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RE: [RFI] shielding answer

To: "RFI List" <rfi@contesting.com>
Subject: RE: [RFI] shielding answer
From: "Jim Brown" <jimbrown.enteract@rcn.com>
Reply-to: Jim Brown <jimbrown.enteract@rcn.com>
Date: Wed, 24 Dec 2003 13:53:50 -0600
List-post: <mailto:rfi@contesting.com>
On Wed, 24 Dec 2003 19:20:31 -0000, David Robbins K1TTT wrote:

>Round 1:  run shielded cables for speakers and add ferrites at receiver
>end, about 12 turns of both wires through a ft-114-77 core. 

Thanks for sharing all of this. 

I don't remember if you mentioned what the original loudspeaker cable
was, but unshielded twisted pair is known to be far superior to zip
cord in rejecting RF. I've never used shielded cable for loudspeakers,
but I've seen a lot of RF coupled into audio gear through the
loudspeaker terminals.  

Was your shielded speaker cable also twisted pair, or was it parallel
conductors?  It is well known that cable shields provide relatively
little magnetic shielding (Ott says almost none except in narrow
frequency ranges), but does provide shielding to the electric field.
Most LF interference is magnetically coupled, while the E-field becomes
more important as frequency increases. On the other hand, twisting
provides considerable rejection of magnetic fields. 

Another question. Lots of electronic gear has a pin 1 problem. Any jack
that isn't bonded to the chassis is probably a pin 1 problem. No, I
don't mean continuity to the chassis through the pc board, I mean the
shell must be in physical contact with the chassis. Pin 1 problems are
a VERY common coupling mechanism for RFI, and it gets worse at higher
frequencies. Did your RCA connectors appear to be bonded to the
chassis, or were they insulated from it? 

Jim K9YC

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