i THE CAVITY DUPLEXER John E Portune W6NBC
[email protected] rev 2019 NOTE FROM AUTHOR This book was written several years ago and based on hardware-store copper water pipe as the source of home-brew duplexer construction materials. Later I began making from spun-aluminum commercial cake pans. Both require no welding. Unfortunately the price of copper is today much higher. Cake pans, however, are still reasonably priced, readily available and very acceptable as the basis especially for VHF cavities. Because of maximum cavity size limit, copper water pipe may still be indicated for UHF and above. In any case, how a duplexer operates is basic physics. No matter what the material, or whether the duplexer is commercial or home brew, the principles herein are universal to duplexer construction, modification and tuning. This book, however, is not finished. Repeater building is no longer my primary interest in ham radio. Some subjects could be added. But as it contains the essentials, I have placed on the internet incomplete. If you reproduce it, be so kind as to give proper author’s credits. W6NBC January 2019 . ii CHAPTER OUTLINE 1. The Mysterious Duplexer • The black box everybody uses but nobody understands • Keys to understanding it • This is not a cookbook 2. Let’s Make a Cavity • Home-brew 2M aluminum cavity • Example for the entire book • The best way to learn 3. Cavities • Mechanical and electrical properties of cavities • Basic structure of a duplexer • Why use cavities • Getting energy in and out: loops, probes, taps and ports • Three cavity types: Bp, Br, Bp/Br • Creating the other types • Helical resonators for 6M and 10M duplexers 4.