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 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
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.

Roy Lewallen, W7EL