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[TowerTalk] Force 12 C31-XR - Long

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Subject: [TowerTalk] Force 12 C31-XR - Long
From: "Gary Jones" <>
Date: Thu, 17 Aug 2006 08:53:55 -0500
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I have had a C331-XR for 5 or 6 years and used it extensively following
a tornado which destroyed my higher/bigger yagis. My C31-XR used to be
mounted on a Hy-gain 70' crank up and is now sitting on top of a 89' US
Towers crank up. I have had a 20 meter stack of Hy-Gain 205CAs
(65'-105') in the past. 

I find the C31XR an excellent yagi. Performance has lived up to my
expectations. The beam is flat on design bands. It also resonates on 17
without an antenna tuner and I have made many 17 meter DX QSOs with it.
It is not designed for 17 but works. It fires sometimes in one direction
on 17 and at other times in the other direction on 17, which is hard to
understand, but there is clear front to back at times, it just changes. 

My current installation is at the top of a hill in a very high point in
very low Louisiana and on the motorized 89' crank up. In this location,
I find the C31-XR to have very flat SWR on all the design bands. My SWR
is 1.4 to one at the bottom of the band and 1.1 through the top of the
band on 20. On 20 meters, the SWR varies from 1.2 to one to 1.1 to 1 at
around 21.295 back up to 1.3 to one at the top of the band, and on 10
meters, the SWR varies slightly up and down around 1.2 to 1 across the

The one issue that you will occasionally see, and used to be a complaint
before Tom made a design change on to this yagi is that the SWR on 15
used to vary rather widely during strong winds. On this yagi, you can
either feed the antenna with a single feedline or three feedlines. Mine
is fed with a single feedline. In this configuration, the 15 meter
driven element is not directly driven in what Force - 12 calls an "open
sleeve" design. The element is positioned physically very close to the
20 meter driven element and is excited by the 20 meter driven element.
Because it is driven by its close coupling, changes in physical distance
between the two elements would change the coupling. In a strong wind, as
the 15 and 20 meter elements would flex due to their differing lengths
and diameters, the distance between them would change and the SWR would
change. In very bad conditions, the elements could touch each other
making much bigger changes to the SWR. 

It is my understanding that in the early designs of the C31-XR, the 15
and 20 meter driven elements simply sat close to each other. In the
later designs like mine, Force-12 had engineered a plastic or rubber
spacer that sits out about half way on the 15 meter element and straps
the 15 meter element to the 20 meter element. This strap then tends to
keep the two elements more exactly parallel. 

The yagi balances nicely and had a nice space near the boom to mast
bracket if it needed to be rotated around a tower or structure. The
construction is very good. My yagi exhibits a slight amount of boom sag
without the boom trusses in place, but is perfectly flat with the
trusses installed correctly. The boom to mast bracket is a very solid
aluminum plate with muffler clamp type U-bolts. The C31-XR made it
through a tornado that passed directly over my property in Mississippi
and took down my 20 meter stack. It was cranked down, but the yagi made
it through unscathed, which was impressive. 

Force - 12 advised to always have at least 100 feet of coax in the feed
system. That caused me some real hours of puzzlement in my most recent
installation at this QTH. I put the MFJ on the yagi at 45 feet with
roughly 80 - 90 feet of coax on it, and the SWR was crazy on all bands,
and 15 was particularly terrible. The resonances were very low. The feed
system uses a coiled hairpin and two parallel wires from the 10 meter
driver to the 20 meter driver that parallel the boom. You are instructed
to compress the coil or stretch it, and to move the parallel wires
around to find your best resonance. Nothing made the yagi look better. I
was doing something that I had never done before... I rented a 65' man
lift to install the yagi on the top of the tower (nested down but could
be done as a one-man installation), I found the man-lift metal cage
produced problems in the SWR and an additional length of coax made
everything come into spec. 

I'm very happy with the performance of the C31-XR. A good friend of mine
K5JZ also runs this same antenna on a crank up at 70' and is very happy
with the antenna. His made it through Katrina, 35 miles inland from the
Gulf, and it survived nicely also (his is the most heavy duty
construction design with the highest wind survival rating). So is mine. 

I'm a top of the honor roll DXer and pretty serious about what I expect
from antenna performance and equipment, and this one has me impressed. 
           Gary    W5FI    

-----Original Message-----
[] On Behalf Of Larry stowell
Sent: Thursday, August 17, 2006 5:26 AM
To: 'Marc Wullaert ON4MA';
Subject: Re: [TowerTalk] f12 c31xr

I had one up for a couple of years, it worked fine on 20 and 10 but it
was very narrow on 15. I paid
for some new(different) coils to no avail. They kept telling me it was
my installation!! Replaced it
with a SteppIR

Larry K1ZW
Durham, NC

-----Original Message-----
[] On Behalf Of Marc
Wullaert ON4MA
Sent: Thursday, August 17, 2006 2:58 AM
Subject: [TowerTalk] f12 c31xr


I'm replacing one of my KT34XA tribanders in september with a F12
The F12 antenna is second hand and about 6 years old.I'm rebuilding it
with new TT  U-bolts for the
elements and mast/boomplate.

Any tips for this antenna ? Would it be better than my trusty KT34XA's
specialy on 20m ?

Marc ON4MA

visit  to read the
story about OP0HQ IARU HF 2006

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