On Fri, 19 Jan 2007 14:41:46 -0800, Roy Lewallen
wrote:



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

I will spend more time here. The last model I worked out with a
perfect ground appeared to produce a 0 degree take off angle.

Mornings are best for me to push my gray matter!
John Ferrell W8CCW