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

[Jim Brown]

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