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Re: [RFI] Common-Mode CHoke

To: rfi@contesting.com
Subject: Re: [RFI] Common-Mode CHoke
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Tue, 13 Jun 2017 09:11:50 -0700
List-post: <mailto:rfi@contesting.com>
On Tue,6/13/2017 8:33 AM, Kimberly Elmore wrote:
I have a naive question: I've come into some FT-240 type 31 toroids. I'd like 
to make them into common mode chokes. I've read about the types that use 6-12 
large toriods with 4-5 turns of coax as common mode chokes. Would 10 bifilar 
turns of 16 ga magnet wire (twisted to a couple of turns per inch so as to keep 
the two wires together and simplify he winding) through a single FT-240 work 
about as well? If not, why not?

Depending on the use, just as well or almost as well.  See (study)

k9yc.com/RFI-Ham.pdf  and http://k9yc.com/KillingReceiveNoise.pdf

which shows a bifilar-wound choke using #12 THHN or #12 magnet wire connected as a 2-wire transmission line. #16 is small if you're running high power, otherwise it's fine. Conductor gauge and spacing affects Zo of that 2-wire line. #12 and #14 magnet wire closely spaced results in about 50 ohms, THHN about 85-95 ohms. Jerry Sevick, W2FMI, published the first data I've seen on this, and I've confirmed those numbers in my lab to reasonable accuracy.

Note that the chokes with many cores stacked are for high power. More turns on fewer cores are fine for low(er) power.

73, Jim K9YC
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