|Subject:||Re: [TowerTalk] modeling help?|
|From:||Jim Lux <email@example.com>|
|Date:||Wed, 24 Nov 2004 15:52:19 -0800|
At 01:04 PM 11/24/2004 -0500, you wrote:
I am not a modeler, and honestly have no desire to go thru the learning curve necessary to have confidence in the results. That said, is there one of you out there who would be willing to do some modeling for me?
I think you need to define what you're looking for in a bit more detail. Here's some things to think about.
For instance, are you looking for accurate predictions of gain? (how accurate? 1 dB, 5 dB? 0.1 dB?)
Are you concerned more with forward gain or with things like F/B or F/R ratios. The relative uncertainty in a model for small signals is much greater than for large signals. That is, if you're computing front/rear ratios, the error in the rear facing power is going to dominate the overall uncertainty. Say you radiate 100 watts in the forward direction, and 1 watt in the backwards direction. Your F/B is 20dB. But say the uncertainty in both those numbers is 0.5 watts. The F/B could be anything from 100.5/0.5 (23dB) to 99.5/1.5 (18 dB), but it's mostly the uncertainty in that backwards power that creates the overall uncertainty. The forward gain only changes by 0.02 dB. (this is why designing antennas with sidelobes down 40dB is a challenge!)
Or, are you looking for a more qualitative planning model to identify possible trouble spots, and to evaluate various physical arrangements (for which you really only need to look at whether the induced currents are "significant").
For instance,HFTA makes some fairly generic assumptions about the antennas you're stacking, ignores any vertically polarized components, and ignores mutual coupling among the antennas. But, as you note, it's easy to fool with different spacings and get a feeling for how the pattern changes.
On the other hand, if you use something like NEC you'll get a good model of the antenna system with mutual coupling among elements(and potentially, any feed system effects). However, none of the terrain effects will be covered (NEC assumes "flat earth").
And, as far as fidelity of modeling goes, NEC (even NEC4) doesn't deal with tapered elements particularly well, so, if you have tapered elements, while the pattern will be reasonably accurate, the feedpoint impedances won't be so accurate (which affects the accuracy of the feed network model, if you're running multiple antennas through a BIP/BOP stacking box).
And, another important point.. what do you want at the end of it? A report on the results, or a model that you can run and alter? If the latter, what modeling tool will you be using (EZNEC, NEC4WIN, 4NEC2, MultiNEC, NEC4, etc.etc.etc.) because the input files aren't all the same.
I suppose what I'm trying to get at is that this could be a trivial project or a hideously complex one, depending on what you expect to get out of it. Just the tedious mechanics of entering the physical model is part of it; although one that you can do a lot of the "grunt work" on, since it's just measuring/researching dimensions, and typing them in. Especially if you're doing a very high fidelity model, the time to enter and validate the geometry is non-trivial. None of the modeling tools have a "Rohn 45" block that you can just drop in. (I should qualify that.. none of the tools commonly used by hams. For all I know HFSS has some fancy stuff like that available, but for $60K a seat, it should)
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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