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The Mysterious Duplexer
By John Portune W6NBC
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Cavity Duplexers Chapter 1 The Mysterious Duplexer By John Portune W6NBC |
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The cavity duplexer is a very familiar part of both amateur and commercial two-way radio. On a typical two-way hilltop, cavity duplexers are as common as the forest of antennas that bristle from the towers. Most repeaters have one. They're well known to repeater owners and builders. Or are they? Don't be so sure.
If you are like many hams and two-way radio professionals, you likely have more questions about duplexers then answers. More than the other parts of a modern repeater, an aura of "black magic" and many "old-wives tales" shroud the cavity duplexer. It might be better to call it the familiar "black box" that everybody uses, but nobody really understands.
What are the keys concepts? Why are there different types? What is the right way to tune one? How can you minimize duplexer losses? And for the aspiring ham repeater builder, "How do I modify a commercial duplexer for an amateur band, and can I possibly build one for myself using hardware-store materials and ordinary home workshop techniques?"
These same questions were running through my head when I first decided to enter the world of repeater ownership. By then I'd already successfully installed and maintained several duplexers. I'd even retuned commercial units to the ham bands. But did I really understand them? Did this kind of ham experience really qualify me as a knowledgeable duplexer user? Definitely not, and frankly I knew it didn't.
Like so many hams, especially those who want to put up their first repeater, my experience had only given me a piece of the picture. It was also the dangerous part - bits and pieces, acquired from other ill-informed hams and several old wives tales. Cavity duplexers were truthfully then still a very big mystery to me, as they currently still are for many.
Purpose of This Book:
My objective is to cut through the common "clouds of fog" that surround the cavity duplexer, and to do so as simply and as non-technically as possible. Duplexers aren't "black magic" You really need only a handful of basic principles to build, modify and tune one from a base of sound knowledge. So I have no intention to write a definitive treatise on duplexers here. We won't be able to avoid some simple mathematics. I will leave out any that is of little value in the practical world, however. Basic principles are the objective. That's what most repeater owner/builders need and normally lack.
Also this book is not a cookbook. There are no cut-and-paste duplexer building plans here. It is your responsibility to translate these concepts into actual constructional designs. But don't be discouraged, there's precisely what you need here to "roll your own," to modify an existing commercial duplexer for an adjacent ham band, or to knowledgably purchase a new unit. For perhaps more than for any other aspect of the modern repeater, working with a duplexer from sound basic principles and not blueprints is very important.
We will, though, build a 2M cavity. The idea is to illustrate the basic principles of cavities in a practical way and to show that they can be built in a home workshop. We'll also look at a few examples from the 440 MHz band. 70 cm was my specific interest when I began this book.
Lastly, there are many related topics of major importance to repeater builder and owners other than duplexers - feed lines, antennas, isolators, to name just a few. Some of these may get mentioned, but my objective is to limit this discussion, as much as possible, to the common type of duplexer used in most modern repeaters. The early chapters apply to all readers, the latter to home builders. All are useful in concept, however.
Origin of The Book:
In my original quest to understand duplexers I begin my search for an understanding of the basic principles of duplexers in the highly technical engineering textbooks. My own engineering background suggested that for me this was a reasonable place to start. And this did yield a degree of useful principle. I also did of course review the available popular amateur sources. There have been a number of moderately informative duplexer/cavity articles published in the ham literature over the years. But to my general disappointment, from both sources, I discovered quite quickly that the duplexer in the modern repeater really remains somewhat of a "mystery" topic.
So I turned to a practical approach to close my understanding gap. I would jump straight in and try to actually build a practical working duplexer for myself. Maybe that was foolish. I'd probably make all sorts of mistakes, but I also sensed that I'd probably learn a lot too. I've always been one who believes that trying to "roll your own" is a good way to acquire practical understanding.
Immediately though, as you might guess, my ham friends loudly proclaimed that I couldn't do it. "Only professionals can build duplexers, not amateurs. Duplexers are beyond the weekend radio warrior. You don't have the proper tools and test equipment." And yes, I will admit that at times it did seem that I'd bitten off more than I could chew. Consequently, by the time I completed my first successful duplexer, I'd gathered an impressive collection hardware-store leftovers.
Today, though, I can say that the struggle was worth the effort and I did save "some" money. Though with the cost of metals these days, the savings in a home-brew duplexer is less than one would like. The valuable part is what I learned. That should be your primary objective in reading this book as well, not saving money or finding cookbook-style plans for a duplexer. Learn the concepts first. For despite how much mystery there seems to be surrounding duplexers in the ham and commercial two-way radio world, they are not as difficult as they seem.
