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

Varying the parameters of the free space vertical, reading the EZNEC
help file and computing various scenarios with the G4FGQ program
RADIAL_3 has satisfied my curiosity for the moment.

I will next look at the physical problems of erecting a vertical for
use on 40/80/160.

Initially I will start with my 28 foot radiator over a radial field of
16 seventy foot radials fed with the SG-237 tuner. If I can work out
the matter of keeping that up with acceptable guys I can step up to a
48 foot radiator out of my existing aluminum supply. Based on that
experience, I will consider buying a little more aluminum to get to
the 66 foot height. That is the reason for the 70 foot radial length.

Most of the problems will be mechanical for a while.
John Ferrell W8CCW