> Jim,
> My feeling is that this is much ado about very little...
First, a 4 degree difference in MODELED takeoff angle on 160
is meaningless...The presence of power lines, house wiring,
plumbing pipes, sewer lines, telephone wires, CATV cable,
and on, and on, within the near field makes the model
nothing more than a rough guide - very rough... Everything
metal out to 3 waves is going to have a major effect on your
pattern...
>From experience...
I'm in gently rolling pastureland in middle GA.
I have a 318 foot insulated base tower. There is really no
significant difference between that tower and a 200 ft tower
or a 1/4 wl vertical. Of course I have a good ground system
of 100 200 ft radials on the 200 ft tower and 50 1/4 wl
radials on the 1/4 wl vertical.
The problem is I don't know how things would compare with a
poor ground system.
My input on the model would be this:
Models amateurs use generally assume FS at some very large
distance from the antenna. You can tell if the model is
doing this by looking at FS at zero degrees. If you see FS
at zero degrees is very low or nearly zero, you know the
model assumes you are interested in FS at a distance of
hundreds of wavelengths or more.
That really isn't how the antenna works in the real world.
The real world isn't flat and on 160 meters the height of
the ionosphere often compares to the distance the model uses
to calculate pattern. The result of this is low angle FS is
often underestimated.
I think this is why good 1/4 wl verticals often compare
nearly equally with high dipoles on 160 and why the two come
reasonably close on 80. When I model a vertical on 160 the
model shows a dipole at 300 feet would beat the vertical at
almost any angle. Yet when I actually installed an ideal
dipole at 300 feet and compared it to a vertical in hundreds
of A-B blind tests the dipole and vertical averaged within a
few dB of each other broadside to the dipole, and the
vertical just naturally smoked the dipole near the ends of
the dipole.
A second issue would be the exaggerated use of TO angle to
pick an antenna. TO angle alone doesn't matter. What matters
is the absolute field strength at the desired angle or
angles. An antenna can have a TO angle of 30 degrees and
have more absolute FS at 10 degrees than an antenna with a
TO angle of 10 degrees.
Models are excellent for antennas some distance above earth,
especially horizontally polarized antennas. They really are
just very rough approximations for vertical antennas near
earth (or even dipoles at very low height).
I wouldn't predict one way or another what would happen in
other locations where a marginal ground system is used, but
I'd never rest 100% on any model when earth is
involved...especially when the models we use don't model the
earth in any accurate detail.
73 Tom
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