[TowerTalk] 2-el 15/20 meter duobander
jim.thom at telus.net
Sat Dec 2 12:32:00 EST 2017
From: Tom Hellem
Sent: Friday, December 01, 2017 6:45 PM
To: Jim Thomson
Cc: towertalk at contesting.com
Subject: Re: [TowerTalk] 2-el 15/20 meter duobander
<I am curious about why you are suggesting that one needs NEC 4 software to
<design a 2 el. duobander.
<FWIW I basically took a C-3 and removed the 10M elements, tweaked it with a
<model in EZnec (NEC 2), built it according to the model. It ended up being
<resonant exactly where the model predicted with an swr of 1.0 at resonance
<on 20. However on 15 it ended up with 2.0-1 swr . Maybe I should have used
## On paper, NEC-2 should be ample for a simple duobander. Its when you
start with the 3-4-5-6 band yagis..all on one boom, that NEC-4 shines.
## Most of the F12 designs were done with NEC-2. The re-designed versions
of the F12 yagis are now being done using NEC-4. Ken at JK ants has re-designed
the infamous F12 5BA.... and done it with at least 2, if not 3, LESS eles. He
also ended up with more space in the middle, in the cases of mounting to a
rotating tower, side mount etc. He ended up with lower swr, more gain,
better FB, better FR, less eles used. Told me there is no way he could have
done that with NEC-2, since it has its limitations.
Even the monobanders were improved with NEC-4. In some cases, the multiple
feed lines have been eliminated on multiple band yagis. The hairpins have been eliminated
for the bigger, longer monobanders. In a lot of cases, the duobanders, using NEC-4, have
slightly more gain than the monoband versions of each band in question, and the monoband versions
were also designed using NEC-4.
But JK ants, innovate, optibeam, and others are designing on a commercial basis, so they may as well
use the best software for the job. JKs on the air results mirror his NEC-4 software, so hes done it right.
The rest of it is the mech design, and there... Yagi mechanical by DX engineering works superb. Then
you waste the least amount of tubing, cut weight, and windload, costs etc, and end up with a robust
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