Roy Lewallen wrote in
treetonline:

Hello Roy,

Thanks for the response.


....
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.

Ok, here is the model I constructed of b) (the coaxial tubes
construction). For simplicity, the upper and lower outer tubes are the
same diameter, the same wire in this model.

CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 47 0 0 0 0 0 15 0.005
GE 1
GN 1
EK
EX 6 1 1 1 0
TL 10 1 1 16 50 5 1e+99 1e+99 0.0001
FR 0 0 0 0 15 0
EN

I have a 3/4 wave vertical over perfect ground, and I have inserted a
quarter wave s/c transmission line into the vertical at 1/3 height. I
have shunted the TL with 10k ohm to represent some loss in the stub.

The currents report shows the currents in the top half wave to be
approximately 180° out of phase with the bottom quarter wave.

The question is whether such a construction yields three quarter waves in
phase, or whether the NEC model is correct that they are not in phase.


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.


My initial feeling is that the wire model of a) is correct. I have not
yet done as you suggest in the previous par.

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.

Is my model above what you suggest?

Appreciate your comments Roy, thanks.

Owen