[TowerTalk] 2.4 inch diameter Type 31 ferrite cores

[Dave Hachadorian]

As previously reported on the reflector, Arrow Electronics is a 
good retail source for these, $5.24 for the 2631803802 in unit 
quantity, but their stock is low right now.

Dave Hachadorian, K6LL
Big Bear Lake, CA



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From: "Tod -MN" <tod at k0to.us>
Sent: Monday, July 26, 2010 6:22 PM
To: <towertalk at contesting.com>
Subject: [TowerTalk] 2.4 inch diameter Type 31 ferrite cores

> 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
> directly.
>
> 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 
> http://www.audiosystemsgroup.com/RFI-Ham.pdf
> . 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
>
> http://k0to.us/HAM/Balun/Experiments%20with%20Type%2031%20core%20materials.p
> df
>
>
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