[TowerTalk] 90 degree coax bend atop a crankup tower

[Roger (K8RI) on TT]

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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