> On a 5 band 4 element quad you have 15 stubs. Each requires
> > lowering
> > antenna and raising it several times. Lots of work.
>
> Amen. You can burn a LOT of time twiddling with the tuning. That's
> why I
> switched to yagis.
If you set the reflector to the lowest possible frequency you plan on
using the antenna on without a driven element nearby, it will be set
at the frequency where F/B really starts to roll off fast.
This works with virtually any parasitic element.
Optimum F/B will occur someplace higher in frequency, but that
always works as a lower limit adjustment with normal element
spacings.
With multiple elements for each band, you could tune with all
reflector elements in place but no other elements on the antenna.
Another trick, if you don't want to set for lowest usable frequency
and want to set for a certain parameter you choose is to model the
antenna for the pattern you want. Then you can extract the element
from the model by either deleting or setting a source or sources to
zero current in the driven element.
When you feed that element as a dipole (or single loop) in the
model, you can then find the projected frequency of resonance.
You simply tune the element that way in real life, close the
feedpoint, and add the driven elements later.
The quad we worked on had a unique problem because it was
large, heavy, and also had some loading. Since the loading wires
were laid right against the fiberglass spreaders, it was impossible
to get dimensions from EZNEC (the dielectric effect was unknown
and could not be put in the model).
Since the goal of the user was to have F/B roll off at a certain area
of the band so he could occasionally work CW, we set the reflector
tuning so it was self-resonant at his chosen cutoff frequency. But
other parameters could have been accounted for.
.
73, Tom W8JI
W8JI@contesting.com
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