Two other cautions for anyone modeling a J-pole with any NEC-2 based
program:

1. Be sure to read the EZNEC manual section "Closely Spaced Wires" --
you'll find it in the index.
2. Run an Average Gain test. (See "Average Gain, Detecting source
placement problems" in the manual index.) The small loop in closed-ended
J-poles can be a problem for NEC-2. If the Average Gain test shows a
problem, using the double precision calculating engine (available in
EZNEC+) might help.

Roy Lewallen, W7EL

David wrote:
Roy,

The model I used for the open stub J-pole was not the same as your 3
element col.

I used a vertical long element and vertical short element. They were
joined by a horizontal element at the base. The source was set at 50%
along the horizontal bar as per your diagram.

I changed the model for free space rather than over real ground but this
has not helped the directional aspects of the plot.

The dimensions were scaled form those shown for a 150MHz open ended
J-pole shown on Arrow Electronics site

http://www.arrowantennas.com/j-pole.html