[TowerTalk] MODEL FOR TOWER

[Gedas]

One thing I have not seen mentioned (maybe I missed it) is the fact that 
a whole lot of time & effort is going to go into generating a super 
accurate model for a complex tower and antenna system but then, what of 
the grounding of the tower itself at the base? Grounded or un-grounded 
or sorta grounded are all going to give different results.

There are going to be deviations from real world results if one models 
with the limitations of objects touching the ground with NEC-2 and even 
with NEC-4.

Gedas, W8BYA

Gallery at http://w8bya.com
Light travels faster than sound....
This is why some people appear bright until you hear them speak.

On 4/25/2019 7:40 PM, jimlux wrote:
> On 4/24/19 5:03 PM, Steve Maki wrote:
>> On 04/24/19 8:40 AM, jimlux wrote:
>>
>>>> I have always just guess when converting a tower to a wire 
>>>> diameter. The original poster seemed concerned that his tower would 
>>>> effect his horizontally polarized HF beams, I have not seen that 
>>>> occur.
>>
>>> I think the question would be about the SSV/BX style tower which is 
>>> larger at the bottom than the top. Rohn 25 or 45 are "small" 
>>> compared to a wavelength in the horizontal direction, so they can be 
>>> modeled as a "fat wire" - just like a cage dipole element, for 
>>> instance.
>>>
>>> The tower in question is 7.5 ft at the bottom and 2 ft at the top 80 
>>> ft high.
>>> The OP was asking about a 20m Yagi to be mounted at 60 ft, where 
>>> you'd effectively have big square loops that are about 3 1/2 ft on a 
>>> side (14 ft total perimeter)  near the antenna, as well as diagonal 
>>> struts of some length.
>>>
>>> The wavelength is 60-70 ft, so those squares are about 1/4 
>>> wavelength in perimeter.  If they were 1/10th wavelength, I'd say 
>>> "model it as a big wire", but that's big enough that there might be 
>>> some interaction, especially since they will be effectively "inside" 
>>> the Yagi.
>>
>> In the scenario where you have a large enough tower that a nearby 
>> horizontal antenna is impacted by the tower's horizontal members - is 
>> there a fundamental difference between a lattice tower compared to a 
>> cylinder of like diameter?
>>
>> I've assumed no, but now you have me wondering.
>>
>> -Steve K8LX
>>
>
>
> Here's what the latest NEC documents say:
> "The wire radius a relative to {lambda} is limited by the 
> approximations used in the kernel of the electric field integral 
> equation. NEC uses the thin-wire approximation, neglecting transverse 
> currents on wires and assuming that the axially directed current is 
> uniformly distributed around the segment surface. The acceptability of 
> these approximations depends on both the value of a/{lambda}  and the 
> tendency of the excitation to produce circumferential current or 
> current variation. Unless 2*pi*a/{lambda}  is much less than 1, the 
> validity of these approximations should be considered."
>
>
> So NEC does not model transverse currents in a conductor - so while 
> you can model a tower as a wire of comparable diameter to the tower, 
> the model will only work for (mostly) fields that are vertically 
> oriented.
>
>
> A further hiccup in modeling a lattice tower might be the "short 
> segments forming loops" problem.
>
> NEC2 doesn't deal well with very short segments. NEC4 deals with them 
> just fine.
>
> However, for loops where the circumference is <0.002 wavelength, the 
> results may not be valid.  IN practical terms.. if you've got a 
> triangular tower with face width 1 foot (perimeter 3 feet), if the 
> wavelength is >1500 feet, you might have a problem.  Topband and Cheap 
> TV antenna lattice *might* get into trouble.
>
> Modeling 4" reinforcing mesh or a dense rebar lattice might also run 
> into troubles.
>
>
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