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Re: [TowerTalk] Modelling a basic stack

To: Pete Smith <>,"Towertalk" <>
Subject: Re: [TowerTalk] Modelling a basic stack
From: Jim Lux <>
Date: Mon, 10 May 2004 11:45:54 -0700
List-post: <>
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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