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Re: [TowerTalk] Antenna modeling

To: "R. Kevin Stover" <>,TowerTalk <>
Subject: Re: [TowerTalk] Antenna modeling
From: Jim Lux <>
Date: Wed, 08 Dec 2004 09:07:30 -0800
List-post: <>
At 09:44 AM 12/8/2004 -0600, R. Kevin Stover wrote:
Hello all.

I'm planning a tower install this next spring and am getting deep into the planning and a have a few questions.

#1. How do you model commercial antennas without the dimensions of said antenna?
I'd like to be able to model the antenna before buying it

How good does the model fidelity need to be? Are you interested in the general pattern, assessing interactions, or what? If you're looking for essentially qualitative data: "Does antenna B screw up the pattern of antenna A when they're stacked 6 feet apart?" then your modeling can be pretty rough and ready, because what you're really looking for is what (if any) currents are induced by one antenna on the other (or the structure).

Some mfrs will provide dimensions, or you can look in the manuals, or, you can ask here on the list.

#2. How do you model something like a Cushcraft X7 with the log feed section?

If you have mechanical dimensions, it can be modeled. It might be hideously tedious because you might have to model each little piece. The manual for that antenna gives most of the mechanical dimensions (
Depending on the fidelity of the model you want, you might be able to model the transmission line segments as round conductors with the same circumference as the cross section perimeter. You'll definitely need to model the boom.

Think carefully about how good a model you really need, because this could be quite a project.

#3. When modeling an antenna with traps how do you know what values to plug in as loads for the traps?

Measurement? Ask mfr? Ask other hams who have one? Model it from physical dimensions, and adjust trap values until the SWR curves look like the ones in the spec sheet.

I was planning on 4el SteppIR and an XM240 on a 70ft AN Wireless tower then I got to thinking about feeding the tower for 160m and using the beams for toploading. That won't work too well if the dimmensions and thus toploading provided by the SteppIR change all of the time so I'm now looking for traditional beam designs. Since Cushcraft is the only manufacturer to my knowlege that provides manuals for the products online, and I can use those dimensions to get an idea of how the system will perform, they've moved to the head of the pack.

Since the SteppIR is such a wonderful antenna, for other reasons, why not do a quick model of the tower with the SteppIR on the top. Model it with the elements at several different lengths (someone here can probably give you some starting points for numbers). Since the SteppIR is non-resonant on 160, it's mostly looking like a capacity hat, and I doubt that millimeter precision is needed.


See: 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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