This is an introductory, practical guide to common-mode chokes (often called "ugly baluns") aimed at everyday amateur radio operators.
The explanations are deliberately simplified to help illustrate what happens when RF current flows on the outside of the coax shield, why this can happen, and how a common-mode choke helps reduce it.
For those interested in the detailed engineering, Steve Hunt G3TXQ (SK) carried out extensive measurements on common-mode chokes and his work remains one of the best practical references available. His data was invaluable when I built my own chokes – link here:
http://www.karinya.net/g3txq/chokes/
As with most things in RF engineering, antenna balance, feedline routing, nearby objects and operating frequency all influence the amount of common-mode current present. The values mentioned in this video are simply those I’ve found to work well on my own station.