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

Jim Brown jim at audiosystemsgroup.com
Mon Sep 23 11:51:00 EDT 2013

On 9/23/2013 7:39 AM, Ray, W4BYG wrote:
> Looking to minimize conducted and radiated noise from switching supply power
> cords and my double conversion (full time) UPS in the shack.  Snap on
> ferrites aren't helping enough.

In my experience, most electronic noise is conducted as a common mode 
signal, not as a differential signal, so conventional filters won't 
solve it.  A common mode choke formed by WINDING TURNS of the conducting 
cable through a suitable ferrite core is the best solution when this is 
the mechanism. Study http://k9yc.com/RFI-Ham.pdf   then use guidelines 
in the Choke Cookbook for the size of coax that corresponds to the size 
of cable that you are trying to choke. You're right that simply snapping 
ferrite cores onto those cables won't make a dent -- if you study the 
data sheets, you will see that they are VHF suppressors.

It's also worth taking the system approach -- replace as many as 
possible of these noise sources with equivalent product that is not 
noisy.  I recently went through the exercise of replacing all of the 
newer switch mode wall warts that came with newer equipment with old 
linear supplies from my junk box. When I ran out, I went shopping at an 
electronic flea market for more and came home with several dozen of 
various voltage and current ratings. Average cost under a buck.

You might also want to re-think that full time UPS.  For at least ten 
years, all the 12V gear in my shack has been running on one of the 
biggest 12V deep cycle batteries that Costco sells for about $100, 
float-charged by a clean, well regulated switch-mode power supply. The 
PSU I'm using is sold by PowerWerx. I use a laptop for logging, and have 
a good standby UPS on the business computer across the room.

I recently augmented the Powerwerx box with four solar panels that a 
neighbor ham rescued from the dumpster at work. And on the advice of 
other local hams, I upgraded from the Costco battery to a pair of 6V 
golf cart batteries with more than twice the Ah rating. I'm using a 
simple hysteresis regulator between the panels and the batteries, so no 
noise from that either.

73, Jim K9YC

More information about the RFI mailing list