[TowerTalk] Half Wave?

[jimlux]

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.