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Re: [TowerTalk] modeling help?

To: "w9ge" <>
Subject: Re: [TowerTalk] modeling help?
From: Jim Lux <>
Date: Wed, 24 Nov 2004 15:52:19 -0800
List-post: <>
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?

My plan is a three tower antenna farm.

Tower 1 is a stack of Opti Beam OB-16-3's at 40-80 and 120 feet, and a 40 yagi at 130 feet all on a rotating 55 tower. Hanging from the top from phyllistran catenary lines will be an 80 meter four square vertical array and I will shunt feed the tower on 160. (currently it is an 80 footer with 40 beam at top, 80 vertical 4 square hanging off it and a 160L attached to the top.)

Tower 2 is a guyed 80 foot 45 tower 120 feet away from the one described that will be converted to a WARC array. This one will hold a pair of M2 5 element 17 beams at 40 and 80 feet, a 4 element M2 12 meter beam at 85 and a 2 element 30 meter yagi at 90 feet. (currently it is an 80 footer with an OB 16-3 at the top and a C3E on a tic ring at 40 feet)

At some point in the future I would put up tower 3; a 40 footer with a C3E fixed on EU, making a triangle of towers with about 100 feet between it and each of the taller sticks.

All guys are/will be phyllistran.

Question is, what if any interactions can be expected and should perhaps I not be considering such a complex trio of arrays so physically close together? I have analyzed the take off angles etc. using HFTA and am impressed with its potential from my flat as a pancake location. I am also very impressed with the single Opti Beam OB-16-3 currently up at 78 feet, and the C3E below it at 40 feet is strong into EU. Anyone wish to offer some help? We will first discuss compensation. Thanks and Happy Thanksgiving. 73 bob de w9ge

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)

Jim, W6RMK


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