[TowerTalk] Force 12

[Jim Lux]

----- Original Message -----
From: "Darrel J. Van Buer" <darrel at vanbuer.net>

> I had a lot of trouble tuning 10m once with a C4XL (and changing element
> length was really bizarre - small changes act as expected, but at some
> point there would be a big jump to a wildly different resonance), they
> after taking it down to replace cable on crankup tower it was fine.  I
> THINK the problem is that the drive cell is really sensitive to spacing
> and parallelism between the elements.  If so, then it means loosening
> the U-bolts and shifting element position.
>
>
> >
> >Seriously,  has anyone had difficulty with their 3 band matching system,
operating on 15m?   There is a C4XL which we finally gave up on after two
years and put in three separate feedlines.   Then, we gave up on that and
replaced it with an optibeam.   Works great.
> >
> >At V26DX, there's a C3 with a 40m kit on it....well, it HAD one.  It
never tuned, either, and we finally gave up on it.   Seemed to tune ok on
20-10, and had
> >reasonable f/b.
> >
> >n2ea


I think Darrel is right.. The drive scheme for these relies on really tight
coupling between the driven element (i.e. the one that's actually connected
to the feedline) and the other parasitic driven elements.  I was doing some
modeling of this scheme to get a better understanding of what Force12 was
doing, and found that very small changes in the spacing had very large
effects on the feedpoint impedance (the performance as an antenna didn't
change all that much).

Perhaps one conceptual model is to think of the three elements as a 3
section LC network (with a lot of coupling between the components).  With
just the right spacings and lengths, all the reactive components cancel out
at the three frequencies of interest (14, 21, and 28).  However, change just
one component, and all the zero reactance points move around.

It's worse than trying to tune a multisection filter, and if your only
tuning tool is a SWR meter, you could spend a very long time trying to make
it work. With some sort of sweeper, and a bucket truck, you might be able to
do it a bit faster.  Very much a case of a clever design, that you pray is
assembled correctly and rigidly.

For what it's worth, at the JPL amateur radio club (W6VIO) we have a C3S
that we used for field day to good avail.  It tuned up fine on 10,15, and
20, but we only used it on 10 and 20, since we had the Cal Tech club's
(W6UE)  4 element monoband on another tower for 15. For all I know, the C3
was a fine dummy load on 15.

Also, for what it's worth, this is an example of an antenna design that
could make good use of a antenna mounted autotuner.  As I mentioned above,
the antenna performance (F/B, gain, etc.) didn't change all that much with
small changes in the element orientation/length/position, it's just the
reactive component at the feedpoint that's the problem.  A well designed (!)
tuner would probably be as low loss as the L/C inherent in the 3 driven
elements.