Posted by
Stephen W. Kercel on
Mar 03, 2007; 7:27pm
URL: http://elecraft.85.s1.nabble.com/Re-Vertical-antennas-tp444447p444448.html
Fellow Elecrafters:
The discussion of verticals has inspired me to do a bit of EZNEC
modeling. I plotted the azimuthal pattern at an 8 degree takeoff
angle for several different 80 meter configurations. In all 3 cases,
I've assumed average ground.
The first case is the classical full size vertical, with a quarter
wave monopole element and 128 quarter wave radials. I've assumed
aluminum conductors on the theory that if I were really going to lay
nearly 2 miles of wire on the ground, I'd use aluminum electric fence
wire and not copper. Also, the monopole element would almost
certainly be made from aluminum tubing; my EZNEC program does not
support mixed conductor types. Hardly anyone would actually build
such a costly configuration, but the performance does give a standard
for comparison. Anyway, the pattern is an omni pattern with a signal
strength at 8 degrees takeoff angle of -2.72 dBi.
The second case is the Force 12 vertical dipole, with no radials. (I
do not have the actual engineering data for the Force 12, but it is
easy to approximate from the promotional materials.The Force 12
people do not recommend using radials, and for good reason. Cebik did
a study that showed that radials under a vertical dipole do virtually
no good whatsoever. The ground losses that affect its performance are
hundreds if not thousands of feet from the antenna. That is why the
spectacular results reported in the Force 12 promotional material are
from operations right on the seashore.) I've assumed aluminum
conductors. There is a very small note in the very fine print of the
Force 12 promotional material that their patterns were run with the
bottom of the antenna elevated 28 feet above the ground. I used that
assumption in my simulation. (The trick with vertical dipoles is
getting the current loop as high as possible above ground.) The
signal at 8 degrees takeoff angle is an omni pattern at -3.09 dBi. In
other words, the Force 12 with its low end 28 feet above ground is an
undetectable quarter dB worse than the ideal full size quarter wave
configuration. The Force 12 appears to be just as good as the
promotional material claims.
How important is the mounting height? It matters. For the same
configuration except with the bottom 1 foot above the ground, the
signal strength at 8 degrees takeoff angle is -5.8 dBi. This is a
quite noticeable >3db degradation from the full featured quarter wave
configuration.
The other configuration is an inverted L. This is a bit of a clunky
design, but it is feasible tom build on my lot. It is a W3DZZ dipole,
with one element vertical and one horizontal, and the feedline coming
off normal to the plane of the L. The height of the feedpoint is 50
feet. The elements of a W3DZZ are longer than 50 feet, thus I've
kinked out the part of the bottom element at a 45 degree angle (in
the plane perpendicular to the horizontal element) so that the end
barely clears the ground. (Yes, I know, if you have kids or dogs, put
a fence around it.) I assume copper conductors, average ground, and
take trap losses into account. Anyway, on 80 m at 8 degrees you get a
near omni pattern that is -0.85 dBi in the strongest direction and
-1.79 dBi in the weakest direction. Anyway, this is a cheap antenna
(provided you happen to have 50 foot high trees at just the right
spots) that outperforms both the full size vertical and the Force 12.
Into the bargain, you get a near omni pattern on 40 meters that at 8
degrees takeoff angle is -1.2 dBi at its strongest direction and
-3.15 dBi at its weakest direction. But wait, there's more; you get
low SWR at both 80 and 40 with no need for a sophisticated matching scheme.
The trick as always is that what really matters is getting the
current loop as high above ground as possible, and configuring the
elements such that the currents in them do not cancel each other out.
As for slightly elevated ground planes with resonant radials, they
work surprisingly well, but not as well as the three configurations
above. However, that is another story for another day.
73,
Steve Kercel
AA4AK
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