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Re: [TowerTalk] Modeling Rohn 25G in K6STI's AO

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Modeling Rohn 25G in K6STI's AO
From: "Lux, Jim" <jim@luxfamily.com>
Date: Sat, 23 Oct 2021 08:05:38 -0700
List-post: <mailto:towertalk@contesting.com>
On 10/23/21 4:48 AM, Mark - N5OT wrote:
Hey youall,

I want to model Rohn 25G as a transmitting antenna.  Has anyone learned how to do that fairly accurately in K6STI's AO program? In addition to some length of regular straight sections, my tower has a factory 8' tapered top section which I would extend with a 2" stinger and a single-point pier base that is basically a 10 foot section with 3 feet of taper to a flange added.  It appears to be "factory" although I have never seen one in person and it could be a one-off.

1. How do I model these tapers?

2. How do I model the tower itself?  Do I call it "10-inch diameter steel?"  Larger?  Smaller?

For NEC (you'd have to ask Brian if AO works the same way, I think not, though)

What you want is a "wire" that has similar electromagnetic characteristics in terms of diameter, conductivity and permeability per unit length.  So you could put a 10" diameter wire, but you'd need to scale the conductivity down so that it matches the tower (i.e. it's not solid steel) - Don't forget that since it's steel, the skin depth is quite shallow - Someone may have figured out the magic numbers to match.  You might be able to use a single smaller wire that's "close enough".  It kind of depends what you're doing with the model - sometimes, a simple approximation works for a sensitivity analysis.  You put the wire in, get some data, take the wire out, get some data, decide that since the data didn't change very much, you neglect the effect of the wire.  Putting the wire in and then changing the diameter or conductivity is a similar exercise.

NEC doesn't model the currents flowing "around" the wire - Maybe you could model three parallel wires for your corner tubes and start with that.

Or, you just model the entire lattice (writing a short program to grind out all the segments is how most people do this kind of thing).  Of course, you will potentially wind up with "segment very much shorter than a wavelength" kinds of issues, but that's more a numerical precision thing.




3. Do I have to re-learn calculus?

I would love to hear from anyone who has either done this or knows how it is done.

Thanks in advance.

73 - Mark N5OT
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