My motorized 72-foot tubular crankup is behind some trees about 250' from
the shack, so I use the remote control feature. My comments on coax are
mostly relevant for those who have motorized crankups operated with remote
control. If you manually crank the tower, it doesn't much matter what you do
with the coax, as long as it can't work its way into the tower between the
lattice members or otherwise get caught above your reach.
I used LMR-400UF (so-called ultra-flex) when I first installed crankup and
very nearly had a disaster. I put a standoff just below the top of each
section, and oriented the standoffs 180-degrees from the winch pulley so the
coax wouldn't get in the way when the tower was tilted. This seemed the most
sensible placement of the standoffs. The coax was fixed to the top standoff
with cable ties and tape. From there it dropped straight through the rest of
the standoffs -- it was not secured to any of the other standoffs. I tried
fixing the coax to the other standoffs, but the resulting loops got twisted
and it was clear that either the coax would be damaged or the loops would
get caught somewhere in the tower (like, on a lower standoff!) Loops may be
OK when manually operating the tower, but not when it's operated out of
sight by remote control.
Unfortunately, the coax was so stiff that it bowed out below the bottom
standoff and worked its way around the tower as it was being lowered. On the
next trip up, the coax snagged on one of the motor housing brackets. As the
antenna peeped above the trees, I saw the tower swaying violently back and
forth as it pulled with all its might on the coax. Just as I was reaching
for the off switch, the TH-7 plastic balun housing failed, which allowed the
coax to yank the SO-239 connector out and the tower was saved. Luckily, the
coax had slipped through the cable ties and the balun wasn't very rugged.
The only damage was a busted balun and some severely stretched coax with a
big gash where it snagged. Whew.
I took a number of steps to correct this. First, I replaced the LMR-400UF
with RG-213. RG-213 is much more flexible and worked well for many years.
However, a couple of years ago I did some tests and found that Buryflex
cable is as flexible as RG-213 (maybe even more flexible), and has a more
slippery jacket. I replaced the 213 with Buryflex and it has worked very
well.
Perhaps more important, I bought an extra standoff and positioned it under
the motor (previously, the bottom standoff was above the motor, at the top
of the bottom section. This way, the coax could not bow out and get snagged
on the motor. I also repositioned the standoffs to be 180-degrees from the
motor housing. Finally, I built a cage out of hardware cloth around the
bottom three feet of the tower to keep the coax away from the rotor area and
base bolts. This arrangement has worked very well for 6-7 years.
I'm not sure if it's a good idea to run hardline up to the top of the bottom
section. The preferred method is to let the coax drop straight to the ground
through the standoffs. If it's fixed at the top of the bottom section, the
loop formed when the tower is lowered could get tangled around the tower. If
it's a lattice tower, the coax could get caught inside the tower.
I've never had a problem with the weight of the coax pulling on the point
where it's attached to the top standoff, but I suppose the best way to avoid
this would be to use a Kellems grip secured to the top standoff. Been
meaning to do that for years, but the only grip I have isn't weatherproof
and I haven't taken the time to locate one that is.
73, Dick WC1M
> -----Original Message-----
> From: Al Williams [mailto:alwilliams@olywa.net]
> Sent: Friday, July 01, 2005 11:49 AM
> To: towertalk@contesting.com
> Subject: [TowerTalk] crankups, coax, and rotator
>
>
> I am about to replace the coax and rotator cabling on my
> LM470 crankup tower.
>
> FYI the reason for replacing is that the m2 Orion rotator
> finally quit working after much losing of its position. I
> found that the pigtail connector pins had literally
> disintegrated and was replaced by a mucky material. I had
> not waterproofed the connector. It only took about two years
> to fail completely.
>
> I didn't use coax stand offs on the mid sections of the
> crankup because my "intuition" told me it might be a problem
> and doesn't help unless the coax was tied securely at each
> standoff (to reduce the weight at the top). I am considering
> using a piece of heavy garden hose to help distribute the
> weight of the coax when it bends at the top toward ground. I
> am also planning to use LM400 or LM600 coax--maybe hardline?
> I have two towers, one with a 200' and the other with a 400'
> run to the shack.
>
> All comments will be appreciated
>
> k7puc
>
_______________________________________________
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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