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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 20:30:38 -0700
List-post: <mailto:towertalk@contesting.com>
On 4/28/20 6:50 PM, David Gilbert wrote:


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.

There's a difference between the theoretical discussion and practical application, especially given that these days, one can do high quality models and run them over a wide range.

The simplified discussions of things like current distribution don't discuss the impact of IR losses, for instance. The antenna books tend to consider a couple bounding cases (half wave dipole, and infinitely small Hertzian dipole) and one or two point designs, and generally make "non-specific" assertions about loss, without actually giving the numbers. A more theoretical text will get into the effects of diameter and give some sample plots of Z vs freq for a few l/d ratios, along with the analytical equation.

And the antenna gods sure know there's a lot of talk about traps and their loss or not.
Or losses in a small loop.


What's not in the usual discussions is a plot of "loading coil position vs performance" (whether directivity, gain, etc.).

One tends to find "point designs" and discussions thereof. Your example of comparing coil at the center vs coils at 50% out is really useful.


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