Ed,
Hate to disagree with a friend, but on the difference between center
(base) loading and mid-element loading, there is not as much difference as
we had previously suspected. I have looked extensively at the current
distribution of dipoles with center, mid-element, capacity hat, and no
loading. The distribution is approximately, but not exactly sinusoidal,
which means that hafway down the line, current levels are still quite
high.
For a given length that requires loading, you can center load a dipole
with a certain required reactance. As you move the loading inductor--now
split into two part) outward, the rquired reactance increases until by
about the mid-element point, the needed reactance of each coil is almost
that needed for a single center-load coil.
For a given Q, the resistance for the loads of the mid-element loaded
dipole will be about double that of the center-loaded dipole. The result
is for a mid-element loaded dipole a gain figure that is only marginally
(a few hundredths of a dB) higher than for a mid-element loaded dipole of
the same ultimate length.
The advantages of mid-element loading lie elsewhere than in raw gain. The
feed impedance is a bit higher than for center-loading. The mid-element
load can be so arranged that the secondary resonance of the antenna can be
engineered to a desired frequency. The loading element can be separated
from "disturbing factors" at the feedpoint, such as support materials.
But for some homebrew projects, center or base loading may be mechanically
easier to implement--but needs to be used with caution.
The next installment of the small beam series in Communications Quarterly,
which addresses capacity hat loading, has a small graphic on current
distribution with various types of loading, which saves me from trying a
very bad ASCII version here. But the exercise can be replicated on any
antenna modeling software--just look at the current tables available in
the program output. Indeed, your note gives me the excuse I needed to
remind antenna modelers that not all of the important information on
antennas is given in far field patterns. The tabular data are also very
useful in understanding why a given antenna puts out the far field it
does.
Hope this is 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://funnelweb.utcc.utk.edu/~cebik/radio.html
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