CHAPTER TWO

WHAT A DUPLEXER DOES

You might think that in this book we'd head straight into how to build duplexers, and that we would find ourselves knee-deep in coaxial filters, coupling loops, and transmission lines. We will certainly get to these, but we really need to do something else first. We need to first capture an overall view of what duplexers do and how they fit in the grand scheme of a ham radio or commercial repeater? What is their reason for being?

No matter how familiar you may be with dupexers, I truly recommend that you read this and the next chapter first. Like me, you may have built or maintained many repeaters. But if you really are like me, you have made some serious mistakes by not clearly understanding this material. Remember. And in the following chapter we'll add something else that’s essential right up front. How well must duplexer must perform its jobs? An even greater number of experienced users lack this knowledge. These two chapters are the real keys to achieving to achieving high performance for minimum cost.

The Big Picture

From a broad perspective, a repeater's duplexer has two basic jobs to perform.

1. It must allow your repeater's receiver and transmitter to operate on a single antenna at the same time.

2. It must keep your repeater from interfering with your neighbors.

JOB ONE: A Single Antenna

Consider, if you will, a typical commercial or amateur repeater. In what terms would we state its output? In watts? That's the normal way. Similarly, how would we specify the performance of the receiver? In microvolts? Again, that would be typical. There is a point here, however.

Here’s a real example. Let’s assume that you have a receiver that can hear an input signal of 0.22 microvolts and that you have a 100 watt transmitter. I've chosen these particular values to make what follows easier. But I think you will agree that both are typical of the real world.

Yet as different as watts and microvolts may normall sound to us, they are equivalent terms, especially on the same antenna. Only the size is different. We can easily convert one into the other. When we do, the first basic job of a duplexer will jump out at you. So, follow me in a little simple math.

To convert watts to microvolts on our combined antenna, we can use Watt's law. In our typical repeater, the resistance that we must use is the characteristic impedance of the antenna and feedlines, that is 50 Ohms. And the equation we need is,

watts = volts2/ohms

100 watts = 71volts2 / 50 ohms

Therefore, a 100 watt transmitter delivers 71 volts to a 50 Ohm antenna. Carrying this a little farther now, Figure 1 is a table of this same equation solved for a wide range of powers. I have also included dBm values. dBm is absolute. It is referenced to 1 milliwatt at the impedeance of the system, in this case 50 Ohms. 1 milliwatt is 0 dBm.
 

Watts 
Volts 
dBm
 
.22 microvolts
-120 dBm
 
.71 microvolts 
-110 dBm
 
2.2 microvolts 
-100 dBm
 
7.1 microvolts 
-90 dBm
 
22 microvolts 
-80 dBm
 
71 microvolts 
-70 dBm
 
220 microvolts
-60 dBm
 
710 microvolts 
-50 dBm
 
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 
.22 volts 
0 dBm
10 milliwatts 
.71 volts 
+10 dBm
100 milliwatts 
2.2 volts 
+20 dBm
1 watt 
7.1 volts 
+30 dBm
10 watts 
22 volts 
+40 dBm
100 watts 
71 volts 
+50 dBm

Fig. 1 Watts, Volts and dBm in a 50 Ohm system.

The important point to notice is that 71 volts from the transmitter is 71 million microvolts to the receiver. This is an astronomical signal to the receiver. In the ordinary transceiver that you may have in your car, the transmitter isn't connected to the receiver. Only one or the other works at any one time. The power is switched off to the one that isn't working. Also, the transmit-receive switch disconnects it from the antenna.

In a repeater, however, both receiver and transmitter must work simultaneously. That's how a repeater "repeats" incoming signals in real time. To do this, the receiver must be able to recover a tiny 0.22 microvolt signal and not be bothered by a bone-crushing 71 million microvolt signal on the same antenna. The difference is colossal. This is the number one responsibility of the duplexer. We'll see at how it's done shortly.

