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