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The Cavity Duplexer By John Portune W6NBC

Note from the 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 duplexers 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

Chapter Outline (click on a title to go to that chapter):

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. Temperature Drift
   • Commercial method - Invar rod
   • Simple, elegant home-brew method
5. Performance
   • Isolation
   • Insertion loss
   • Measuring hilltop noise
   • The importance of hilltop noise
   • Receiver sensitivity and selectivity, how to measure
   • Transmitter purity, how to measure
   • Pitfalls of preamps and power amplifiers
6. Tuning a Duplexer
   • The simple equipment
   • The basic process
7. Loops
   • Position
   • Placement
   • Materials
   • How critical?
8. Losses
   • Skin effect
   • Cavity size limits
   • Bandwidth vs. insertion loss
9. Lines
   • Lines between cavities
   • Rescaling a commercial duplexer's lines

Contact Information:

The author can be contacted at: jportune [ at ] aol [ dot ] com.

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This article created on Wednesday 09-Jan-2019.

Article text, images and photographs © Copyright 2019 by John Portune W6NBC.
Layout and conversion to HTML © Copyright 2019 by Robert W. Meister WA1MIK.

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