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
All good topband ops know fine whiskey is a daylight beverage.
_________________
Topband Reflector
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