[RFI] AC Line filters

Jim Brown jim at audiosystemsgroup.com
Thu Apr 3 13:06:54 EDT 2008

On Thu, 03 Apr 2008 16:25:11 +0000, w70m at comcast.net wrote:

> Is there another source for these relative inexpensive AC line filters.

You can, indeed, roll your own, following guidelines in my RFI tutorial. 
You can also buy pretty good line filters from a surplus house called 
Halted Specialties (also known as HSC). On the link below, search on "line 
filters" and then google the part numbers to see what they are. I've never 
dealt with them on the internet, but am in their warehouse-sized store 
about once a month. The filters you see pictures of and data for in the 
tutorial came from them, all for $10 or so. 



It is VERY important that the filter case be bonded with a VERY short lead 
to the chassis of the noise source, and that the line cord between the unit 
and the filter is VERY short. In other words, you probably need to cut the 
line cord and insert the filter. 

In addition to this filter, I would also wrap some turns of the line cord 
after it leaves the filter through a #31 ferrite toroid of the big clamp-on 
shown in the tutorial. How many turns will depend on what bands you operate 
and where the noise is the worst. See the graphs of choke impedance vs 

As noted, both filters will knock down the RF coupled to the AC power line. 
It is NOT correct, however, to only call that RF "conducted" even though 
some test standards may do so -- the power line is acting as an antenna 
that RADIATES that RF as a common mode signal. It isn't the differential 
signal tha bites you, it's the common mode, because the power line is a 
wonderful long wire antenna at ham frequencies. 

These filters wil NOT affect RF radiated from the unit iself due to 
inadequate shielding. Nothing will fix that other than adequate shielding 
and improved circuit design that reduces the size of current loops carrying 
the noise, and lengthening the rise time of the square waves generating the 


Jim Brown K9YC

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