JOB TWO: The Neighbors

Most repeaters, unfortunately, have to live out their days in a very bad neighborhood. The RF occupants of a typical repeater site, the other radios in the same building, are often very unsavory characters. The rogue’s gallery often includes,

• Other commercial and amateur repeaters

• Broadcast radio and TV transmitters

• Paging and telemetry transmitters

• Microwave link transmitters

• Data transmitters

The second major responsibility of a duplexer is to keep all these bad customers from interfering with your receiver. Similarly, like a good neighbor, it must keep your transmitter from bothering them.

As we work out way through this book, we will see that these two main jobs of a duplexer are not easy. In all duplexers, we are skating on thin ice. As much as we would like, there is no "fits-all duplexer", that performs both jobs perfectly in all cases To achieve performance with economy, we have to evaluate each site in light of the two fundamental responsibilities of a duplexer. We will return to these two concepts again and again in this book.

Basic Duplexer Configuration

Simply then, how a does a duplexer fulfill its two responsibilities. Figure 2, is the basic design. It is the way that we construct almost all duplexers.

Figure 2 The basic layout of a duplexer

As you can see, a duplexer is a three-port combiner. The receiver and transmitter both connect to the single antenna at a "tee." On both sides are filters. One set isolates the receiver from the transmitter. The other isolates the transmitter from the receiver.

The receive filters reject the transmit frequency, and let the receive frequency pass. The transmit filters do just the opposite. They reject the receive frequency, and let the transmit frequency pass. This is a vital concept. Remember it.

Imagine. if you will, that you are a receiver or a transmitter looking into your side of the duplexer. You would "see" only the antenna. You would not see the other side of the duplexer. Because of the filters, it would look like an open circuit. In essence, we've disconnected the receiver from the transmitter. Each thinks that it stands alone on its own antenna.

Of course, this is the ideal situation. In practice we only have to achieve it partially. The amount is the subject of the next chapter. Before we leave the basic concept of a duplexer, let's briefly talk about two important duplexer terms that we will need throughout the book. They are (1) isolation and (2) insertion loss.

Isolation

As I just mentioned, one side of the duplexer does not have to be totally invisible to the other. The filters only have to weaken the unwanted signal. In practice we only need a few tens of dB. We call this amount, the isolation. One of the two important ways that we rate duplexer filters is in how many dB the filters reduce unwanted signals from other side.

Look again at Figure 1. Notice that the receiver must be able to successfully hear an incoming signal in the presence of a transmitter that is a thousand million, million times stronger. As you can see, 100 watts is 150 dB stronger than 0.22 microvolts.

You might think that the transmit filters would have to isolate the receiver by exactly this amount, or 150 dB. This isn't so. The amount is much less lot less. The receiver has selectivity.

Remember, the transmitter is operating on a different frequency than the receiver. We call this the repeater's frequency split. As we know it is at least several hundred kHz. It varies from band to band, but on the 420-470 MHz spectrum it is 5 MHz.

Because of its selectivity, the receiver does not hear the transmit frequency as well as it does its own frequency. Receiver selectivity actually performs most of the isolation in a repeater. The duplexer only has to provide additional isolation to allow the receiver to live on the same antenna as the transmitter. This is a critical factor in repeater design. Don't forget it.

Similarly, on the opposite side of the duplexer, the transmit filters only have to remove unwanted transmitter energy on the receive frequency. For a good transmitter this will again normally only be a few tens of dBm. The transmit filters only have to reduce these signals below the receiver's ability to hear them. This too is an important factor in repeater design.

We'll put some real numbers to isolation the next chapter. For the moment just realize that each situation is different. There is no economical "fits-all" solution. Too little isolation is obviously bad, too much is a waste of money.

Insertion Loss

The other major main performance figure for duplexer filters is called insertion loss. That's the amount of signal that we have to unwillingly give up in the filters? We will look into the causes in detail in later chapters. They are skin-effect resistance, filter Q, coupling factor and the transmission lines between the cavities.

For the moment we only issue is important. Insertion loss is not always bad. Repeater owners often unwisely attempt to excessively minimize it. Insertion loss is the one performance characteristic of a duplexer that is most open to knowledgeable compromise.

One can often derive big performance dividends by intentionally increasing insertion loss. In several cases I've been able to improve the ability of a repeater to hear weak signals by increasing the insertion loss. We will give major attention to this topic in later chapters.