[TowerTalk] NON-INSULLATION STRIKES AGAIN! RG174 ANOMALLY

[Michael Tope]

Larry - K7SV wrote:

>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 
>  
>
Larry,

How thick is the black conductive layer underneath the stranded coppper 
shield? I wonder if that black conductive layer is thin enough to be 
equivalent to the foil shield that you see on double-shielded coax (as 
on RG-6 or LMR-400) ?

Mike, W4EF...........