I agree with Dave. We had a similar discussion about this (using monopoles)
nearly 20 years ago in the rec.radio.amateur.antenna forum. I wrote a little
paper at the time; I see it's available at:
https://www.qsl.net/n7ws/Loaded_Antennas.pdf ; Of note is my reference #1.
Wes N7WS
On 4/28/2020 6:50 PM, David Gilbert 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
On 4/28/2020 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.
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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