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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: David Gilbert <xdavid@cis-broadband.com>
Date: Tue, 28 Apr 2020 18:50:40 -0700
List-post: <mailto:towertalk@contesting.com>


Whether NEC accurately models the current isn't the point.  You don't need a model to know that if you push the coil out away from the center the portion of the current between the coil and the center is higher than it would be between that same point and the center if the coil was at the center.

And to a point, the further way from the center for the current distribution the better the pattern.  Consider how colinear dipoles with a shared feed work ... the total area under the curve for the current is that same as for a single dipole except that the current is more widely separated.

The problem with putting the coil TOO far from the center is that the current there eventually becomes low enough that the coil needs to be really large.  As an extreme, a coil at the end of a dipole is useless for affecting the tuning other than whatever capacitance it might have to the wire.  Coils need current to have any effect.

I don't understand why I have to be explaining this stuff.  It's pretty basic and described in just about every antenna book that discusses current distributions.

73,
Dave   AB7E



On 4/28/2020 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.

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.

73, Jim K9YC

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