[TowerTalk] 90 degree coax bend atop a crankup tower

[Patrick Greenlee]

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 at 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 at 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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