NEC-LIST:Modelling the J-Pole Antenna

I have been attempting to model the J-pole antenna using both NEC and the
EZNEC version. Example is: stub length= 20", overall length =64", Conductor
diameter =0.5", stub spacing = 2". Model is 7 wires, for z: wires 1,3, 0 to
4
inches; 2,4, 4 to 20 inches; 5, 20 to 64 inches; 6, 0 to 0 inches; 7, 4 to
4 inches, where 1,2,5 are at y=0, 2,4 are at y=2, and 6,7 go from y=0 to
y=2. Segments are 3,21,3,21,45,3,3.

Resonant frequency of stub section and antenna section are close to 150 MHz.
At this frequency, some drive results are:
    feed 6 mid: 21.4+j70.9
    feed 7 mid:13.25+j3.0
    feed 5 nom 0: 14.76+j68.8
    feed 4 nom 0: 34.9+j83.8.
Moving wire 7 higher increases the drive impedance, with a change in
resonant
frequency. If it is lower, impedance decreases , and the change of j from +
to _,
which marks resonance can disappear. Feeding both wires 2,4 gives negative
power on one, with the algebraic sum about equal to a single feed on wire 4.

It appears that the antenna is partly driven by conduction (90%) and partly
by coupling to the stub (10%). Current at the junction 4,5 does not go to
zero,
but is about 20% of maximum current.

Any suggestion as to the way to derive the correct value of the drive
impedance of the usual tap across the stub?

bob haviland, PE, W4MB.

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