I was using RG-174 as a shielded lead in a product we build and
experienced exactly the same problem as Larry describes. This was
from a brand-new, very expensive roll of Belden RG-174. This was to
the grid of a tube, so even a very high resistance had an effect. We
have changed to a Teflon-insulated coax and that solved the problem.
Doug K3KW
>Date: Fri, 4 Sep 2009 06:55:15 -0400
>From: "Larry - K7SV" <k7sv@comcast.net>
>Subject: [TowerTalk] NON-INSULLATION STRIKES AGAIN! RG174 ANOMALLY
>To: <towertalk@contesting.com>
>
>Some of you may remember my posting about our experience with beverages
>using plastic boxes to house matching transformers. The black plastic boxes
>turned out to be conductive which obviously led to a lot of head scratching.
>
>Well, I've been working a small project using RG-174. I kept finding shorts
>after putting connectors on the ends. Nothing was making sense so it got to
>the point that I cut a foot long piece of the stuff, stripped back about an
>inch of the outer insulated jacket at both ends and pushed the shield away
>from the ends. I then took resistance measurements between the shield and
>the ends (didn't strip back the inner insulation, just stuck the meter probe
>in the end). So I'm still seeing resistance between the shield and the
>center conductor.
>
>On a whim I connected a meter probe to the inner "insulation" at both ends
>of the piece. Sure enough it was a conductive material. I picked up a couple
>hundred feet of this stuff from Mendelson's at Dayton a few years ago. As it
>turns out construction consists of an insulated black outer jacket, stranded
>copper shield, a black conductive material, a light opaque insulation and
>then the stranded copper inner conductor.
>
>My problem obviously existed anywhere that the inner black "insulated"
>material was touching the shield.
>
>This is the first time I've experienced something like this with RG-174.
>Actually as I think about it, the stuff must have really great isolation
>between the center conductor and the braid! I don't think this is typical
>RG-174!
>
>73 de Lar K7SV
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