Friends,
I thought I would share this with everyone because I really should have
known better!
On Friday night a very strong front came through upstate New York with winds
in excess of 66 mph at my hilltop QTH. During this wind attack, my stack of
two C3E's on a rotating pole stopped playing antenna. This meant that I had
the "honor" of doing the DX contest with a ground mounted vertical instead
of the stack.
Today, I went outside with a friend to fix the stack and we quickly isolated
the problem to the coaxial cable running up the tower between the Heliax and
the Stackmatch. I immediately cut off the ends to put new connectors (one
an "N" and one a PL259) and was simply shocked to see that the nice new
cable installed last spring was 9913 with its solid conductor. A dummy load
and an analyzer quickly determined that the cable had failed. A quick slice
session with a case knife located the problem spot where the center
conductor had pulled apart after breaking.
The solid center 9913 had been installed with a large loop at the bottom of
my pole (called Stosh) and was attached with strain relief at the top AND
was prevented from swinging in the breeze by a Rube Goldberg adaptation of a
rope harness attached to the top of each telescoping tower section.
The failure occurred due to the flexing of the rotating pole, not the
turning of the tower as the rotor is at the bottom. The wind storm of one
month ago (82 mph winds) and the weekends blow were enough to flex or bend
the tower repeatedly, therefore moving the attached 9913 and ultimately
twisting the solid center insulator apart.
What I have learned from this event is never to use 9913 with a solid center
conductor in an application where any movement can be encountered. I know a
rotor loop of RG213 or RG8 is needed after using 9913, but I never thought
subtle repeated movement would lead to failure. Even though the 9913 was
secured to the pole, there was enough movement to cause failure.
73
Natan W6XR/2
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