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 http://www.cebik.com/amod/amod1.html 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 http://www.eznec.com/demoinfo.htm, 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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