A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing

This tutorial, authored by Jim Brown K9YC, provides a detailed guide to understanding and solving radio frequency interference (RFI) problems in ham radio setups. It covers fundamental concepts of RFI, including how RF interference is coupled into equipment and detected, particularly focusing on detection at semiconductor junctions and the antenna action of system wiring. The guide explains that virtually all detection causing RFI follows a square law, meaning a 6 dB reduction in RF signal can lead to a 12 dB drop in detected audio. The content is based on the author's engineering education, 60 years in ham radio, work as vice-chair of the AES Standards Committee working group on EMC, and extensive research in pro audio. It is intended for amateur radio operators seeking to mitigate interference issues. The latest revision, dated January 7, 2019, moved much of the material on common mode chokes for transmitting to a separate applications note available at k9yc.com/2018Cookbook.pdf.