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I want to build a common mode choke for my end-fed wire antenna, using turns of coax through an FT140 or FT240 ferrite. I can find various finished products for sale, but I want to make my own. I found plenty of YouTube videos telling me how they built theirs, but I cannot seem to figure out how they knew what to build. How do I determine, given a target band or set of bands, the appropriate ferrite size, ferrite mix, and coax turn count?

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The very best source of information that you'll find about making common-mode chokes is available at Jim Brown, K9YC's site http://k9yc.com/publish.htm. Download his PDF http://k9yc.com/RFI-Ham.pdf.

RFI, Ferrites, and Common Mode Chokes For Hams Most recent update April 2019. This tutorial is directed specifically to RFI in ham radio applications. It includes an extended discussion of the use of common mode chokes in antenna systems and for suppression of RFI.

No matter what band or application, you'll find it all there.

Note, however, that the impedance of an end-fed antenna will change drastically on different bands.

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    $\begingroup$ And note that most of the graphs in that pdf are based on measured data. e.g. if you really want to know, with your set of specific ingredients for your specific situation, build and measure... $\endgroup$ – hotpaw2 Jan 3 at 22:21
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You build a few and measure them with a vector impedance meter.

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