Thanks grant. Apparently I was set to worrying over a non problem. I
have seen but not used the Chinese finger cuff things for pulling wires
through conduit using a winch and 3/4 inch rope. I hadn't seen the
lace-on variety but those sound really handy.
Thanks again, a little experience trumps a whole lot of speculation.
Patrick NJ5G
On 6/5/2015 9:39 AM, Grant Saviers wrote:
Tis the simple answer. Use a Kellems grip and then there is no stress
on the bend. They are available to slide over the end of the cable or
as "lace on" so they can go on an established length or where a
connector on the cable is larger than the expanded diameter. LMR400,
Buryflex, LMR600UF and the like have a specified tensile strength of
350 to 400# and a weight per foot of 0.07 to .16#/ft so one grip at
the top handles up to 110' crank up with better than a 2:1 safety factor.
Grant KZ1W
On 6/5/2015 4:32 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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