The short answer: Use Buryflex. It's the most flexible of the low-loss
cables.
My experience with coax on a crankup is both similar and different from
Barry's.
In 1997, I installed a 72' US Tower motorized rotating crankup with remote
control. I had a TH-7 on top at the time. I used LMR-400UF and the three
coax standoffs that came with the tower. I tried tying the coax to the
standoffs, but it tended to twist and get tangled up with itself when the
tower was lowered, and it was clear the coax would likely be damaged when
the tower was raised. So I let the cable drop straight through the standoffs
and coil on the ground.
The tower location is about 250' from the house and behind a small stand of
trees. At the time, the bottom of the tower wasn't visible from the house.
But the remote control feature worked well for about a week. Then one day I
was demonstrating the remote control to a friend. In those days, the trees
were shorter and I was able to see the antenna and top of the tower after it
got up to about 50'. When the tower neared full extension, it began to swar
violently (and sickeningly.) I was momentarily so shocked that I froze. The
tower gave one final violent lurch, then stopped. I immediately killed power
and went to see what had happened. Turns out the too-stiff coax had wandered
around the tower and snagged on the motor mounting brackets. The only thing
that saved the tower was that the SO-239 connector on the TH-7 balun pulled
out (actually, the plastic balun box exploded), releasing the tension on the
coax. The coax was stripped and ruined, and the balun was a goner. But the
much-more-expensive tower and its all-important aircraft cable survived.
U.S. Tower was concerned that the cable might have stretched and sent me a
replacement, but I never got around to changing it out (a major job.) It's
been 14 years, and the original cable is still in good shape.
What I did to correct this was:
1. Changed the LMR-400UF to RG-213, which is much more flexible. Later I
changed the 213 to Buryflex, which is even more flexible and lower loss.
2. Added a fourth standoff below the motor. This prevented the cable from
wandering around the tower.
3. Changed the position of the standoffs the be 180 degrees from the motor
(they were previously closer to the motor housing.)
4. Built a cage out of hardware cloth and used it to enc lose the lower part
of the tower so the coax couldn't get in there and snag on the rotor and
related parts.
I probably ran the tower up and down a hundred times to make sure the coax
couldn't snag on anything. The cable drops down onto the ground, which is
slighly sloped away from the tower, and never tangles. I've run that tower
up and down hundreds of times over the past 14 years, without being able to
see it, and have never had a problem. The one caveat is that when we have
snow on the ground I have to make sure the cable isn't under ice. Usually
during the winter I have to take several trips to the tower to pull the coax
out from under a sheet of ice so it can unwind freely. But that's it.
Of course, YMMV.
73, DIck WC1M
-----Original Message-----
From: Barry Merrill, W5GN [mailto:w5gn@mxg.com]
Sent: Thursday, October 27, 2011 1:11 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Solid center conductor LMR400 for crankup tower?
Don't expect the arms to keep the cable cluster from being blown into the
telescoping tower sections, if you tarry a bit and don't start lowering the
tower before the storm arrives.
Early on, I used the standard US Tower arms on my HDBX 72 to "stand off" the
cluster of three coax and two rotator cables, but as soon as the tower was
lowered just a few feet, the slack in the cable cluster would be dancing all
around when the wind got to only 20 or 25 mph.
Several times, I found I had to stop lowering the tower after Mr. North Gust
had pushed that slack loop INTO the tower; sometimes I'd have to raise the
tower to tighten up the cable and thus to pull the slack "loop" out of the
tower, and then I had to wait-for/hope-for a Mr. South Gust to come along to
push the cable away from the tower sections, as it was stiff enough to have
some memory, and only then, could I restart the lowering. And then,
sometimes along would come an unwanted Mr East, pushing a lower section of
cable into the tower from a different direction.
After the last and most harrowing experience, where the wind was pushing 60
mph and it took a long time to get the tower down, I gave up on using the
standoffs, and simply tied three rope lines to the center of each section of
the cable cluster.
Now, when the tower is up, the cable cluster hangs from the only remaining
arm, at the top. The cable cluster hangs down to 15 feet and then has a
drip loop back up to 20 feet, and then the cable cluster goes horizontal to
the shack.
Then, when I lower the tower, I can pull individual tension as needed on
each of those three rope lines that are tied to the cable cluster at those
three points, and I can thus keep the cable cluster out of the tower while
it lowers.
I also mounted three pulleys for those rope lines at 20 feet and ran those
three lines thru those pulleys. When the tower is down, I can pull up those
lines thru their pulleys, and keep the entire cable cluster hanging off the
ground, the weight on those three lines.
And, I had much earlier given up on using the installed remote
raising/lowering feature of the tower, even though it was way cool to flip
the $1000 remote switch on, and then step out onto the porch of the second
floor radio shack, and watch the tower, about 85 feet away, begin to lower.
Kewl it was, to be remote, but only until the first time when it abruptly
stopped lowering, with a dull bang from the vicinity of the tower base, but
I was too far away to have any idea of the source of the noise or what
happened.
(Attempt to restart failed, no power, as the circuit breaker was the
bang. Reset breaker, retried at the tower base, and could hear the
motor whine and see it stall and then heard the circuit breaker bang.
So I decided it was wise to ALWAYS be at the base and to ALWAYS manually
control AND ALWAYS WATCH the lowering and raising of the tower, from the
base of the tower.
73
Barry, W5GN
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of W0MU Mike Fatchett
Sent: Wednesday, October 26, 2011 9:12 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Solid center conductor LMR400 for crankup tower?
I don't think using a solid conductor on crank up is a good idea. I don't
believe the cable was designed for those types of applications.
Mik0MU
J6/W0MU November 21 - December 1 2011 CQ WW DX CW
W0MU-1 CC Cluster w0mu.net
On 10/26/2011 7:58 PM, krishna kanakasapapathi wrote:
> Howdie folks,
> I am installing the DB18 on a 75ft crankup. Have a roll of LMR400,
> the solid center conductor type.
> I thought i might just ask before i cut the cable from the roll.
> Any possible issues with
> using the LMR400 on a crankup?
>
> I have arms mounted on each of the 3 sections to keep the cables
> away from the telescoping sections.
>
> 73s
> krish
> w4vku
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
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