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

[ZR]

>> 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

CATV installers use almost 100% quad shield in order to keep the signals 
inside and not cause interfering leakage (egress); FCC specs are adamant 
about that. These specs go back to the 70's. In more recent years the cable 
also must keep local RFI (ingress) out.

For topband Id suspect that the foil  thickness is so thin that some skin 
depth currents are on the outside. The foil is BONDED to the foam dielectric 
and is not supposed to be folded back; I dont know where that misinformation 
originated.

Id suggest actually reading the connector manufacturers and major 
installation professionals cable prep instructions and all the ones Ive seen 
specifically states to NOT fold back that foil.

For QS the outer braid is folded back, the next layer is the outer foil and 
that is removed, and then the inner braid is folded back. The inner foil 
goes thru the connector tube and both braids go between the outer and inner 
tubes.

Carl
KM1H