> >
> > So my question is, what are the REASONABLE limits, say, if you have a 3
> > element beam (monobander? tribander?), of adjusting only the driven
> > element length (which you can usually get to on top of the tower)and not
> > diminish the ability of the antenna to achieve what it's supposed to?
In most Yagi designs, adjusting the driven element will have the sole
effect of changing the feed point reactance. In some designs, the
feedpoint resistance wil also change as a function of length, but the
degree of change can be quite variable, being a little on some designs and
a lot on others.
In most designs, the gain and front-to-back ratio will not change more
than a tenth of a dB with large changes in driver length. Hence, changing
the length of the driver will affect things like SWR, but will not
ordinarily affect gain and F-B. To improve those figures, access to the
other elements, especially the director(s) is necessary.
Notice that I keep repeating, "in most designs." There may be some
designs in which driver changes more radically affect performance other
than SWR, but in the dozens of designs in my files (not including scalings
of the same design for other frequencies), I have not found one. However,
with close-spaced elements on very short boom designs, the interactions
are stronger and the likelihood of performance alteration, especially off
the peak of gain or F-B, may be greater. I have found this indicated on
some 2-element beams where the driver is resonated rather than operated
with a capacitive or inductive reactance for use with a Tee, beta, or
gamma matching system. The effect is largely to broaden or narrow the
performance curves rather than drastically affect peak values.
I hope these notes are useful.
-73-
LB, W4RNL
L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215
1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335
Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509
37938-4443 USA / \ \ \ \ || cebik@utk.edu
URL: http://web.utk.edu/~cebik/radio.html
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