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Re: [RFI] shielding question

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] shielding question
From: "Jim Brown" <jimbrown.enteract@rcn.com>
Reply-to: Jim Brown <jimbrown.enteract@rcn.com>
Date: Mon, 22 Dec 2003 19:15:57 -0600
List-post: <mailto:rfi@contesting.com>
On Mon, 22 Dec 2003 19:51:41 -0500, Tom Rauch wrote:

>I'm fascinated by the RF part of this, since the RF part runs contrary to
>what I have measured.
>No question the drain COULD carry considerable common mode current at low
>frequencies, but it strikes me very odd it could at frequencies where the
>foil is more than several skin depths thick. Even if the drain would
>slightly perturb balance, I don't see how this would apply in a case like
>this where skin depth isolates everything inside the cable from the outside.

If you go high enough in frequency it appears to do exactly what you
think. It's just that it appears to simplify to that at a higher
frequency than you think.  

>Especially to the extent that the shield would be noticeably degraded,
>making the shielding worthless or ill-advised. 

Yes. When I have time, I intend to look at unshielded CAT5/6 using one
pair to carry audio and both conductors of another pair to replace the
shield as a return for phantom power for microphones. I'll look both at
SCIN performance (which I expect to be quite good), and also see how
much RF it picks up as compared to shielded cables. 

Although neither Bill Whitlock (my co author) has modeled the physics
of it, our conclusions are based on looking at the data, and trying to
correlate that to the physical realities. 

>That's a big site. How would one go about finding the paper on that site?
>Maybe there is something I'm missing. 

Go to publications and then Convention Preprints. Then search on my
name in the 114th Amsterdam and 115th NY papers. You can also use the
"AES Preprints Database Search and Shop Utility" at the top of that
page, then search by author name. That will bring up both convention
preprints and JAES publications. 

You can find Neil's paper in the June 1995 JAES, along with some other
very good EMC papers. That issue was quite popular and sold out quite a
few years ago, but it has been reprinted and can be ordered from the
website too. The papers in that issue by Neil and by Bill Whitlock
changed part of the way we think about EMC in pro audio. 

Jim Brown K9YC

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