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[TowerTalk] 2.4 inch diameter Type 31 ferrite cores

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Subject: [TowerTalk] 2.4 inch diameter Type 31 ferrite cores
From: "Tod -MN" <>
Date: Mon, 26 Jul 2010 20:22:16 -0500
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I am organizing a large lot purchase of the Fair-Rite 2.4 inch toroid cores.
If we can accumulate an order for 500+ cores we can capture a price of about
$4 per core FOB the factory in New York. If you or your radio club would be
interested in participating in this mass purchase please contact me

Tod, K0TO


Jim, K9YC, has provided a significant amount of information about the use of
such cores in fabricating RF Cokes to reduce the RF energy flowing on the
outside of coax feed lines. [common mode current]. Type 31 ferrite is
particularly useful for constructing RF Chokes for use in the frequency
range of 1.8 - 10 MHz. They are NOT useful for making RF transformers
because of the high resistive component of the impedance of the winding when
this type of ferrite is used.

You can reference Jim's work at
. Look at pages 35-36 in "Chapter 7 K9YC's Choke Cookbook" to get an idea of
how you would use these toroid cores to fabricate RF Chokes.

Last weekend I fabricated a test jig which was similar to the one Jim
describes on p.31 of his text. Jim was interested in measuring the impedance
of the RF Choke. I felt that I would like to get an estimate of the
attenuation of RF energy that might be passing along the coax. Instead of
measuring the voltage and calculating the impedance as Jim did, I used a
calibrated power meter to measure the change in power level across the input
of the power meter. Using that setup I measured the attenuation provided by
seven turns of coax through six cores as well as the attenuation provided by
fewer turns through the same set of cores.  I was pleasantly surprised to
find that there was 40 dB of attenuation at both 160m and at 80m for the
seven turn unit.

The experimental data and curves can be viewed at


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