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Re: [RFI] RFI Solved After 2 Years

To: rfi@contesting.com
Subject: Re: [RFI] RFI Solved After 2 Years
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Fri, 10 Jul 2020 11:11:53 -0700
List-post: <mailto:rfi@contesting.com>
On 7/10/2020 10:56 AM, Michael Aust via RFI wrote:
CorCom filters take care of Sym (In-Phase) & ASym (Out of Phase) RF EMI Issues which 
can be betterthan ferrite type 31 cores on AC lines try to justattenuate Common Mode 
suppression !MikeWB6DJI RF EMI Engineer

In my experience, common mode is dominant from an RFI perspective. When a traditional AC line filter like Corcom products fixes an RFI problem, it is when the filter is installed in such a manner that it causes the Green wire to be properly terminated at the noise source, wheras without the filter it would not be, due to th design of the fixture.

The principle here is that common mode is what radiates. A definition is appropriate. What EMC regulations define as common mode is voltage between neutral and the Green wire. What RADIATES is the common mode current on the three wires, and most of this is on the Green Wire due to it's improper (or non-) termination within equipment. The Green Wire passes through traditional line filters, so they do nothing to suppress this current.

Another fundamental principle is that twisted pair (or triplet) inherently suppresses differential mode radiation, and, in my experience, also helps with common mode. When I lived in Chicago, I solved a lot of RFI from VHF TV transmitters to home stereo systems by replacing the zip cord feeding the loudspeakers with twisted pair.

73, Jim K9YC
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