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Re: [Antennaware] antenna project and software...

Subject: Re: [Antennaware] antenna project and software...
From: Terry Conboy <>
Date: Tue, 13 Feb 2007 05:22:33 -0800
List-post: <>
At 08:53 PM 2007-02-12, Ed K4SB wrote:
 >Terry Conboy wrote:
 >> Not only will the $80 version of EZNEC model this array, but even the
 >> free version of EZNEC (limited to 20 segments) will work well unless
 >> the elements exceed 1/2 wl long.

 >You absolutely CANNOT model an antenna of this sort with the free
 >version. Consider that at a minimum, you will have about 140' of
 >elements. With only 20 segments, you would be looking at the currents
 >every 7' along the element.
 >W4RNL's site has a good article on proper segmentation.

The absolute length of a segment really isn't the issue, it's the 
fraction of a wavelength.  With NEC-2 based models, you can get 
reasonable accuracy for simple antennas (dipoles, verticals, inverted 
Ls, 2-el yagis, etc.) with only 10 segments per 
half-wavelength.  However, it's also true that very closely spaced 
wires can require more segments.  Accurately modeling structures such 
as transmission lines are an extreme case of this.

W4RNL at states:
"Because we are impatient, a bad habit has set in among many program 
users: to divide the antenna elements into as few segments as 
possible, again within certain generalizations in the instruction 
manuals. For MININEC, the recommended minimum number of segments per 
half-wavelength is 10, while for NEC-2 it is 9 or 11. (When center 
feeding an antenna element, use an even number of segments for 
MININEC and an odd number of segments for NEC.)"

On that page, L.B. gives an example of a 20m half-wavelength dipole 
above ground.  The apparent gain rises from 7.97 to 7.99 dBi as the 
number of segments is increased from 11 to 45.  The impedance changes 
from 67.51 - j0.22 to 67.56 + j0.56 ohms.  These differences are 
certainly far less than the variations due to a real world 
environment (conductor sag, ground conductivity variations, wire 
insulation, end insulators, bird droppings, etc.) and exceed most 
hams' measurement capabilities.

Here's what W7EL says in the EZNEC Help file under 'Segmentation':
"A useful rule of thumb is 10 segments per half wavelength for 
pattern/gain analysis, and perhaps twice that number if really 
accurate impedance values are required. Wires joining at very acute 
angles may require more segments (see Acute Angles). If in doubt, a 
straightforward way of telling whether you've specified enough is to 
increase the number and see how much the results change."

And on, Roy says:
"Even with the 20 segment limit, the demo program does a very 
respectable job analyzing a two-element quad, two- and four-element 
phased vertical arrays, simple Yagi, W8JK, and many other antennas"

I often recommend the free demo version of EZNEC for new modelers to 
get them "hooked".  W7EL's customer support is impeccable and his 
program's user interface is the result of years of user feedback, so 
the full versions of EZNEC are a bargain, in my opinion.  (I've been 
using Roy's programs since ELNEC 1.0).  My only complaint is that 
they keep me up late at night building models (and some do use all 
1500 segments in EZNEC 4.0+)!

73, Terry N6RY

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