On 4/28/20 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.
There's a difference between the theoretical discussion and practical
application, especially given that these days, one can do high quality
models and run them over a wide range.
The simplified discussions of things like current distribution don't
discuss the impact of IR losses, for instance. The antenna books tend to
consider a couple bounding cases (half wave dipole, and infinitely small
Hertzian dipole) and one or two point designs, and generally make
"non-specific" assertions about loss, without actually giving the
numbers. A more theoretical text will get into the effects of diameter
and give some sample plots of Z vs freq for a few l/d ratios, along with
the analytical equation.
And the antenna gods sure know there's a lot of talk about traps and
their loss or not.
Or losses in a small loop.
What's not in the usual discussions is a plot of "loading coil position
vs performance" (whether directivity, gain, etc.).
One tends to find "point designs" and discussions thereof. Your example
of comparing coil at the center vs coils at 50% out is really useful.
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