The cavity duplexer is a well-known device in the two-way commercial and amateur radio worlds. On an average radio mountain top, duplexers are as common as the antennas that bristle from the towers. Virtually every repeater has one. They are a known quantity, or are they?

If you are like most in the amateur or profesional two-way radio world, you may have more questions then answers about duplexers. For more than any other part of a modern repeater, an aura of black magic shrouds the cavity duplexer. It's that "black box" that everybody uses, but few truly understand.

How do they really work? What factors determine their performance? What is the best type to use in a given application? How do you properly tune one? Can you easily modify one for another band? Could you possible build one for yourself? Sound familiar?

That's how I felt a few years ago. Like many, I'd had a lot of practical experience with duplexers. I'd purchased, installed and mantained them. I was able get them to perform properly. I had modified several for other bands. Even so, I still couldn't say that I really understood them. Like many of my associates, I only had a part of the picture. The duplexer was still the least understood part of the repeater. How about you?

I also wondered why they cost so much. I wasn't sure if it was always necessary spend a lot of money to get good results. How could I knowledgably compromise? And as a ham radio repeater builder, I wanted to know if it was possible to build a high quality duplexer for myself? These, and many other questions, often cross the minds of amateur and professional duplexer users alike.

Purpose of The Book

My objective in this book is to cut through the mysteries that surround the cavity duplexer. Duplexers, frankly, are not "black magic." There really only are a few key principles. And this what I intend to share with you here.

For late at night, on a cold and windy mountain top, when you are trying to repair a broken repeater, you need the essentials. A lot of complex engineering theory, or worse yet, a bunch of "old wives' tales" learned from your ham buddies simply won't do.

This book, however, is not a superficial "how-to" cookbook, either. A practical working knowledge of duplexers demands that we do talk serious theory and examine some moderately complex principles. And I don't apologize for this. I will, however, promise to stay away from unnecessary mathematics, needless jargon and complex theoretical issues that have no real value in the practical world.

Layout of the Book

Chapters 2-3, the big picture. We'll look at what a duplexer must do, and how well it must do it. This is incredibly valuable stuff. Lacking this, you'll make big mistakes.

Chapters 4-8, the inner workings of a duplexer, both mechanical and electrical. This is where we'll dispel the mysteries. 

Chapter 9, do-it-yourself. You'll learn how to construct an actual duplexer. And the good news is that all you will need is common hand tools and ordinary hardware store parts - no machining.

If you are a commercial reader, don't let the home construction concern you. It really is not the primary focus of this book. It was just my own way of digging out and understanding the important basics, when I began. But if you are a ham builder, you will find this chapter useful. 

Origin of The Book

Before we begin, though, indulge me. Let me tell you a story. It was frankly simple curiosity that started me looking into duplexers. You may be the same. "Forbidden" black boxes have always annoyed me. I hate being told that I shouldn't try to understand something.

I suppose I could have continued using duplexers in the same old hit-and-miss way, but I wanted to understand what makes them tick. For I had a suspicion that a lot of what I had been told about these mysterious devices was actually half true, and that this was getting me into a lot of trouble. Or at least it was only causing me to spend too much money.

I began my original search in the engineering textbooks. My technical education had taught me that this was possibly the best place to start. I am not completely sure now, looking back, that this maxim is ever true. But it was a start. I did, however. also look at the amateur radio sources. To my surprise, I discovered that there really isn't much good clear published information on duplexers in either. This actually is a "mystery" topic, I thought. You may feel the same.

So I decided to try another approach. I'd try to build a duplexer for myself. I'd learn from practical experience. At least that was the thought. I admit that I took this tack mainly because I am the type of compulsively-practical person who believes that you must "roll your own" if you really want to understand something.

Immediately, though, my ham buddies told me that I couldn't. They said that only the commercial technical wizards could build duplexers. Duplexers are beyond amateurs. To be very honest with you, I will admit, at times that it did seem I had bitten off more than I could chew. For by the time I completed my first successful duplexer, my strugglings had left me with a very impressive collection of copper pipes and other hardware store "goodies."

Today, though, I can say that the struggle was very much worth the effort. The savings alone have been significant. I'll let you guess how much my ham radio group has saved on the more that ten home-built duplexers that are currently in service. But the truly valuable part was the knowledge I gained. That's mostly what I will pass along to you here. Cavity duplexers are not as difficult as they seem. The practical designs should also be very useful, too.

Amateur vs. Commercial

On my first project, I carried out all testing on the amateur radio bands, 420-450 MHz in particular. Experimenting on the ham bands is easy. One does not need special licenses or FCC type acceptance. Ham band testing does, however, apply directly the commercial bands. The ham bands HF through UHF are generally immediately adjacent to commercial bands. The amateur 440 MHz band, for example at 420-450 MHz is right next to the commercial 450 MHz band at 450-470 MHz.

If your interest lies in other bands -- no problem. Except for a very few issues, the principles and examples that I will give apply to all bands. Generally, you will only have to change the physical size of the components in direct proportion to the wavelength. A few other characteristics do change differently with frequency or wavelength, but I will cover these as well. 

A word of caution, though. This book applies to frequencies roughly only to 1 GHz and below. The basic concepts mostly still apply at higher frequencies, but other important factors do come into play that I may not mention.

A Comfort Zone

When you have completed this book, you won't know everything there is to know about duplexers, but I promise you that they will never again mystify you. And you will be able to successful build duplexers yourself. The cavity duplexer, you will find is actually an uncomplicated, relatively-forgiving device. Only a handful of basic, easy-to-understand principles determine everything important. HF, VHF and UHF cavity duplexers, for both the commercial and amateur bands, are easy to understand and build once you know the rules.