Re: [TowerTalk] Modelling a basic stack

[Jim Lux <jimlux@earthlink.net>]

At 01:51 PM 5/10/2004 -0400, Pete Smith wrote:
> At 09:17 AM 5/10/2004, Jim Lux wrote:
> > The desktop computer might be powerful enough (especially if you're willing
> > to wait overnight), but I think you're also looking for a modeling
> > program/system that is reasonably inexpensive and doesn't take hundreds of
> > hours to use effectively.
> >
> > There are a number of approaches, none cheap in either time and money, that
> > exist today.  Ohio State's NEC-BSC has been used (10-20 years ago?) to model
> > HF and VHF propagation over real terrain with real dielectric properties.
> > Companies like EDX provide tools for very accurate propagation analysis in a
> > variety of environments (they'll even sell you a database of buildings in
> > some cities).
> >
> > Of course, once you have the terrain model, you still need the earth
> > properties for that model.
>
> Thanks for the info.  Yes, I was thinking in terms of something that "we" 
> (amateurs) could both afford and put to reasonable use.
>
> I had heard of NEC-BSC, but had the impression that it was principally 
> used for problems like antennas placed in the superstructure of ships, 
> rather than above real ground.  A quick Googling found only that sort of 
> application.

There was a paper about 15-20 years ago (I'll find the reference if you're 
intested) where they used BSC to model the terrain as a set of flat plates. 
You can define the EM properties of the surfaces in BSC, and it does the 
solving correctly.  NEC-BSC, of course, isn't free.



> Jim speculates about the effects of earth properties on the modeling 
> fidelity.  I tend to rely on Dave Leeson on this.  If I understand what he 
> writes on page 10-2ff of "Physical Design of Yagi Antennas," this is 
> relatively unimportant.  He writes, "The reflection is from the conductive 
> or dielectric discontinuity between the air and the surface of the ground 
> (giving effect to skin depth), not from some magical underground water layer."

Sort of... The effect of changing the em properties of the surface changes 
the phase, magnitude, and polarization of the reflected wave, which would 
have an effect on the overall propagated wave.  If nothing else, consider 
the difference for vertical and horizontally oriented antennas, 
particularly in directions "off boresight", where the polarization is not 
necessarily aligned with the antenna (a dipole, probably the most common 
amateur antenna, would be a good example, particularly if drooping or with 
any coupling to the feedline).




> Jim also comments (in part of his message I did not quote) on the 
> potential usefulness of data from Landsat Thematic Mapper.  HFTA's 2-D 
> terrain profiles are generated from USGS's seamless data server.  Its 1 
> and 3-meter resolution data are derived from Thematic Mapper and Shuttle 
> radar mission data, if I understand correctly.  In any case, a whale of a 
> lot better than drawing pencil lines on a topo map.


SRTM generated accurate terrain data, but didn't measure the 
electromagnetic properties of the surface, which may have an effect. I was 
thinking about using the Land Use/Land Cover datasets which are gridded 
pretty coarsely (1km?) to estimate EM properties, once you know the 
terrain. There's also data sets at that sort of resolution from AVHRR, 
which could give you variations over time.


Jim, W6RMK 

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