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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|