[TowerTalk] MODEL FOR TOWER

[jimlux]

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?
> 


It depends on how the modeling code treats the segment.  I'll have to 
check, but I think NEC is a "thin wire" code (someone correct me if I'm 
wrong) and it models only currents flowing "along" the wire, not 
"around" the wire.

For instance, when you print out the segment currents, there's only one 
number for the current along the segment. Not two (along + around).

And, when you model a skew dipole over a "plane" made as a grid, you 
have to have wires running both directions in the box surface.