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.
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