George,
Force 12 has a description of the operative theories behind their
antennas--if not at their site, then in their main catalog.
Joe's account of the feed system, often called open-sleeve coupling, about
covers the case at that end of the antenna. Given the context of a
multi-band Yagi, the exact spacing used between the fed and the coupled
driver is able to be modeled but still requires field setting. Element
taper also plays a role in the exact setting, you have two variables:
elements lengths and spacing, and you need to hit for the upper band a set
of values that gives the correct feedpoint impedance on that band without
disrupting the feedpoint impedance on the directly fed band. In a
tri-band Yagi, which side of the fed driver you place the "slaves" can
make a performance difference.
You can also use what some have called closed-sleeve coupling, with a pair
of parallel wires between the main and the subsidiary element. Because of
very close spacing, the wires do not act wholly as a standard transmission
line--indeed, the coupling tends to predominate.
A second effect that many current multiband Yagis take advantage of is
that the elements on the unused bands are not wholly inactive. Hence,
longer elements add reflector effects and shorter elements add director
effects--and this tends to "supplement" the gain of the m,ain elements for
each band. Part of this effect is somtimes called "forward stagger."
There are some notes on this on a two-band full monoband performance on
each band design at my site, along with an ingenious 5-band design from
Belgium in another note. Where elements are interlaced, they can limit
bandwidth. A forward 20-meter director will be active on 10 meters as a 1
wl element--and because it will be low in frequency without end effects
every 1/2 wl, it may force the performance on 10 meters to diminish
somewhat suddenly in the 28.8 - 29 MHz region, rather than show the slow
tapering off of a monoband beam. How much effect depends on all of the
element spacings involved.
A third effect is the addition of elements to set impedance levels--as in
the NW3Z 48' 20 meter design. Although not used in the small Force 12
beams (to my knowledge), I suspect that the X31 has at least one element
with this kind of function.
My hat is tipped to the designers of the current crop of tri-banders from
all sources, whether they use log cells, open-sleeve coupling, or other
feed methods. A multi-band Yagi is serious piece of engineering, and it
takes a good set of artist's intuitions as well as solid Yagi science to
make one work.
Incidentally, the WARC band use of these beams is simply
surrendipity--they happen to show matchable SWRs and with virtually any
Yagi, There is some latent or remnant directivity on bands adjacent to the
main ones. (My old GAP 6 is dandy on 6 and 2, although my understanding
is that performance on these bands was more of a discovery rather than a
design objective. So good stuff does happen--sometimes--when Murphy is
napping.)
Hope Joe's notes, these notes, and the web and book references help you
get a handle on the Force 12 and other current multi-banders.
-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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