Owen Duffy wrote:

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

Sorry, my mistake when looking at the source. Your model is just as I
described. I apologize for the error.


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.

I've been running your model without the loss, and I'm seeing currents
in the upper and lower wires which are nearly 180 degrees out of phase.

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

For sure -- maximum gain is about 46 degrees above the horizon.

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.

You're right, you didn't. My mistake.

. . .

In playing with the model, I noticed something surprising -- length and
Z0 of the transmission line have very little effect on the pattern, even
over wide ranges (5 to 5000 ohm Z0, lengths from essentially zero to one
wavelength). In fact, try removing the transmission line altogether,
leaving the wires connected directly together and look at the pattern.
Then try changing one wire end slightly to break the connection between
them -- again, very little change in the pattern. The fact is that the
junction of the two wires is at a point of very little current, so you
can connect or disconnect them with almost no change. Likewise, you can
insert just about anything (of zero physical size), including an ideal
transmission line of any length, without any real effect. So the
transmission line stub doesn't really do anything significant at all.
What I don't understand yet is exactly why the wire stub does what it
does. It sure doesn't work like the simplified explanations imply.

Roy Lewallen, W7EL