[RFI] EMI/RFI AC line filter design info?

Dale svetanoff at earthlink.net
Thu Sep 26 00:32:17 EDT 2013

Christopher and Ray,

Keep something in mind as to what Jim Brown, K9YC said: these conventional 2-line filters will NOT help if the 3rd wire ground is also carrying common mode current.  That is because the "normal" filters pass the 3rd wire straight thru, no filtering.

Before spending any time or money on buying or making filters, make some serious effort to see if the ground wire of the device is, in fact, radiating signal.  You can do that by "sniffing" methods, wrapping the 2 main conductors around a core, then trying all 3 around the core, etc.  If you find that the 3rd wire is "hot" with common mode, then you need a 3-line filter, and you need either inside the offending device or external to it, but placed very close so as to minimize the exposed unfiltered wire length.  So, where to get a 3-line filter?  They are made for 3-phase power applications.  They will not be anywhere near as plentiful, or cheap, as conventional 2-wire filters.  However, if your problem can not be solved by use of the appropriate toroids, beads, or rods as discussed in Jim's excellent tutorials, then a 3-line filter might be the most effective solution to the problem.

One more comment: Jim talked about filter details that you don't see in schematics or books.  I'll add one more: capacitor construction and type.  You should note that the standard filters form Corcom, and others, are usually built with discrete wire leaded by-pass caps.  (You can see them if you open up the filter.)  These are fine for meeting the FCC Part 15 conducted emission requirements, and similar applications.  The problem with getting better performance from these filtered is that the self-inductance of the cap leads limits their upper frequency limit due to Xl, which increases with frequency.  In tough cases, higher performance filters maybe needed, and the "secret" to those is not different parts values, but better parts.  The higher performing filters, frequently referred to as "facility" filters, is the by-pass caps have NO leads at all - they use coaxial (or "tubular") caps that are mounted into bulkhead dividers within the filter can.  These caps (examples of which can be seen in many pieces of old military equipment from WWII days) have essentially no lead inductance, and can thus exhibit very low effective by-pass impedances clear into the UHF and microwave frequency ranges.

I totally agree with the comments made previously about not trying to homebrew line filters to save a few bucks.  Safety is the main reason, but also performance.  Even if you get the inductor and cap value right, you must also use proper construction techniques.  Do NOT use aluminum mini-boxes for housings; rather, make housings out of either brass or tin plate so that the housings can be soldered together.  A filter made with leaky seams is not a filter - it's just another antenna that radiates.

73, Dale

-----Original Message-----
>From: Christopher Brown <cbrown at woods.net>
>Sent: Sep 25, 2013 1:33 AM
>To: rfi at contesting.com
>Subject: Re: [RFI] EMI/RFI AC line filter design info?
>On 9/24/13 6:21 AM, Ray, W4BYG wrote:
>> Thanks to all who replied to my question.  I got some good responses.  I
>> appreciate those of you who took the time to respond.
>> The Corcom info was interesting but at $55 apiece totaled more than I want
>> to spend.  I need a couple.   Thus, I prefer save the money and build/test
>> them myself.  
>I tend to troll fleabay for Corcom filter, and lookup the P/N for
>anything looking interesting 99.9% junk or useless, but there are _alot_
>of them out there.
>Have a stack of 30VK6 filters on the desk I pick up for $2 each, and
>several boxes of the better 1, 3 and 6 amp filters all picked up for 1 -
>3 $ a pop.  Just takes a bit of shopping.
>RFI mailing list
>RFI at contesting.com

More information about the RFI mailing list