On 1/17/19 3:03 PM, Bob Shohet, KQ2M wrote:
That’s a REALLY presumptuous response on your part Joe. I have spent MONTHS of ACTUAL
TIME time modeling antennas of all types over all types of terrain at multiple qth’s
over the past 28 years with at least five different modeling programs and I guaranty that I
have run far more antenna/terrain models of antennas than you have and far more than most of
the people on this reflector.
I don’t live on flat ground and most of the people that I know don’t either; but even
if they did have a flat qth, their patterns would still be affected by the topography around them as
well as soil conductivity, which is why you should always model your ACTUAL antennas over ACTUAL
terrain or else your model won’t be reasonably accurate and not likely to be particularly
useful.
This is actually a very difficult modeling problem. NEC and it's ilk
model the soil surface as a plane, with uniform soil properties, with an
optional step or cliff.
Some other method of moments codes might allow a layered or non-uniform
soil. That would get you your near field effects..
But the far field effects of uneven terrain are a bit more complex to
model - HFTA assumes horizontal polarization - and that's not an
unreasonable assumption for most dipoles and yagis, although once you
throw in guy wires, towers, and uneven soil surface under the antenna,
I'm not sure that the polarization purity is as good.
If there are significant scatterers in the "near far field" (say, within
100 wavelengths) that's a bit more tricky to model. Vertical trees
probably not a big deal, but horizontal structural components, power
lines, etc. are something to consider.
There's some literature on HF propagation through forests and other
layered media (Brazil, for instance, has tall forests, with roads and
clearings, and they use low VHF and HF links, so there's some research
out there) but probably not at 7 MHz. One can probably approximate a
forest as a set of fairly uniform layers using a bulk volume average of
tree and air dielectric properties.
But then, I don't know of many "3D" modeling codes intended for general
HF use - we've got some special purpose FDTD codes at JPL we used for
modeling low microwave (2-4 GHz) propagation through soil, rubble, etc.
- but those are hardly general use.
So, overall, modeling is good for comparisons and seeing if there's
something that sticks out in a phenomenological way, but I'd be leery of
claims for "better than 1dB" kind of accuracy in an absolute sense for
HF in any sort of "real" environment with lumps and bumps.
If you want to compare "significant" differences in height for a simple
antenna (dipole or Yagi), then NEC and HFTA do just fine, and are worth
spending some serious time on.
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