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Re: [TowerTalk] Coax Arms for Tri-Ex Crank-Up Towers

To: "'jimlux'" <>, "'Larry Essary'" <>
Subject: Re: [TowerTalk] Coax Arms for Tri-Ex Crank-Up Towers
From: "Tod - ID" <>
Date: Thu, 29 Oct 2009 10:03:51 -0600
List-post: <">>

I have been dealing with that problem for the past 10 days.  

I use a small Kellem type grip to hold four coax lines and one rotator line
to the top coax arm. The grip is sized and I selected one that will handle
between 1.25" and 1.75" of cable diameter. I bought it from an electrical
supply store in town and as I recall it was less than $25. The pressure on
the coax is distributed along a 10" sleeve so that one does not damage the
coax or rotator cable. I have a loop of cables that go from the Kellem grip
to a leg at the top of the tower and there I use a SS worm clamp, gently
tightened to hold the coax lines in place and orient them so they are going
vertically as they go up the tower to the thrust bearing and mast.

The inside of the coax arm loops on the US Tower that I have were very rough
as a result of hot dip galvanizing. I found some clear plastic tubing at
Home Depot with the correct  inner diameter and now have that threaded on
the loops at each level. This really makes the coax 'slip' through the loop

With the tower fully extended I used a ladder to reach the lowest coax arm.
I put some colored tape on the coax bundle about three feet below the arm.
Then I retracted the tower [for safety] and went up the ladder again. I used
an SS worm clamp to fasten the cable bundle to the coax arm . I fastened it
so that the tape was at the point fastened to the arm. Once that was done I
had a drop that went down the tower to the base that did not change position
because it was part of the first 20 feet or so of the coax going up from the
base. I clamped this lowest end to the bottom of the tower with an SS worm
clamp so that it would be out of the way with respect to the descending coax
bundle. I also now had a loop going from opposite side of the arm fastening
clamp that was descending to the bottom of the tower before it then went
back up through the 1st coax arm loop to the top coax arm. This descending
loop was a bit more than 20 feet long in my case. If I raise my tower by
about 12 feet the bottom of the descending loop is just above the bottom of
the tower base. The result is that I have about 60 feet of coax that is very
well oriented and is not piling up on the ground. At 12 feet higher than the
minimum height of the tower the wind stress on the tower is low enough so I
am not concerned about it. [The stress on the mast does not change as a
function of height; only as a function of the number of square feet of
antenna fastened to the mast].

In summary:. 

When the tower is extended,  the lowest section of the coax bundle does not
change its position and can be fastened to the tower at the lowest coax arm
level. Once fastened, only the coax above the lowest arm will move and you
have about 40 feet of tower lowering before the bottom of the loop will hit
the ground. You do want to be sure that the descending loop does not catch
on anything when you are raising the tower. For me that means instead of
lowering the tower by 40 feet I lower it by say 36 feet. That keeps the
bottom of the loop free from the base. 

My tower is a US Tower HDX-572 but the principle should be the same for any
telescoping tower with coax arms.


> As for the tension on the cable.  They make nice woven grips 
> that hold the cable, and then it's a matter of choosing a 
> suitable cable with sufficient strength or running a strength 
> member along side the cable (like cable TV drops that have 
> the steel messenger cable in the jacket). 
>   There are flexible low loss cables made for this kind of 
> application, although I'll bet they're not cheap.
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