> > An advantage of LMR-400 or any similarly constructed cable
> is that it's
> > double shielded, and literally 100% shielded by the inner
> foil shield.
>
> Why is that important?
I'm sure I'll take some heat on this, but oh well.
First, I would consider LMR-400 to have a dual shield, not to be double
shielded. It has a foil shield which likely provides 100% shield
covered with a braided shield. Personally I believe this provides a
very good shield for a reasonable cost. Double shielded coax such as
RG-214 and RG-142 have two braided shields over the dielectric, my hunch
is that no braid is tight enough to ensure 100% shielded so one over the
other pretty much gives you that while still maintaining the
flexibility.
Second, the importance of the shield has to do with the use of the
product. I hope we can agree that the shield around the dielectric and
center conductor is a necessary component to the coax, it is apparently
there for a reason. I also hope that we can agree that the amount of
shielding can change the characteristics of the coax. To the extreme, a
5% braid around the dielectric is going to produce a different product
than a 95% braid. These products will produce different results at
varying frequencies and power levels. There are reasons that higher
power (lower loss) feedlines go to a solid shield.
I think Tom's point is that you don't need to have dual shield or 100%
shield for many applications, and frankly spending the extra money for
it may be a waste. I've had electrical engineers spec quad shield RG6
coax inside a grounded metallic raceway to 'reduce interference'. I'm
no PE, but what will that 5th shield catch that the first 4 don't?
Third, I kind of like the idea of as much shielding as practical on the
outer jacket. My experience with CATV and other services taught me that
a dual shield was considerably better at keeping signal in the wire than
was a 90% shield, so it makes sense to me for some installations.
73
Daron J. Wilson, RCDD ) )
Telecom Manager ( (
LH Morris Electric, Inc. ) )
(541) 265-8067 office _|****| mmm!
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daron.wilson@lhmorris.com |____|
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