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
>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????
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