HFTA is a great tool, no doubt about it.
But there are certain preconditions that must be met if HFTA results are to be
trusted.
Attempts to use HFTA outside of its domain are a waste of time.
The following preconditions are due to HFTA using a geometrical optics (GO)
approximation:
* the wave incident on the terrain must be a plane wave;
this translates to the wave source (antenna or a diffraction point)
being many wavelengths away from the observation point;
* each reflection point must be surrounded by a large enough area (Fresnel
zone)
in order to result in a reflection that obeys the geometrical "law of
reflection";
this translates to the terrain being composed of flat planes large compared
to wavelength
The following precondition is due to the use of UTD to augment GO:
* the distance between successive diffraction points must be many wavelengths.
The following precondition is due to HFTA using discrete ray-tracing:
* your terrain must be smooth enough because HFTA only uses
about 100-200 sampled points from your profile;
if the profile is not smooth, then the sampled points will not be
representative of the surrounding terrain, and you'll get random results.
The following precondition is due to HFTA using 2D representation of the
terrain:
* your terrain must be representable well enough as a series of wide flat
plates ("stairs")
i.e. parallel terrain profiles taken in the same direction, but laterally
displaced, must be identical.
If your terrain profile satisfies ALL of the above conditions,
then you'll get a reliable result from HFTA.
If not, you must look for another tool,
such as experimentation, 3D EM software, or hope.
In addition, HFTA does not address the question of horizon obstructions,
which is at least as important as ground reflections.
73,
Sinisa YT1NT, VE3EA
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
|