[TowerTalk] Force12 C3XL : help
( Sante ) IK0HBN
ik0hbn@isa.it
Sat, 17 Jan 1998 01:03:37 +0100
At 16.49 12/01/98 -0800, you wrote:
>Good afternoon.
>
>Appreciate your asking about this situation. The condition of the PVC
>melting on the 20 mtr reflector cannot happen under typical operating, even
>running several KW. If any element would have a problem, it would be the
>driver.
>
>An important question is, "What happened when it "melted"?
>
>The reason this is important is because the antenna will work the same. It
>does not matter if the reflector is insulated or not from the boom. It can
>be either insulated OR grounded to the boom. Insulated or not, melted or
>not, the reflector will still operate properly. The issue of the PVC melting
>indicates something else is happening.
>
>
>The driver element in the 20 mtr portion of the C-3XL will have 100% of the
>maximum current. This 100% will occur right at the center of the driver. The
>reflector has about 49% of the maximum. The director has about 47% of the
>maximum. This gives us a clue. Something happened to the reflector center
>insulator. Nothing happened to the driver, which has twice the current.
>
>
>We need to look for some unique condition here.
>
>
>How could sufficient current flow from the element through the PVC to melt
>it?
>
>The only way that comes to mind is to have an external source. Two
>suggestions are:
>1) look at the antenna to see if the reflector element (i.e. the tip)
>is coming into contact with something. For example, maybe an AC high voltage
>power line that can pass high current/voltage through the element, through
>the PVC, to the boom, to ground.
>2) ground the reflector directly to the boom. Whatever is causing this
>strange condition will then have a direct path to ground.
>
>
>Hope to know what is the cause.
>
>
>
>
>Have a good day and 73,
> Tom, N6BT
> Force 12 Antennas and Systems
> (Home Page http://www.QTH.com/force12 )
>
Hi everybody,
thanks to all tried helping me. Here below some considerations could help
in understanding the problem.
1) Working conditions here are an IC 765 and an Alpha 87A, so maximum
delivered power should be 1,5 Kw, BUT, being my amplifier very expensive,
for safety reasons Never I run more than 1 KW (Scott, of Italian Power...hi)
2) The C3XL has mounted on a crunk up tower at about 11 meters from the
ground. That tower is mounted directly on the ground and near it there is
nothing. On the same tower, at 8 meters from ground (so 3 meters under the
beam) there is a dipole for 30 meter band and it never caused troubles to
the beam, in fact, removing it, SWR doesn't change.
3) NO electric power lines in the radius of a mile.
4) Reflector tips are at 4 meters from the nearest tree.
Other consideration could help:
In the C3XL boxes, I found all the insulators egual (gray color), less
which one of 20 meter reflector which was white and much less thick of the
others. Surely this reason could be the easiest explanation to the fault.
And now the matters:
During the CQWW CW, when running on 20 meters band, the Alpha went in self
protection for high ROS. Useless the tempative to restore it: SWR was
higher than 3:1, so to force me to qrt on 20 meter. The weather was foggy
(so umidity could be another reason), but no storm nor lightning to let me
think at a discharge).
The following week, with the help of 2 hams come from Rome, we erected a
tall ladder and we proceeded to check the system. Below the exact order of
our check:
Coax line: it has been tested at the balun connection using a dummy load:
NO SWR.
Balun (by Force12, model 3KW) it was put offline and tested with an
ohmmeter: no shorts, by the way it was substitute with another, but SWR
remained high as before).
Radiator: for what we could do due to the elevated hight with no protection
for the climber, we were looking for something burnt, but the radiator
seemed OK.
Only at this point we noted one U bolt at REFLECTOR turned brown. I asked
to my friend to check and what at first seemed to be only rust at 3 meters
distance, afterwards turned to be the white insulator melted upon it.
After having dismanted the reflector, we found the insulator melted and
burnt for 3 Cm.( the white colour has become black) at the point in which
the U bolt clamped it.
I must say that it was mounted as the book says, or better with the slit at
45 degrees, pointing toward bottom, but not vertical to prevent any
shortage as manual (and your message) rightly says.
At moment I have not a camera to take a picture and send to you, but hope
in a week to find one to do the job.
The insulator has been substitute with another thicker, lets say more and
less the thickness of all the others used in the beam.
The slit has been full up with Silicone to make an higher insulation (may
be we wrong in doing this...I don't know). The insulator in the U bolt has
been mounted as before. Generose amount of self-vulcanizating tape has been
used at both end of the insulator, covered afterwards with normal tape.
Finally the antenna restarted working with a flat bandwith as before.
Last sunday, during another foggy day, when moving from 15 to 20 meter, the
Alpha rewent in protection. During the following days I tested again the
beam with dry weather but SWR never came down 3:1.
At this point, it could be the reflector again (I fear) or something else.
Till next Sunday morning, no way to go up, but the idea, in the event the
fault would be the same, of removing insulator at reflector is very
intriguing for me.
What,I humbly ask, is I do not want my beam turned in a compromise antenna.
Since its born here, it has given me a lot of satisfaction, and if the
effort of looking for another type of insulation could avoid a degradation
of any of its performance, sincerely I should prefer waiting for another
insulator ( still to be looked for it). I do not want renounce to his gain,
F/B ratio, his very beautiful bandwidth. I mean that it was put in the KIT,
probably it was useful for something....))
Many many thanks and sorry for the bandwidth.
73 Sante, IK0HBN
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