|To:||Pete Smith <email@example.com>,"Towertalk" <firstname.lastname@example.org>|
|Subject:||Re: [TowerTalk] Modelling a basic stack|
|From:||Jim Lux <email@example.com>|
|Date:||Mon, 10 May 2004 11:45:54 -0700|
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 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.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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