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

[john at kk9a.com]

Excellent post Dave. I found this to be an interesting towertalk  
discussion with some initial surprising model findings. I do recall  
modeling a loading coil far out on the element and found it to be less  
effective from a loading standpoint. I never tried the center loading  
model that Jim Lux did. N6BT used to sell a Q series multiband Yagi  
and dipole with I believe had center switchable loading coils but  
generally every manufacture of 30m and 40m monobanders places the  
loading coil out on the element, perhaps 1/3 of the way out, for the  
reasons in your post.

John KK9A


David Gilbert AB7E 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.

73,
Dave   AB7E