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Re: [RFI] Common mode chokes

To: "RFI List" <rfi@contesting.com>
Subject: Re: [RFI] Common mode chokes
From: "Jim Brown" <jim@audiosystemsgroup.com>
Date: Thu, 18 Mar 2010 22:14:46 -0700
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>

You're missing the point of how ferrite chokes work. IT'S NOT ABOUT 
INDUCTANCE, IT'S RESISTANCE! An effective ferrite choke is a VERY low Q 
parallel resonant circuit tuned to the frequency you want to suppress.  

Study the material on ferrite chokes in my RFI tutorial. The tutorial 
explains why, in considerable detail, but without a lot of math. I don't 
know anything about the "J" material, so to answer Chuck's question you 
must measure chokes having a varying number of turns on that material and 
plot the results(which is how the curves in my tutorial were generated).  


Also -- permeability is NOT a single-valued number, it is a COMPLEX 
VARIABLE (that is, real and imaginary, both varying with frequency). The 
number you are quoting is the mu for very low frequency (i.e., audio). 

And -- these materials are different chemical compositions developed by 
various manufacturers, and NAMED by them. #31 is a relatively new one 
developed around 2000 by Fair-Rite. It is, by far, the most useful one for 
suppression at LF, MF, and low HF, and is nearly as good as #43 atm mid- to 
high-HF, VHF and UHF. Fair-Rite really hit the jackpot with it. If you 
compare their current catalog with their older ones, you'll notice that #31 
has replaced all of their older materials for suppression at HF and below. 


Jim Brown K9YC

On Thu, 18 Mar 2010 19:43:46 -0600, Larry Benko wrote:

>Material "J" also known as material "75" generally has a permeability of 
>5000 while "31" material has a permeability of 1500.  I would leave well 
>enough alone.

>Larry, W0QE

>Chuck Hutton wrote:
>> Hi - 
>> I've been using up some FT240-J cores to wind common mode chokes for 
Beverage antenna feedlines. I use these antennas only for LW, MW and 160 m 
listening. In looking at the Fair-Rite data for #31 cores, it looks like 
there is not much difference between the two for MW and 160 m usage. 
>> #31 seems to be the favorite these days, but given that my chokes are 
already wound isthere much point to udoing them and using #31????
>> Chuck
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