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Re: [TowerTalk] 40m 4el KLM - replacing linear loading with coils

To: towertalk@contesting.com
Subject: Re: [TowerTalk] 40m 4el KLM - replacing linear loading with coils
From: jimlux <jimlux@earthlink.net>
Date: Tue, 28 Apr 2020 16:57:25 -0700
List-post: <mailto:towertalk@contesting.com>
On 4/28/20 4:18 PM, Jim Brown wrote:
On 4/28/2020 3:20 PM, David Gilbert wrote:
There is some benefit, however, to use two loading coils on either side of center for each element (instead of center coils) because that gives better current distribution along the elements.  Most modern Shorty-40's do this, and it's the same reason why some mobile vertical antennas use center loading instead of base loading.

There was an excellent 2-part piece in QEX 4-5 years ago showing by well-controlled measurements of real mobile antennas that NEC does not accurately model current distribution in an antenna with lumped inductors placed in a segment. The error is that it fails to account for current change through the inductor -- it models the current as the same on both sides of the inductor.

That's because the NEC loading inductor is lumped, and of zero physical length, and has no interaction with the antenna's fields. It *has* to have the same current on either side.

There's two ways to approach this. One is to break your inductance up into little segments (which NEC4 has no problem with), but you're still going to have trouble because there's mutual inductance between segments that is not accounted for. You might, though, be able to get close by approximating with 10-15 segments.

The other approach is as you describe below, use the GH card.

Since all modern versions of NEC have the ability to handle thousands of segments and still have decent run time, that might be the best approach. Model the helix with 10-15 turns, divided into 8 segments/turn, and that's only 100 segments, which is nothing. (I run 4000 segment models all the time these days, about 120 seconds to fill, 15 seconds to factor, over the fancy GN3 ground on a recent MacbookPro)



NEC DOES, however, include an option to model inductors as a helix, which does account for the change in current and voltage through the inductor, but you need a version that allows a LOT of segments. I'm using a version of W7EL's EZNEC for which I paid about $500 ten years ago, and I've done that for a few portable designs that W6GJB and I were working on together.
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