Using the Ham Bands:
For my first duplexer in the 1990's, my interest was the 420-450 MHz amateur UHF band, 70cm. At that time, common hardware store copper water pipe and fittings were reasonably priced and were very suitable for building UHF cavities. Today, aluminum is generally my preference, as we will see in the next chapter. That why this book shows an aluminum 2M cavity built using home-workshop tools and techniques. Again, the object is not to save money, but to give an example fundamental duplexer principles and construction concepts.
If your interest lies on another band - no problem. Simply re-scale this 2M cavity in direct proportion to wavelength. An important word of caution here: Do not attempt to simply re-scale the coaxial lines between the cavities. It is much more complex than that. I will, though, show you how to easily deal with the inter-cavity lines in a later chapter. It isn't difficult either.
The home-brew 2M cavity in this book demonstrates one of the easiest way to construct very-workable cavities in the home workshop. They are large in size, but perform very well. I have actually used the technique for successful duplexers on 2M, 70cm and 6M. I will even suggest a possible way to use the design on 10M.
Vital Opening Concepts:
We really do need to begin with two very basic principles. Don't mistake these for "fluff," however. In my view they are vital to one's broad working knowledge of duplexers. They have to do with, "Why do we need a duplexer in a repeater in the first place?" Simple, you say? Yes it is, but many mistakes begin here.
At the most basic level, a repeater requires a duplexer for two reasons: (1) to allow a very sensitive receiver, and a transmitter making power, to operate at the very same time on the very same antenna. Also it must (2) isolate a repeater from other radios I paid way too much attention to old wives tales in both these areas in my early. So let's very quickly get the essentials here. Every repeater owner or builder must be "easy" with both.
DUPLEXER JOB ONE: The Same Antenna:
Consider, if you will, a typical repeater. How do we state its output? In Watts, of course. Similarly, how do we specify its receiver sensitivity? This time in microvolts. Simple, you say, but don't pass too quickly over the difference.
Let's take a real example by creating a theoretically repeater here in this opening chapter. It will serve as an example throughout the entire book; we'll return to it often. So first let's assume that the receiver can just hear a 0.22 microvolts from a weak distant user station. I chose this specific value, as we'll see in a moment, to simplify the easy math that follows. I think, though, that you will agree that 0.22 microvolts is a reasonable as a minimum receive sensitivity for a modern 2M repeater.
The important issue now is this. As different as Watts and microvolts might seem, they tare the same entity. Merely the size is different. We know this because we can convert one into the other. It's much like saying that a temperature of 23 C is also 68 F. We can quite literally also state the Watts from a repeater's transmitter stated in microvolts, even though we normally don't do so. So indulge me. This conversion will emphasize a very important point about all repeaters.
To convert Watts in a repeater's antenna system to microvolts we use Watt's law:
For our sample repeater, we'll assume 100 Watts. But as you can see, we need a resistance in Ohms to solve the equation. No problem. It is automatically specified by the impedance of the antenna system. The 50 Ohm feed-line impedance gives us this value. Computing the equation we get:
Work through the math yourself if you like, but notice that a 100 Watt transmitter produces a 71 Volts in a 50 Ohm antenna system. That's 71 million microvolts compared to the receiver's sensitivity of 0.22 microvolts Clearly, the transmitter is making a signal very much too large for the receiver to handle. To illustrate this point, which will become very fundamental to an understanding of duplexers, is the reason we made this comparison. Just to drive home the massive difference between the working signals associated with both the transmitter and the receiver on the very same antenna, consider Table 1-1. It lists the entire possible range, in power steps of 10, of the signal levels possible between the two. It should make the equivalence of Watts, microvolts and dBm as used in repeater discussions even clearer.
| Watts | Volts | dBm | |
|---|---|---|---|
| 10-15 Watts | 0.22 microvolts | -120 dBm | Our receiver |
| 10-14 Watts | 0.71 microvolts | -110 dBm | |
| 10-13 Watts | 2.2 microvolts | -100 dBm | |
| 10-12 Watts | 7.1 microvolts | -90 dBm | |
| 10-11 Watts | 22 microvolts | -80 dBm | |
| 10-10 Watts | 71 microvolts | -70 dBm | |
| 10-9 Watts | 220 microvolts | -60 dBm | |
| 10-8 Watts | 710 microvolts | -50 dBm | |
| 10-7 Watts | 2.2 millivolts | -40 dBm | |
| 1 microwatt | 7.1 millivolts | -30 dBm | |
| 10 microwatts | 22 millivolts | -20 dBm | |
| 100 microwatts | 71 millivolts | -10 dBm | |
| 1 milliwatt | 0.22 volts | 0 dBm | dBm reference |
| 10 milliwatts | 0.71 volts | +10 dBm | |
| 100 milliwatts | 2.2 volts | +20 dBm | |
| 1 watt | 7.1 volts | +30 dBm | |
| 10 watts | 22 volts | +20 dBm | |
| 100 watts | 71 volts | +30 dBm | Our transmitter |
Table 1-1: Watts, Volts and dBm in a 50 Ohm antenna system
Remember from electrical theory that Volts are proportional to the square root of the power. In any case, you should now be able to see why I chose 0.22 microvolts above.
