[TowerTalk] Force 12

[Jerry Keller]

----- Original Message ----- 
From: "Gene Smar" <ersmar at comcast.net>
To: "Jim Lux" <jimlux at earthlink.net>; "Darrel J. Van Buer" <darrel at vanbuer.net>; 
<jimjarvis at comcast.net>
Cc: "Towertalk" <towertalk at contesting.com>
Sent: Wednesday, November 10, 2004 12:41 AM
Subject: Re: [TowerTalk] Force 12


> 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.
>>
>> _______________________________________________
>>
>> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
> 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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>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless 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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