[RFI] ISOBAR

Jim Brown jim at audiosystemsgroup.com
Sat Sep 15 21:46:51 EDT 2012


On 9/15/2012 5:26 PM, Scott Yost wrote:
> Is Isobar still considered a decent product for RFI reduction or has it been eclipsed by some other manufacturer?  The Isobar website seems thin on specs for noise suppresion data.  Interested in specs especially at LF.

I can't comment about RF filtering, but it's an MOV box, which is BAD 
NEWS on a branch circuit. Also, none of the commercial line filters do 
anything to kill common mode current on the AC cable (which is on all 
three conductors, INCLUDING THE GREEN WIRE), and the green wire must, by 
law, go through the box without putting anything in line with it.  
That's bad news, because much (most?) RF trash on house wiring is COMMON 
MODE.

The most effective common mode filter I know of is to simply wind lots 
of turns around a #31 core. I can get at least 14 turns of most common 
IEC power cables through a 1.4-in i.d. #31 core without removing a plug. 
14 turns is quite effective down to at least 80M, and is still good on 
160M.  More turns is better if you need to go lower,  or you could wind 
turns through two cores in a stack. Two or more cores in a stack also 
helps if you're using larger diameter cable and can't fit as many 
turns.  As a rule of thumb, 10 turns with two cores is roughly 
equivalent to 12 turns through one core. That's because inductance is 
roughly proportional to the length of the path through the core 
material, but proportional to the SQUARE of the number of turns, and 
capacitance is proportional to turns.

73, Jim K9YC.


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