I use a run of Chinese Grip braid, that grabs the whole run/length of all
tower top wires and lets them down in a ZERO stress point affair...look at
this site toward the end and see the approach
Bill - KC4PE
http://www.kc4pe.com/amateurshack.htm
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Patrick Greenlee
Sent: Friday, June 05, 2015 10:21 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] 90 degree coax bend atop a crankup tower
However we slice it the weight of a fairly long run of coax has to be
suspended somehow on crankup tiltover towers such that they are free to
crank up and down as well as tilt over and up. There are lots of approaches
to performing this task but one inescapable constant is the need to
distribute the "hanging weight" over a sufficiently large area so as to
reduce damage to the coax.
I haven't done/tried the following but solicit comments from those who have:
Use a large radius of curvature 90 degree section of conduit to increase the
area of contact between the coax and conduit. Use an adhesive (spray
adhesive sold for attaching auto trim or other good adhesive) to line the
conduit and increase the friction available via the capstan effect.
Alternatively, how about filling the space left between the coax(es) and the
90 degree bend conduit with epoxy or other similar stuff. This would
distribute the force over a large area and reduce deformation of the coax
and potential for the center conductor to "wander" over toward the shield as
it can when highly stressed.
Once installed, coax is typically not taken down and put back up very often
except for replacement so the epoxy approach shouldn't be a problem. Years
later when replacing the coax you might find it easier to replace the epoxy
filled bent conduit than trying to clean it out for reuse.
When bending conduit (and lots of other tubing) especially in larger
diameters it can be very helpful to fill the tubing to be bent with sand.
You can put tape over the ends to keep the sand in. Sand filled tubing is
far less likely to collapse/kink during bending than without the sand.
Removing the sand is easy, just pour it out.
Patrick NJ5G
On 6/5/2015 6:32 AM, ve4xt@mymts.net wrote:
> Wouldn't holding the coax with a sling-type grip in such a way that the
bend itself is a tension-free loop, solve both problems?
>
> The issue in both cases is the unsupported weight of the coax pulling on
the bend, yes? So if you took all the weight off the bend...
>
> Or am I missing something?
>
> 73, Kelly
> ve4xt
>
>
> Sent from my iPad
>
>> On Jun 5, 2015, at 12:15 AM, "Roger (K8RI) on TT"
<K8RI-on-TowerTalk@tm.net> wrote:
>>
>> Coax has two bend radius listed. A repeatable radius and a one time bend.
It's a good idea to stay well away from both of these. The one time bend
can be a very short radius. Short enough that the center conductor will want
to migrate off center given enough time just laying on a flat surface. The
repeatable bend radius is usually quite a bit larger and "should not damage
the coax over a given number of bends. No, I don't know the number, but the
minimum repeatable bend radius can still stress the coax.
>>
>> Now hang a load on the coax in a vertical run. The center conductor will
tend to migrate downward. In a 100 ft vertical run it's often enough for N
type connectors to lose continuity. If the coax is supported only from the
top with a relatively sharp bend, the center conductor will migrate off
center. That's why several small loops evenly spaced up the side of the
tower will remove the load from that top support.
>>
>> There are several methods of supporting the coax from the top on crank up
towers. Wire mesh and rope slings do a good job of holding the coax, but
do nothing for the linear migration of the center conductor. Several
commercial ones linked to on here appear to do a pretty good job,
>>
>> Too sharp a bend causes the off center migration, while too wide a bend
will allow the linear migration. There has to be "a best radius" for each
size and type of coax. I'd choose about twice the repeatable bend radius.
No concrete proof, it just looks right. Not exactly a scientific approach.
>>
>> Tall, crank up towers put a lot of stress on the coax supported only from
the top. Unfortunately supporting the coax at intermediate heights brings a
whole new can of worms.
>>
>> 73
>>
>> Roger (K8RI)
>>
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