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@contesting.com
Reply To: jim@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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