It is my understanding that the bending radius quoted in the spec is the
tightest you can bend it in a permanent installation and the tightest you
can put it on a reel.
There are two issues here (at least), repetitive bending where the failure
mode is metal fatigue in the wire. And the other primary issue is the
migration of the center conductor through to dielectric towards the
"inside" of the bend and its shield. The metal fatigue issue is solved
by using a stranded center conductor -- the more strands the better.
The rate of the center conductor migration depends on the dielectric material
as well as the temperature. Foam polyethylene tends to be a real problem,
especially at higher temps (environmental or due to RF losses),
solid polyethylene is much better, and Teflon is the best.
HOW the cable is dressed around the rotator is also important -- ie, does
the cable just get repetitive bends or is there some twist of the
cable as well.
I have been pleased with making multiple LOOPs of cable that sits on top of
the tower bearing plate. Like a cable-style RF choke. As the tower rotates
the diameter goes through a small change as the turns count changes by
one turn. In other words, I make a flat coil of coax about a foot in diameter
and of 5 or six turns. As the antenna rotates the coil will change by one
turn and the coil diameter will change by a couple of inches. I have
never had a problem with cable failure using this technique and the coils
tend to make yet another RF choke to reduce currents flowing on the
shield. The diameter of this coil is usually MUCH greater than the minimum
diameter spec.
--John W0UN
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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