[TowerTalk] Force 12

[Gene Smar]

TT:

     I have a Bencher Skyhawk with the older open sleeve feed system
(similar to F12's.  Ahem!)  Both F12 and Bencher Yagi's have available a set
of phenolic spacers that keep the DEs in the same spatial relationship over
their entire lengths.  The need for these spacers is to minimize SWR
fluctuations during high winds that cause the elements to whip around,
dramatically changing the feed impedance on the various bands.

     Just a thought but maybe F12 folk whose Yagis don't include these
spacers ought to install a set to maintain the design impedance during windy
conditions.

73 de
Gene Smar  AD3F

----- Original Message ----- 
From: "Jim Lux" <jimlux at earthlink.net>
To: "Darrel J. Van Buer" <darrel at vanbuer.net>; <jimjarvis at comcast.net>
Cc: "Towertalk" <towertalk at contesting.com>
Sent: Tuesday, November 09, 2004 10:56 PM
Subject: Re: [TowerTalk] Force 12


>
> ----- 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.
>
> _______________________________________________
>
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Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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