John Ferrell wrote:

On my "Things to do List" I plan to spend some time with EzNec and a
ground plane in free space.

What I hope to better understand is the effect of the the quality of
the ground plan in the are between a modeled "real ground" and a
modeled "perfect ground".

I don't know the free space model will be valid, but I hope to learn
from the exercise.

I could just ask another series of "dumb questions" here but I fear
some of those who are so generous will tire of my noise!

When I modeled the vertical with a perfect ground I was expecting half
a vertical dipole and that is not what I got!

John Ferrell W8CCW

If you model a vertical connected to perfect ground, you should get
almost exactly half the impedance of a dipole of twice the length in
free space, with the same radiation pattern except 3 dB greater in
amplitude and of course with the lower half missing. If you use an odd
number of segments and a single source for the dipole, there will be a
small difference because of the difference in source placement between
the dipole and vertical. This difference will become less as the number
of segments is increased in both models.

However, you can make the models virtually identical by using a "split
source". Here's an example you can even do with the demo program:

Open the EZNEC example file Vert1.ez or d_Vert1.ez. Click Src Dat to see
that the source Z is 36.65 + j2.971. (It's using MININEC type ground, so
the source Z is the same as for perfect ground.) Then change the Ground
Type to Free Space. Add a second wire with end coordinates 0, 0, 0 and
0, 0, -10.3 meters, 40 mm diameter, 10 segments, to be an exact mirror
image of the vertical. Then open the Sources Window and change the
source type to SV (split voltage) or SI (split current) and click Src
Dat. The reported Z is now 73.3 + j5.942, exactly twice the Z of the
vertical. To compare patterns, you'll need to use Perfect ground for the
vertical rather than MININEC type ground.

Roy Lewallen, W7EL