[TowerTalk] [Bulk] Re: 90 degree coax bend atop a crankup tower

Fri Jun 5 23:00:53 EDT 2015

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 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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-- 

73

Roger (K8RI)

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