[TowerTalk] Strange coax failure

[Roger (K8RI) on TT]

Warm?

I live in Mi, which isn't usually considered warm, but we get the 
occasional summer day over 100.  I know that's shy of the temps "down 
South", but black jacketed coax can easily become too hot to handle.  
That and given, under some conditions the center tends to migrate in 
foam coax at so called normal temps, some thought should be used when 
installing foam coax.

"Any" and I emphasize the "any" position that causes side pressure on 
the center conductor should be avoided.  IE:  static turns approaching 
the repeatable bend radius, let alone smaller are "likely" to cause 
center conductor migration off center.  Even worse are turns with weight 
on them.  IE: A choke balun at the top of the tower with the vertical 
coax run hanging from the balun, or a few turns of coax tapped to the 
tower with the vertical run hanging from it.  NOTE: The turns not only 
distribute the weight, they resist the linear migration

Another problem is linear migration of the center conductor on vertical 
runs, particularly when using N-type connectors. This leads to the 
necessity of using a contradictory approach.(A couple of turns taped to 
a tower leg every 25 or 50 feet)  Use turns with a radius of at least 
twice the repeatable bend radius limit.  This distributes the lateral 
load between a number of coils, reducing the lateral force on the center 
conductor.  With the coax shield grounded at the top and bottom of the 
tower, the turns at the top and middle will not cause problems.  Turns 
at the bottom will resist the linear migration.

73

Roger (K8RI)

On 12/15/2014 12:22 PM, w5gn at mxg.com wrote:
> I believe that's why foam coax baluns are unwise in warm climates; I seem to
> recall several disaster stories in the early 70s around Dallas where the weight
> of the coax and the hot sun deformed and in a couple of cases, I think, actually
> shorted the coax.
>
> 73
>
> Barry, W5GN


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