Owen Duffy wrote:
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S S
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Fig a) above is an attempt to portray a colinear vertical over infinite
ground with a source at "S". The configuration is easy enough to model in
NEC with sensible results.

The common explanation for operation of a) is that the U shaped section
is a quarter wave s/c stub, that it is responsible for delivering direct
in-phase drive to the upper section, and that it plays no part itself in
radiation ie, that the common mode current on the pair of conductors is
zero at all points. Notwithstanding the conventional wisdom, it seems
unlikely that there is no common mode current on that section, and NEC
models suggest that there is, and that it accounts for some small
asymmetric distortion of the pattern.

Fig b) above is an attempt to represent a coaxial arrangement of tubes
where the lower end of the tubes are connected together, and that is fed
at S against an infinite ground.

My questions a

1. To what extent is b) equivalent to a)?

I can't answer that question right off, except that at first glance they
look quite similar in operation. I'd build both models with EZNEC, then
take a look at the reported currents in the View Antenna display. You
can get the same information from tabular NEC results, but most people
find the graphical display quicker and easier to interpret.

You can see the significance of the seemingly small common mode current
on the a) model stub by replacing it with a transmission line model stub
which of course has zero common mode current. The results are quite
different than for the wire model stub.

2. How is b) modelled in NEC?

A coaxial line can be modeled as a combination of a transmission line
(for the inside of the coax) and a wire (for the outside of the coax).
Download the EZNEC demo program and look in the manual index under
Coaxial Cable, Modeling. It'll direct you to one of the furnished
example files which illustrates how. Then you can do the same thing with
NEC if you're so inclined.

Roy Lewallen, W7EL