[TowerTalk] ez nec help

[jimlux]

On 5/3/20 3:49 PM, Wes wrote:
> There's some misinformation here.  Rather than reinventing the wheel, I 
> will suggest reading the EZNEC help section: "Stepped Diameter 
> Correction Overview".
> 
> Wes  N7WS
> 
> 
> On 5/3/2020 12:19 PM, jimlux wrote:
>>
>> Excellent point - tapered means "conical section" (i.e. smoothly 
>> varying radius), and most antennas are actually stepped.
>>
>> In all the NEC engines, there's no problem connecting a "wire" with 
>> radius X to a wire with radius Y, especially if they're in a straight 
>> line, and the segment length isn't less than the diameter.
>>
> \

I stand corrected on the ability of NEC2 to handle the steps..


from that manual:

"NEC-2 is known to be inaccurate in modeling connected wires having 
different diameters. (Note: This is sometimes called being "tapered". It 
shouldn't be confused with EZNEC's segment length tapering.) The problem 
gets worse as the segments near the junctions get shorter and as the 
diameter difference gets larger."
..
It goes on to describe the well known Leeson method for creating a 
constant diameter element that models the stepped diameter.
..
" The accuracy of NEC-2 is worst in the presence of stepped
diameters when the segment length/diameter ratio is small. "
...
"The inaccuracy of NEC-2 in the presence of large steps in diameter 
typically shows up as an incorrect reactance"

---
this is all correct - and the reactance is important because the 
relative phasing of the elements is what gives the antenna decent 
side/backlobe performance


I guess the problem I have is that this is pretty fuzzy in terms of 
"what is large or small" in this context, and what the source of the 
modeling error is.


  NEC-4 is substantially improved in terms of the "basis functions" for 
the current in a segment, so it handles short segments better, and steps 
better. Although there are things it can't do - I need to recover my 
password so I can go read
"NEC-4.1: Limitations of Importance to Hams" in QEX