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Re: [RFI] shielding question

To: "Jim Smith" <jimsmith@shaw.ca>, "RFI List" <rfi@contesting.com>,"Tom Rauch" <w8ji@contesting.com>
Subject: Re: [RFI] shielding question
From: "Cortland Richmond" <ka5s@earthlink.net>
Reply-to: ka5s@earthlink.net
Date: Tue, 23 Dec 2003 8:48:37 -0800
List-post: <mailto:rfi@contesting.com>
I think a lot depends how the drain wire is treated and terminated.

If it's bare wire, and in contact with the shield - usually the case - then
if it is to conduct external signals into the cable, the split between wire
and shield might indeed be pretty well described by the ratio of the
resistance of the two.  Since it's possible to make a metallized coating
very thin indeed, at frequencies where the coating thickness is on the
order of a skin depth, this is unsurprising.  

If it's NOT in contact with the shield - if the foil is only on the outside
of the Mylar, say -- then it will act as if it were simply another wire
passing through the shield, with no return path for its current. And if you
use it to terminate the shield in THAT case -- it won't.  However, this
could be viewed as merely a limiting case of the example above, where the
resistance between the shield and drain wire is very high.  

Telco folks view wires as if a few feet of wire were really ground; drain
wire grounding is endemic and darned hard to change. I have seen tons of
money spent on ferrite beads because a customer was unwilling to adopt
decent EMI shielding practices.

--- Cortland Richmond
--- ka5s@earthlink.net
--- EarthLink: The #1 provider of the Real Internet.

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