[TowerTalk] Coax Mishap on Rotating Crankup

"Dick Green". dick.green@valley.net
Sat, 11 Oct 1997 21:47:23 -0400

Hi, folks. I need some help again. Less that a week ago I finished
installing my U.S. Tower 72' motorized rotating tubular crankup with TH7.
After a number of interesting problems (which I will report in due course),
everything was working just fine. Then today I had my first mishap.
Fortunately, it was minor, but it knock me off the air for a few days.

The mishap was that the coax got snagged on a metal strut that holds the
motor housing. When the tower got near the top, it began gyrating wildly and
before I realized what was happening, the coax pulled out of the balun.
Actually, it pulled the SO-239 connector clean out of the balun, popping the
cover and unwinding the coil in the process. Although the balun was pretty
much destroyed, I'm glad it gave way before something else did. Fortunately,
the tower retracted fine. There doesn't appear to be any damage to the
cables, although I noticed that one had jumped out of its pulley guide and
will have to be put back in before the tower can be operated again. I feel
lucky that it wasn't more serious.

My question for the group, especially anyone owning this type of tower, is:
How do I dress the coax to avoid this???

Please note that I used Times Microwave LMR400 Ultraflex coax. It's probably
not as flexible as RG/213, but it's *much* more flexible than 9913 (it has a
stranded center conductor but the inner shield is pretty stiff.) I'd
appreciate any comments on whether this coax is appropriate for this
applictaion. If not, what's the most flexible low-loss coax out there?

After mounting the beam, my installer and I tried a couple of
configurations. First, we tried fixing the coax to the standoffs so that
loops would be formed when the tower is retracted. That would probably be
the best solution because I live in a frequent snow area and the coax will
probably get buried under a heavy blanket of snow and/or ice numerous times
during the winter. However, we did not like the way the coax loops twisted
when the tower was lowered. It looked almost certain that they could snag on
a limit switch or pulley bracket. However, I should point out that we did
not stagger the coax arms around the tower for this test. It didn't appear
that this would help much, although it might be a different story with more
flexible coax.

We settled on fixing the coax to the highest standoff and letting the coax
fall through the remaining three standoffs (the tower has four sections,
with one standoff per section.) I can live with digging the coax out of the
snow, I guess. The standoffs are lined up a little more than 90 degrees from
the motor housing, where there's a nice clear spot for the coax to drop.
This worked pretty well for a week. The coax, being somewhat stiff, doesn't
fall into a neat pile, but tends to form large loops that sit up off the
ground. I was concerned that one of these loops would snag under the tower
where the rotor is, but that didn't happen. It appears that the mishap
occurred when a higher section of coax found it's way around the tower to
the motor support strut, probably due to movement caused by the wind and
rotating the tower when it was only partly extended (perhaps both happened
at the same time.)

It appears that lining up the coax arms 180 degrees from the motor would
work better. Also, adding a coax arm just above the motor housing might help
as well (and would not hurt.) I've also considered another standoff below
the motor, but haven't measured yet to see if it will clear the base. It
looks like a pretty easy job to build a shield out of plywood that would
prevent the coax from going under the tower and getting snagged on the rotor
and supports down there. Perhaps a more flexible coax would complete the

Any ideas? Has anybody figured out how to dress coax successfully on these
towers? Any help would be greatly appreciated.

73, Dick, WC1M

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