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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