I had 5 100' vertical runs of LMR-400 on the corner of a 45G. All were
securely taped every 3 feet. at the top and bottom were bulkhead
connectors to ground the shield. In less than 5 years "all" of the
center conductors had migrated far enough that the N connectors were
intermittent.
During the same period, the 5 LMR-400UF 28' long pigtails had the
jackets deteriorate where they laid on the tower top plate to where as
much as 6" of shield was exposed. Only one did not have shield showing,
but when I went to make jumpers out of it, I discovered that much of the
foil shield was badly corroded.
I took the 5 100' runs LMR-400 down. I reinstalled it and the connectors
with single turn stress release coils at roughly 50 feet and 90 feet.
This left the runs about 6 to 8 feet short. (12" Diameter coil = 37"
...16" dia = 50") I moved the bracket for the bulkhead grounding
connectors down to the ends of the coax and made the pigtails about 6'
longer.
I had no further problems with coax center migration.
The point is what ever you use for support including supports made to
grab the coax, "finger cuff style" does nothing to support the center
conductor. My coax was secured every three feet, (far better than any
crank up supports) yet the center migrated to the point where it had to
be fixed. Adding the two simple one turn coils fixed the problem.
The one, half way up, would present a problem on the crank up.
My experience shows "to me" that those using N-type connectors on
vertical runs without problems are quite lucky.
A few years later, I had to take the antennas down because the second
TB3 thrust bearing had failed. (I knew I was over stressing them)
http://www.rogerhalstead.com/ham_files/Tower29.htm (BTW my wife took
the photo, not me) As long as they were down, I replaced the LMR-400
with LMR-600 and the pigtails with BuryFlex(TM). The two one turn coils
in the LMR-600 add almost 5 feet each (4'9")so each 100' run becomes 110
feet. The grounding connectors were moved back to the top of the tower.
( Circumference = Pi * d )
Although they sound good, I've had no luck supporting only the outside
of the coax. In practice I had to fix every vertical run. BTW I had to
replace the connectors on both ends of those runs. That's 10 connectors
in my case, not counting those on the pigtails.
73
Roger (K8RI)
On 6/5/2015 11:03 AM, Patrick Greenlee wrote:
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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Roger (K8RI)
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