K7LXC wrote:
> Yeah, don't do it. The only way to insure that a
> crank-up goes up or down successfully is to be in the direct
> vicinity - preferably standing next to it - watching it as
> it's moving. Otherwise a cable can snag and the result can
> be breaking a haul cable and watching the whole thing crash down.
Well, some of us have to do it. My crankup tower is 265' from the shack
behind a tall stand of trees. If I squint real hard, I can sometimes see one
of the antenna elements through the foliage. But most of the time I can't
see anything until the antenna is nearly at max height. Every year, as the
trees grow, the amount of tower I can see diminishes. Pretty soon, I won't
be able to see it at all.
For a number of reasons, I use my crankup very sparingly -- pretty-much only
for major contests and for hunting very rare DX (only need two more
countries, so that doesn't happen very often.) I could probably live with
standing next to it when raising or lowering it, but I want the option to
lower it immediately when the wind speed gets too high. Every now and then,
I need to raise the tower when there's snow up to my waist and it wouldn't
be much fun trudging down a steep hill in the dead of winter to raise the
tower, especially at night. Same is true when there's a big rainstorm. There
are just some occasions when I want the option of operating the tower from
the comfort of my shack.
But Steve is right that remote control is a potential tower-killer. Nine
years ago, just a couple of weeks after I got my U.S. Tower MA770MDP
installed, I was demonstrating it to a friend. It was October, the leaves
had fallen, and we could see the tower rising through the trees. When the
tower got near to the full-up position, it started swaying violently back
and forth. And I mean violently! For a couple of seconds I was completely
stunned and didn't know what could be happening. Suddenly, the violent
swaying stopped. I shook off the shock and hit the OFF switch real quick. I
knew something terrible had happened.
Upon examination, I found that, despite using standoffs, the coax had
migrated around the tower and had gotten snagged by one of the brackets on
the back of the utility box containing the tower control electronics. About
six inches of the coax jacket had been gouged off. The coax underneath was
crushed. I looked up, and saw that the coax had pulled out of the balun on
the TH-7. In fact, the balun had exploded. This was lucky. Basically, the
plastic balun housing failed, releasing the SO-239 connector and saving my
tower before the steel cables or pulleys failed, or before the motor burned
out.
I determined that the failure was caused by several related factors. I had
positioned the coax standoffs at the top of each section. The motor and
control box were mounted at the middle of the bottom section, well below the
last standoff. The lack of a standoff below the motor mounting point allowed
the coax to move around the tower. This was compounded by using Times
Microwave LMR400UF, which is anything but "Ultra Flexible". The stiff coax
bent into a huge loop as it hit the ground, and walked itself around the
tower to the motor and control box. I also had the standoffs less than 180
degrees from the motor, reducing the distance the coax had to travel.
The solution was to put standoffs above and below the motor (especially
below.) That way, the coax could not work its way around the tower. I
repositioned the standoffs 180 degrees from the motor, allowing the coax to
"flow" downhill away from the tower. I also switched to much more flexible
RG-213 (I later switched to Buryflex, which is even more flexible than
RG-213 and has lower losses.) Finally, I built a cage out of hardware cloth
to encase the bottom three feet of the tower, where the coax might snag on
the rotor, rotor shelf, rotor mounting bolts, anchor bolts, tower base
plate, etc., etc. I ran the tower up and down dozens of times to make sure
there was no way the coax could snag anything. The coax sort of bounces off
the hardware cage and coils on the ground.
I've not had any more snagging incidents over the intervening nine years.
I've watched the coax coil and uncoil many times, and I'm satisfied that
it's safe to raise and lower the tower without being present. One caveat: in
the winter it's possible for the coax to get buried under the snow, and for
the snow to develop a hard crust. I make sure to check for this whenever
conditions warrant, often having to pull the coax out from under the crust.
The ice has never been thick enough for this to require much force, and I'm
satisfied that even if the tower pulled on the coax the only damage might be
to the coax itself, and not the tower.
So, if you don't have to operate your tower by remote control, don't. But if
you have to do it, be meticulous in how you deal with all the potential snag
hazards.
N.B. Some may be wondering whether I attached the coax to the standoffs or
let it drop through. U.S. Tower has no guidelines for this, though an old
Wilson manual I have shows the coax tied to each standoff. I tried doing
that when the tower was first installed, and it was a no-go. This caused the
coax to form itself into big loops between the standoffs, which would then
twist around on themselves. That was a snagging hazard in itself, as was the
very real chance that one of the loops could get caught on a pulley. The
only rational solution was to tie the coax to the top standoff and let it
drop through the lower standoffs. This has always worked fine.
73, Dick WC1M
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