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

[Lux, Jim]

On 10/23/21 8:59 AM, Mark - N5OT wrote:
> Thanks Jim.
>
> I was hoping someone had done this the easy way - Dave says "8.5 
> inches should work" and I'm getting predictable and usable results at 
> the moment using 10 inches.  Mostly I'm trying to get a reality check 
> at this point on the lengths of the top load wires. Certainly if I can 
> get it to accept the 160 meter energy it will radiate it. I'm getting 
> wrapped around ropes and wires.
>
> 73 - Mark N5OT
>

NEC, in general, treats the conductor as the current flowing either at 
the center of the wire or as a infinitely thin layer of current on the 
surface. The latter is called the "extended thin wire kernel".  In both 
cases, it assumes the current is flowing ONLY axially.

It also uses the diameter  to calculate the resistance at the frequency 
being used.


Where one might get jammed up with the "axial current only" is if you've 
got something like a h-pol yagi next to a tower or the tower goes 
through the elements AND the tower is big enough that significant 
currents would be induced on the horizontal members of the tower (or the 
diagonal struts).   If you used a single wire (or multiple wires in a 
cage) this effect won't be modeled.


I always just start with either 1 big wire, or 3 smaller wires in a 
triangle, and see if it makes a big difference. Then I put horizontal 
segments between the three wires to make triangles. And if *that* makes 
a difference, then you know you've got coupling from the antenna to your 
support structure.  You can do this pretty easily with the GM geometry 
move card if you pick the wire numbers appropriately.


>
> On 10/23/2021 10:05 AM, Lux, Jim wrote:
>> 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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>>
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