Now notice the additional column on the chart, dBm. This is only another way to specify signal strength. All three are exact equivalents. Which term we choose to use in a repeater discussion depends mostly on which aspect of a repeater we are discussing. For receiver sensitivity, microvolts is more convenient for output power, Watts is best. dBm, being a logarithmic scale is good for both. Hams are often intimidated by dBm so may avoid the term. But as Table 1-1 clearly shows, one can state the sensitivity of a receiver, or specify how much power a transmitter is making in dBm just as easily. I'll therefore use all three terms throughout this book. Just keep in mind that all are the same entity, though. Use Table 1-1 for easy conversion.
A Little More on Signal Strength in dBm:
"Plain" dB are not absolute. dB compare only the relative strengths of two signals. dB say nothing about how many Watts or microvolts, for example, a signal actually is. dBm does, however, specify the actual power or voltage of a signal.
To change dB into dBm we must give an actual value to one of the two signals being compared (1 milliwatt is the usual practice for dBm). We must also specify a system impedance, (typically 50 Ohms for radio antenna systems).
Therefore, in a typical repeater system,
You should now grasp the right column of Table 1-1. Our receiver has a sensitivity of -120 dBm and our transmitter an output of +30 dBm. Again this is saying exactly the same thing as 0.22 microvolts and 100 Watts.
Don't, however, let all of this math confuse you. The issue here is not the technical terms or the math. Though as a repeater owner/builder it is usually worthwhile to master the math of dBm, if for no other reason than to not be intimidated when the term comes up in repeater discussions.
What matters is that 100 Watts from our sample transmitter is equivalent to a colossal 71 million microvolt signal (+30 dBm) on the very same antenna as our receiver. Such an immense signal can never be feed directly into the input of our sensitive receiver. So as you have already likely guessed, preventing the majority of our transmitter's +30 dBm signal getting directly to our -120 dBm receiver's input on the same antenna system is the duplexer's number one job.
To make this point more evident, think about your car or home transceiver. Here the transmitter is NEVER connected to the same antenna at the same time as the receiver. That's the reason for the T-R switch in a transceiver. Never do receiver and transmitter operate at the same time. Therefore, the receiver never has to look at the transmitter's signal even though it is connected to the same antenna, at least some of the time.
Again, this isn't true for a repeater. Here it is necessary for the repeater's receiver and its transmitter to operate on the same antenna simultaneously, in real time. Otherwise the repeater could not "repeat" the signal it hears from a user, also in real time. Saying this one more time, but this time in actual numbers, the receiver must efficiently be able to detect a tiny 0.22 microvolt signal on the very same antenna that is simultaneously carrying a bone-crushing 71 million microvolt transmitter signal.
In relative dB this is a 150 dB difference. So to say one more time, solving much of this difference IS the number one responsibility of a duplexer. In other words, the duplexer provides a major part of a real-time isolation of roughly 150 dB between the -120 dBm receiver and the +30 dBm transmitter. This amount of total isolation is very typical of a modern repeater, and the duplexer provides a major portion of it.
DUPLEXER JOB TWO: The Neighbors:
Unfortunately, most mountaintop repeaters live on the "shady" side of town. The RF occupants of a typical repeater site, the other radios in the same building, are very often "bad" characters. The rogue's gallery includes:
Often overlooked by repeater builders, the secondary job of a duplexer is to keep all these potentially bad customers from disturbing your repeater, and like a good neighbor to keep your transmitter from bothering them. This isn't frankly always a cut-and-dried matter. Too often repeater builder/owners pay little attention to this. They thoughtlessly assume that if the duplexer worked well on the test bench it will work the same on the hilltop. In this they forget that the other hilltop transmitters are also making millions of microvolts for your antenna to pick up. The potential intermix problems can be severe and also be quite a surprise at a new repeater installation. This is the main reason low-cost duplexers won't work on all hilltops.
There's also the real possibility that hilltop repeaters, including yours, may be "out of adjustment." These two ever-present factors regularly cause every repeater owner and its users, to be very familiar with "intermod." Keeping "grunge" minimized therefore is the second main responsibility or a duplexer. And it is here that correctly determining how good a duplexer to use is vital.
Therefore, frankly, never can a repeater owner/builder ignore the other radios on a hilltop. As you master the principles of this book, you will gain the tools to correctly deal with these challenges. Many factors make up our arsenal of defense. What's more, you will also learn the practical lesson that sometimes small compromises in transmitter output power and receiver sensitivity are much better than living with grunge or being a bad neighbor on your hilltop.
So these are the two responsibilities of a duplexer:
Neither is a simple, but is a challenge a repeater builder/owner MUST face. The following chapters will give you the keys.
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.
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