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

[David Gilbert]

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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