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?
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.