[TowerTalk] Modelling a basic stack

[Pete Smith]

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.

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."

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.

73, Pete N4ZR
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