[TowerTalk] Tips for Modelling swaged antenna tubing sections

[jimlux]

On 9/29/20 4:29 PM, jimlux wrote:
> On 9/29/20 1:24 PM, john at kk9a.com wrote:
>> With NEC2 I avoid using short wires. If I had a swedged element I 
>> would use the length of the unswedge portion of the larger tube and 
>> for the next wire I would just add the length of the swedge portion of 
>> this tube to the smaller tube that fits inside it. The 1/8 difference 
>> in diameter for the small overlapping area should not make a 
>> difference on HF and I feel that the segments are more equal and that 
>> the model will be reasonably accurate.  Does NEC4 model short segments 
>> more accurately?
> 
> Yes, there's an improved basis function for the fields along the segment 
> so it models short segs more accurately.  That said, I'd model it as 3 
> segments: tube from center, slightly larger diameter for overlap, 
> skinnier tube continuing out.
> 
> 

from nec4userman.pdf

The main electrical consideration is segment length ∆ relative to the 
wavelength λ. Generally, ∆ should be less than about 0.1λ at the desired 
frequency. Somewhat longer segments may be acceptable on long wires with 
no abrupt changes while shorter segments, 0.05λ or less, may be needed 
in modeling critical regions of an antenna. The size of the segments 
determines the resolution in solving for the current on the model, since 
the current is computed at the center of each segment. Earlier versions 
of NEC suffered a loss of precision or complete failure of the solution 
when very short segments were used, but this problem has been corrected 
in NEC–4. The extremely short segments can be used with NEC–4, subject 
to limitations related to the wire radius as discussed below.


The wire radius a relative to λ 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/λ and the tendency of the excitation to 
produce circumferential current or current variation. Unless 2πa/λ is 
much less than 1, the validity of these approximations should be considered.







> 
>>
>> John KK9A
>>
>>
>>
>> jimlux wrote:
>>
>> but does the model actually show much difference? One can get way down
>> in the weeds with this - put a tapered segment in that's 1 cm long, etc.
>> But if the wavelength is 20 meters, a 1 cm transition is 0.0005
>> wavelength.  I'd worry more about numerical instability than model
>> accuracy at that point.
>>
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