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Re: [TowerTalk] Modelling a basic stack

To: "Towertalk" <>
Subject: Re: [TowerTalk] Modelling a basic stack
From: "Jim Lux" <>
Date: Mon, 10 May 2004 06:17:10 -0700
List-post: <>
----- Original Message -----
From: "

> Perhaps one day we will have a true "Swiss Army Knife" -- a modeling
> program that can optimize actual antennas or arrays, over real
> terrain.  Our desktop computers are certainly powerful enough now.
> 73, Pete N4ZR

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. There is some airborne/spaceborne VHF sounder/SAR
data around, but nowhere near the coverage you need for this kind of thing
(unless you're lucky), and you'd have to do some work to transform VHF/UHF
data down to equivalent properties at HF (although there is some literature
on how to do this).

What would be interesting is to do a sensitivity analysis with one of the
existing tools to see just how much difference poor knowledge of earth
properties makes.  It might turn out that you could ingest some sort of
rough approximation (say, land use/land cover data from Landsat Thematic
Mapper which then is used for a lookup table: if it's an agricultural field
use these two numbers, if it's residential housing, use these other two).
Heck, even distinguishing between urban, suburban, and rural might be
enough, and then using a generic earth properties for the overall region.

For that matter, it might turn out that going from conductivities of 3 to 20
or dielectric constant of 2 to 40 doesn't make that much difference, so you
could use the 5/13 numbers that everyone plugs in blindly.


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