NEC-LIST: Some newbie questions on NEC2

Hello people.

I've been playing with nec2 for a while and never had any real problems.
Yesterday, I tried to simulate a j-pole though, and had the weirdest
experiences:

- I defined the feedpoint as a short wire (as it was only 9 mm, I
defined just one segment) and defined an EX card (type 0 voltage
source) that pointed to that segment.

- I had put the feedpoint a few cm above the shortcircuited end, and found
that the resistive part was very near to zero, and a very healthy reactive
part. The current in the bottom part of the antenna was very large.

- I tried with a 3 segment wire at the feedpoint, and tried to define the
voltage source in each of the three segments. I found wildly varying
impedances, none of them really logical (much too low).

- In another example I found a short text about type 5 voltage sources,
and tried with a 2 segment wire - bingo! The impedance went to near 50 Ohm
and a few Ohms of reactance.

I thought that the type 0 voltage source was a source between the end
nodes of the segment defined in the EX card. But reviewing a lot of nec2
example files I found the strangest things - the spider quad has a
7-segment and defined the exitation in segment 3 of the botton conductor
(even though the Antenna Handbook specifies top feed). So that's hardly
symmetrical. Many examples use an even number of segments and still use
type 0 sources to define voltage sources.

Using nec2, I found the optimal feedpoint to be at 61 mm, so I scaled that
using the velocity factor of twinlead (0.86, measured), but found
experimentally that the correct point was much lower... (30 mm instead of
51). Ideas?

Can someone point me in the right direction on either or both issues? Any
literature on this?

Thanks in advance.

John Coppens
ON6JC/LW3HAZ

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