I used RG11 (foam) for a few traps on an attic antenna (very hot in the
summer) that was used for rtty contesting (plenty of heat being dissipated
by the trap). The trick is to get a big enough radius to prevent the
migration that Roger is talking about. The failure was almost always at the
point near the end where the coax was tied to the connection. Once I got to
about an 8" diameter form, I had no further problem - as long as the wire
was not forced off the center line at the connection point.
Since then I have built a bunch of the K9YC style coax chokes out of Bury
Flex and kept the radius about 1' - no problems with those either - although
I think this is a much less demanding application than the attic coax-type
traps.
73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie
-----Original Message-----
From: Roger (K8RI) on TT
Sent: Thursday, December 18, 2014 2:07 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Strange coax failure
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@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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