[TowerTalk] MODEL FOR TOWER

[jimlux]

On 4/23/19 1:05 PM, Jim Brown wrote:
> There is also at least one equation I found on the web that computes the 
> effective diameter of a triangular tower based on it's cross-section. It 
> might be possible to get a reasonable approximation of the tower by 
> breaking the tower down into multiple sections.
> 
> The equation I found is tricky to express via email.  The equivalent 
> diameter is the cube root of (D x Fsquared /2), where D is the diameter 
> of a leg and F is the face width.

This doesn't do the horizontal crossing pieces (which are actually the 
most likely to cause problems - they're long, and they're aligned with 
the antenna elements) and the diagonal struts.


> 
> There may be a comparable equation for a tower with a square cross section.
> 
> Elaborating on Jim's method, EZNEC has a command that allows wires to be 
> duplicated rotated about some center point. That would allow one of the 
> vertical members to be modeled, then duplicated in 90 or 120 degree 
> increments. Cross-members could then be added between the intersections 
> of the "wires."

  NEC has the GM card which can create copies rotated around a center, 
and that will do the wires.  And it can do the horizontal members, too: 
you build one "corner" of the structure, and tell NEC to rotate it 90 
degrees and create 3 copies.
The diagonal struts, though, are the challenge.

When I've done this kind of thing, I've always just wound up writing a 
short program in whatever language (basic, matlab, python) that just 
cranks out GW cards for all the required members.

A program like 4nec2 can help visualize the model once you've built it.

You could, if you're ambitious, actually write a program in the symbol 
handling language of 4nec2 to allow you to specify height and size at 
top and bottom, and do all the algebra to get the endpoints...
But it's a pain doing that.
By the time you've done that, it's just as easy to write a program in 
something else, and a LOT easier to debug it.

you wind up having a lot of statements that look like this
f.write("GW %d %d %f %f %f %f %f %f 
%f\n"%(wirenum,nsegs,x1,y1,z1,x2,y2,z2,radius))

(I was actually writing python code this afternoon that does just this 
and that's the actual line out of the code)

If your structure DOES have symmetry, then building part and using GM to 
replicate it does model a bit faster.

I find that computers run so fast now, it's not always worth doing, but 
if you're getting up to 1000s of segments, maybe it's worth while.




> 
> In my professional life, I generated models of large and small 
> performance spaces to aid in the design sound systems by studying their 
> coverage of the audience and their interaction with the acoustic 
> environment. The largest and most complex space I "built" was the Staple 
> Center in Los Angeles. Those models use the same x, y, z coordinate 
> system, and the surfaces are all planes with their acoustic 
> characteristics defined. In building those models, I made a table of x, 
> y, and z coordinates of the points defining the surfaces before trying 
> to build the model.  A similar approach would be applicable here.


Yeah, the same basic strategy for NEC.

The holy grail of EM modeling is something that will take a mechanical 
cad drawing and "grid it" - whether into wires for a Method of Moments 
code like NEC or into triangles for modeling tools that want patches.



> 
> 73, Jim K9YC
> 
> On 4/23/2019 11:11 AM, jimlux wrote:
>> On 4/22/19 8:53 PM, Greg Best via TowerTalk wrote:
>>> I have an 80 ft Rohn windmill tower that is equivalent or stronger 
>>> than the same height SSV. The base dimension is 7’ 7” and the tower 
>>> tapers to 24 “ at the top . I am wondering if anyone has a NEC model 
>>> for this tower (or a very similar one)?  I have a 20 m 4 el yagi I am 
>>> considering installing at abt 60 ft and I am concerned about the 
>>> tower impact on the pattern. The yagi at 60 ft would be in a stack 
>>> with another yagi at 95 ft that is mounted on a mast.
>>>
>>>
>>
>> This is one of those things that might be easiest to generate with a 
>> short program in the language of your choice to generate the lattice 
>> of vertical, horizontal, and diagonal members.   I've done that to 
>> generate a model for a truss - it was easier to do that than to try 
>> and figure out the coordinates by hand.
>>
>> There *might* be a cad program in which you could quickly draw the 
>> tower, and export the segment endpoints.
>>
>> Because it's tapered, it doesn't lend itself to using a GM card to 
>> replicate segments.
>>
>> You'd model it as a wire for each structural member, and pick the 
>> diameter of the wire to have the same circumference as the perimeter 
>> of the structural member (or, heck, just make em all 2" diameter wires).
>>
>>
>> for each section do:
>>     calculate the coordinates of the 8 corners
>>         (or 6 corners if it's three sided)
>>     generate a wire for the 4 verticals corners
>>     generate wires for each of the horizontals
>>     generate wires for the diagonal braces
>>
>>
>>
>>
>>
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