[TowerTalk] Antenna modeling

[Jim Lux]

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 
(http://www.cushcraft.com/support/pdf/x7.pdf).
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.