[RFI] RFI Solved After 2 Years

Jim Brown jim at audiosystemsgroup.com
Fri Jul 10 14:11:53 EDT 2020

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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