This didn't make it out to the list the last time around so I will try again.
I have been reading this thread but have held off on posting anything until
now. I am assuming that all the modeling that has taken place has assumed
equal magnitudes for each element of then and the varying of the phase angle of
the lagging element to achieve the desired cardiod pattern. Obviously, when
you suppress the radiation on the ground in the null area of the pattern on
the ground the radiated signal has to go somewhere. Some if it goes to
generating the gain of the antenna in the main lobe of the pattern and some if
it
is radiated at some of the higher vertical angles in the null area of the
pattern that result in less attenuation of the signal at vertical angles at 30
degrees and higher and therefore effect the F/B of the antenna at these
angles. You can see this if you plot the vertical radiation pattern of the
directional pattern.
When I designed my W8AV 80 meter triangle array, I took this into account
and used a shunt coil in reference element (the one that does not have the
phasing line attached to it) to suppress the signal at the upper vertical
angles
in the null area of the pattern and make it as close as possible to be uniform
at all vertical angles. From experimentation and from experience I gained
by working for 26 years in the broadcast field, I found that an magnitude of
about 0.5 on the reference element gave me the best results. This same
principal will work with a two element array. You can model it in NEC to
determine the best number for your particular application by playing with the
magnitude on the reference element and plotting the results. You can also
vary the
phase delay on the lagging element to in conjunction with the magnitude to
achieve the radiation pattern you desire. Once you get the desired pattern
you
can install a coil in parallel with the phasing line at the common feed
point and tap down the coil to get the desired magnitude. This is called a
parallel or shunt power divider and is used in one way or another in all AM
directional antenna systems to develop the required directional patterns.
The caveat, as mentioned before, is that YMMV if there are any re-radiators
in the immediate vicinity of the array that will disrupt the pattern,
especially if they are in the null area. In addition, as previously
mentioned, the
"real world" values can differ greatly from the theoretical values obtained
from NEC and you may have to tweak the phasing line and/or magnitude to
achieve your pattern. My W8AV Triangle uses three wire elements suspended on
ropes from the base insulated tower that serves as my quarter-wave 160 meter
vertical antenna. Because of this, I had to detune the 160 antenna on 80
meters
to achieve the desired pattern. As a note, the big tower wouldn't have had
as much impact on the pattern if the tower was a grounded tower as opposed to
being an insulated one. You can also model nearby radiators in NEC to see
what impact they will have on your pattern.
73............de Goose, W8AV
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