the 1994 paper by Breakall, et al.

J.K. Breakall, J.S. Young, G.H. Hagn, R.W. Adler, D.L. Faust, D.H.
Werner, "The Modeling and Measurement of HF Antenna Skywave Radiation
Patterns in Irregular Terrain", IEEE Trans. Ant. Prop. V42, #7, July
1994, pp936-945



Abstract is below..

Basically they used GTD with the current distribution in the antenna as 
the source.  They also (as expected) found that for horizontal 
polarization, you can just assume that the terrain is perfectly 
conducting (which I believe HFTA does), while for vertical pol, you need 
to know what the dielectric properties are.


It's true that this work modeled the terrain in sort of a 2 1/2 D way.. 
that is, the terrain was a row of strips with the long axis of the strip 
perpendicular to the direction of propagation.

Of course, computational horsepower available now is a LOT more than 
they had in the early 90s, so some of the analysis they couldn't do for 
compute time reasons might well be feasible and reasonable today. 
Likewise, getting the terrain model is probably much easier today, what 
with downloadable DEMs.

One of the authors (J.S. Young) was developing codes to do the combined 
MoM and GTD, and maybe if someone can track him down, it might be 
productive. Gerry Burke published a paper in the same year about using 
Physical Optics for looking at HF propagation in irregular terrain.  A 
google for "Burke Physical Optics 1994" will probably find it.



All in all it's a pretty complex area, and one would have to ask whether 
the work involved in doing the more sophisticated model (and it would be 
a lot... I'd say multiple work years off hand, by the time you get it 
working and validated) would be worth it.


Jim, W6RMK


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Here's the abstract for the Breakall paper.

"The Method of Moments (MOM) was used in conjunction with the Geometric 
Theory of Diffraction (GTD) for predicting the elevation-plane radiation 
patterns of simple high frequency (HF) vertical monopoles and horizontal 
dipoles situated in irregular terrain. The three-dimensional terrain was 
approximated by seven connected flat plates that were very wide relative 
to the largest wavelength of interest. The plate length along the 
terrain profile was the longest possible that still adequately followed 
the shape of the path on the azimuth of the elevation pattern of 
interest and no shorter than 1 wavelength at the lowest frequency of 
interest. The MOM model was used to determine the antenna currents under 
the assumption that the terrain was planar (i.e., locally flat) over the 
distance pertinent to establishing the input impedance. The currents 
thus derived were used as inputs to the GTD model to determine the gain 
versus elevation angle of the antennas for HF skywave when situated in 
the irregular terrain. The surface wave solution for groundwave was not 
included since this does not appreciably contribute any effect to the 
skywave far-field patterns at HF in this case. The model predictions 
were made using perfect electric conducting (PEC) plates and using thin 
plates made of lossy dielectric material with the same conductivity and 
relative permittivity as measured for the soil. These computed results 
were compared with experimental elevation-plane pattern data obtained 
using a single-frequency helicopter-borne beacon transmitter towed on a 
long dielectric rope in the far field on a linear path directly over the 
antennas. The monopoles and dipoles were situated in front of, on top 
of, and behind a hill whose elevation above the flat surrounding terrain 
was about 45 m. The patterns of all of the antenna types and sitings 
exhibited diffraction effects caused by the irregular terrain, with the 
largest effects being observed at the highest measurement frequency (27 
MHz). The results for the PEC plates and the lossy dielectric plates 
were essentially identical for the horizontal dipoles, whereas the lossy 
dielectric plates were required to properly match the measured results 
for the vertical monopoles. The gain of the antennas in irregular 
terrain and the gain of the same antennas situated in flat, open terrain 
differed by up to 20 dB at the lower elevation angles (e.g., 3'4"). This 
difference in gain is significant for most HF systems.