[RFI] ebay line filters

[Doug Powell]

Good points,

It is indeed true that common mode noise is with reference to the green wire ground, but back at the measurement point and not the case of the filter. In the case if Mil Std filters this may be on a grounded sheet metal table.  In the case of European approvals this measurement point is on the floor of the semi-anechoic chamber ‎and the test sample is 80 cm above on an insulated table. Establishing an RF ground up there is extremely difficult and gets worse as you go up in frequency. So, if you buy a filter approved to IEC, CISPR, or EN standards you are likely to get better results than one that is compliant with Mil Std.

All the best, Doug


  Original Message  
From: Jim Brown
Sent: Monday, August 15, 2016 4:24 PM
To: rfi at contesting.com
Reply To: jim at audiosystemsgroup.com
Subject: Re: [RFI] ebay line filters

On Mon,8/15/2016 6:49 AM, Doug Powell wrote:
> One thing you should realize about line filters is they are optimized for conducted line emissions coming from the product where they are built in. This means the line / load labeling ‎will determine your best orientation for connection.

There's another MAJOR problem with commercial line filters -- most of 
the trash conducted from equipment is on the GREEN WIRE, which goes 
right past the filter. The specs for these filters CLAIM common mode 
suppression, but what they're calling common mode is voltage between 
neutral and ground. That's NOT what WE call common mode, and what WE 
call common mode is what causes most RFI -- radiation from the power 
conductors as a long wire antenna.

The most effective line filter for MF and HF radio is a common mode 
choke formed by winding multiple turns around a #31 Fair-Rite ferrite 
core, using the guidelines in k9yc.com/RFI-Ham.pdf for coax the same 
size as the power cable. See Chapter Seven the "Choke Cookbook." Also 
see http://nccc.cc/pdf/CQP-RFI2013-2.pdf

73, Jim K9YC

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