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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