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[TowerTalk] Shields

To: "Daron J. Wilson" <>
Subject: [TowerTalk] Shields
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Tue, 12 Oct 2004 21:00:07 -0400
List-post: <>
> I'm sure I'll take some heat on this, but oh well.

Hey, just put the heat shields up. There seem to be enough
to go around.

> 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%
> 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
> flexibility.

I think the problem comes from the fact we confuse light
with 14MHz signals Daron. We can see through the shield, so
we assume RF can leak through. Fortunately our eyes don't
run at radio frequencies, and that large .005 inch gap looks
pretty damn small to a wave that is 2 meters long, let alone
one that is 40 meters long.

> varying frequencies and power levels.  There are reasons
that higher
> power (lower loss) feedlines go to a solid shield.

A large part of the reason, if not the largest, is
resistance in the shield. Woven conductors even with a good
lay and untarnished conductors has a few times the
resistance of a smooth solid conductor.

Get any moisture in the cable and the loss becomes much
worse. As a matter of fact shield corrosion is the bulk of
the reason a cable gets lossy after moisture ingress.
Current tries to stay inside the shield, and every time the
lay wraps outside the current tries to migrate across the
poor connection to the incoming stands.

The braid of RG-8U cable, when pressure is removed between
lays, has about the same resistance per foot  as  #14 or
smaller solid wire.

If you lay smooth foil against the dielectric, corrosion
doesn't matter nearly as much. As matter of fact, it makes
almost no change at HF.

> 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?

Nothing. It probably doesn't catch anything the first shield
doesn't in most cases.

> 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

I use a dual shield too. The inner foil makes the nearly
perfect shield, and the light outer shield allows my
connectors to get a "bite" on the foil. Think about the end
connection problem. It is impossible to guarantee a good
connection to the foil, but the braid is easy and reliable
to sandwich in a crimp connection. The trusty foil that does
all the shielding only lays inside the connector shell. The
metal shell crimps down only on the braid, and the braid
actually couples to the foil.

I think we are mostly on the same page. If you can see
through the shield light can certainly get through. Lower
frequencies still have a pretty tough time.

Save the extra shield money for something useful, like a
gallon of gas.

73 Tom


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