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

[jimlux]

On 4/28/20 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.

That's because the NEC loading inductor is lumped, and of zero physical 
length, and has no interaction with the antenna's fields. It *has* to 
have the same current on either side.

There's two ways to approach this. One is to break your inductance up 
into little segments (which NEC4 has no problem with), but you're still 
going to have trouble because there's mutual inductance between segments 
that is not accounted for. You might, though, be able to get close by 
approximating with 10-15 segments.

The other approach is as you describe below, use the GH card.

Since all modern versions of NEC have the ability to handle thousands of 
segments and still have decent run time, that might be the best 
approach. Model the helix with 10-15 turns, divided into 8 
segments/turn, and that's only 100 segments, which is nothing.
(I run 4000 segment models all the time these days, about 120 seconds to 
fill, 15 seconds to factor, over the fancy GN3 ground on a recent 
MacbookPro)


> 
> 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.