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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