Topband: Problem with compression F connectors on Quad RG-6

[Pete Smith N4ZR]

Back on the subject of RG-6, Ideal specifies folding back the top layer 
of braid, cutting off the first foil layer, then folding back the second 
layer of braid.  The trouble is that on my cable, the second foil layer 
is mylar, aluminized on the outside.  On the one hand this means good 
contact between the aluminum layer and the inner braid, but when you put 
on the connector it folds the inner mylar back, exposing the dielectric 
and putting the blue mylar side up over at least part of the 
circumference.  I can see that in a worst case situation that could 
insulate the inner aluminized mylar from the connector, but don't know 
if it would really matter since the inner braid is folded back and 
making a good compression connection both to the inner "foil" and to the 
connector shell.

What role does skin effect have in this situation - at 1.8 MHz?

73, Pete N4ZR
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On 5/5/2013 11:08 PM, Tom W8JI wrote:
>> How about folding the shields back a little? I think that's a 
>> superior way
>> of doing it as opposed to cutting them all flush with the jacket.
>
> Be careful doing that or using any non-approved assembly method, or 
> using improper connectors. Many cables (I'm not sure exactly what 
> percentage, but I saw a lot of them) aluminize mylar to form the foil 
> shield. You can usually see the mylar on close inspection, it often is 
> blue or a blue tint. This insulates one side of the foil.
>
> The shield that must have the best integrity at connectors is the 
> shield just outside to the center conductor. Nearly all shield current 
> in on the inside of that shield. If you do not get a good solid 
> connection to the INSIDE wall of that shield, the cable will have all 
> sorts of issues. It doesn't matter how solid outside shield 
> connections are, because the innermost surface of the innermost shield 
> does all of the real work.
>
> The inner wall connection can be, and usually is, by conduction across 
> the cut end of the shield. Say the inner shield is mylar on the 
> dielectric side, or bonded to the dielectric. The bare outside 
> contacts the braid with pressure. The current just travels across the 
> cut end edge (a very short path) to the inside of the inner shield.
>
> If you do something to miss that good solid end connection to the 
> inner foil edge, like folding a mylar shield over so blue side is 
> out,  the connection is by stray coupling over what can be a pretty 
> long length of cable, adding many feet to the shield connection 
> path.    Or you might have no connection at all.
>
> I generally avoid quad shield, because the extra layers are 
> unnecessary and can often cause connection problems. This is 
> especially true outside with lightning and age.
>
>> I asked a question here some months ago about whether or not that 
>> should be
>> done or not on my flooded quad-shield F-6 (the CATV alum. shield 
>> version of
>> RG-6) that I use for my Beverages and to feed my inverted-L. Some people
>> said "absolutely not!" and others said "absolutely they should!".
>
> People do all sorts of strange things with shields. With copper braid, 
> aluminum braid, or solid aluminum foil (not aluminized mylar) you can 
> do almost anything at HF and get away with it. I see people fold the 
> shield back over RG-8 and screw the connector over it! Just because it 
> works in some cases, that doesn't mean it is a good idea.
>
> One cable that is really misused is LMR400. If you solder to the braid 
> on LMR400, you set yourself up for shield connection problems. This is 
> because the inner foil, and that is the real shield, often moves away 
> from the braid and makes a sloppy connection. Sometimes wiggling the 
> cable will make the electrical length of the cable change, and shield 
> integrity is all over the place, when the cable is soldered. This 
> generally won't hurt with dipoles, but it can with critical 
> applications. Crimps actually make a better connection.
>
>> Since folding them back is the only way of being sure that the braids 
>> are
>> all making contact with the shell of the F connector, I now fold the
>> shields back a little in my snap-and-seal F connectors. How can that 
>> hurt
>> anything? I think it's a better way to do it.
>
> I would always use the correct connector, and install the connectors 
> the way the connector and cable manufacturer say. They usually know 
> more about their products than we do.  :-)
>
> It is a bad idea to "improve" installation instructions without 
> understanding the product in precise detail. How many people do you 
> think understand the issue caused by overlaying a foil shield with 
> braid, and soldering to the braid?
>
> 73 Tom
> All good topband ops know fine whiskey is a daylight beverage.
> _________________
> Topband Reflector
>