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

[Tom W8JI]

> 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