Roy Lewallen wrote in
treetonline:

Owen Duffy wrote:

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

. . .

Is my model above what you suggest?

No. But I did take the time to see what would be necessary to actually
model it. And what I ended up with is identical to a) except that the
wire stub is replaced by the shorted transmission line model, and the
lower wire has become the outside of the coaxial structure so is
increased in diameter. So those are the two differences between a) and
b). As Tom mentioned and I alluded to, there's some interaction

I think that is what I had done, but I used the same diameter top to
bottom.

Here is a revised deck with different diameters:

CM
CE
GW 10 1 0 -2 2 0 -2 2.1 0.005
GW 1 15 0 0 0 0 0 5 0.015
GW 2 30 0 0 5 0 0 15 0.005
GE 1
GN 1
EK
EX 0 1 1 1 0
TL 10 1 2 1 50 5 1e+99 1e+99 0.0001
FR 0 0 0 0 15 0
EN

In the above, the lower conductor is three times the diameter of the
upper conductor. The TL is wired into the lowest segment of the upper
conductor. Again, I have shunted the TL with 10k R to represent loss in a
real TL.

This model does not show in phase currents in upper and lower parts of
the vertical.

between the wire stub and the antenna which doesn't exist between the
ideal transmission line and the antenna, so performance is different.

You might as well leave your source open circuited as to connect it to
the shorted end of the transmission line stub. The current into one

I don't think I did that.

transmission line conductor always equals the current out of the
other, so if the two are shorted, no more current can go into or out
of the shorted end. Therefore, any external connection to it looks
like an open circuit since no current will flow through the external
connection.

What's a type 6 source (EX 6)? The NEC-2 and NEC-4 documentation I
have defines only types 1 - 5.

I have been playing with this in EZNEC and 4NEC2. The deck I offered was
from 4NEC2 as my EZNEC files are binaries and couldn't go inline. The EX
6 is an extension for a current source. It is immaterial in this case,
and the 6 can be changed to a 0.

Thanks.
Owen