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Cavity Duplexers Chapter 7 Loops By John Portune W6NBC

Before I began my study, the coupling loops in cavities were especially mysterious. I could not find much about how to design or modify them in the published literature. Why were there so many variations? I'd seen fat loops, thin loops, wire loops, strap loops, side loops and top loops. Why had the designers made these choices? Is one better than another for the home builder, I wondered? I couldn't see a pattern. That's why I devoted a great deal of time to coupling loops in my early experiments.

First, is the shape critical, and what about the placement? Do either of these require great precision? How about loop material? How important is that? My intention in answering these questions in this chapter will be to take you through the experiments that gave me the answers. The principles here present a practical picture of how to design your own loops for peak performance.

Loop Shape:

My first question was, is there a magic shape for cavity coupling loops? As I mentioned, I had inspected many duplexers, and the loops came in a baffling variety. I wanted to know what effect loop shape would have on duplexer performance. So I built a cavity, something like the one illustrated in an earlier chapter and I began to experiment.

When I first began experimenting with loop shape, I kept confusing several factors. For examples, two loops of different geometry have a different inductance even if you use the same amount of wire. They also have a different geometric center. Since the magnetic field in the cavity is not uniform, two different loops will couple differently to the field.

Fortunately, though, I found a way around this. Just keep changing two loops until they exhibit the same bandwidth and losses and you eliminate everything but loop shape. So using this technique I then tested circular loops, rectangular loops and loops of irregular shape. What I discovered was that the shape of the loop makes little difference, provided it is made to perform the same as a loop of another shape.

This led me to realize that the only characteristic of a coupling loop that matter very much is the total area of the loop. I later found in an engineering book that coupling is proportional to the square root of that area. Therefore, if two loops have the same area, they will perform much the same place in a cavity even if shape and the amount of wire is quite different. This simple generalization has limits of course, but for practical purposes, loop shape and size is not significant factors in cavity design. Only the area of the loop and how it is oriented in the cavity determines how much it will couple to the magnetic field.

Where Do You Put the Connectors:

Another factor I wanted to know about, was where to put the connectors attached to the loops? I had seen a lot of variations in commercial and amateur-built duplexers. Two locations seemed to be common. The connectors were typically installed either in the shorted end of the cavity or a short distance down the side wall. Is one better than the other, or will it change how well the cavity performs, I wondered?

Once again, the answer to these two question is no. For any given loop, it does not make any significant difference whether its connector is in the side of the cavity or in the end. As long as it ends up in the same place in the cavity, it performs the same.

So why the difference in connector location in cavities? Why should you select one over another? It is my opinion it is only a matter of convenience. If it is handier for the connectors to be in the side, then put them there. Where you put the loops in a cavity also does not turn out to matter very much. We'll see this in a moment. But as far as connector position goes, if a designing a single cavity application I often employ the side position. For a group of cavities, such as for a duplexer, the end is generally easier. Though take your choice.

Loop Construction Materials:

Next I wanted to know if the piece of metal or wire used to form the loop matters. I knew for example that conductors of different dimensions have different characteristic impedances when used in transmission lines. Does this affect the loops in a cavity? For, example, in that the loop in a cavity is fed with a 50 Ohm transmission line, does it also perhaps have to look like a 50 Ohm line section?

Again I began experimenting. I tried round wires of widely differing diameters. As before, I adjusted all factors until the performance of each loop was equal to those of another loop under comparison. Then I tried flat metal straps bent into loops. I did this in that I had seen loops made of flat strap in commercial cavities. But again, after trying all these variations, while always keeping performance equal, I came to the conclusion that loop conductor size, shape or material has little significant affect of loop performance. Ordinary wire is perfectly acceptable. In fact, it is probably the best choice.

There is, however, one factor that does matter in the material used for loop construction - current handling capacity. Notice Table 7-1. It shows RF current in the transmission lines (and loops) of a repeater at different power levels.

1 watt	0.14 Amps
3 watts	0.25 Amps
10 watts	0.44 Amps
30 watts	0.77 Amps
100 watts	1.4 Amps
300 watts	2.5 Amps
1000 watts	4.4 Amps

Table 7-1: Loop current vs. transmitter power at 50 Ohms

These values may not seem high if you think in DC terms, but RF conductors need to be much larger conductors due to skin effect. We will go into the problems caused by skin effect in a later chapter, but as a general rule, above about 100 watts, loops should be made from heavy wire. Below that power level, 16 AWG wire is completely adequate.

Flat strap actually turns out not to be a good choice. It has skin effect problems compared to round wire. But it's easier to bend into loops for cavities used at higher power levels, and if made a bit larger, it works fine. It is a common choice in commercial cavities.

Loop Placement:

In an earlier chapter we saw how a loop must be placed in a cavity in order to couple to the magnetic field. The H or magnetic field, as you will recall, lies concentrically around the center conductor. The loop couples best when it is perpendicular to the H field. This is along the cavity's length and radial along its diameter. We also learned that the magnetic field is strongest near the shorted end of the cavity and close to the center conductor.

If, however the loop is moved within the cavity or it is rotated compared to the magnetic field, coupling will be less. But does that matter? Must loops always be perpendicular to and placed where the magnetic field is maximum in order to work well? I spent a lot of time researching this too, and once again concluded that the answer is no. I experimented with loops at several locations, near the shorted end, away from the shorted end, near the center conductor and away from the center conductor.

I also experimented with rotating a loop so that it was not perpendicular to the field. In fact, making a loop so that it can conveniently be rotated from outside a cavity is a useful feature found in some commercial cavities, especially large bandpass cavities. To accomplish this, the connector and loop are installed on a small circular plate. The plate can be rotated in the field and locked down with a set screw. This adjustment permits the user to determine how much insertion loss and bandwidth the cavity will exhibit. As we saw in an earlier chapter, the decrease in bandwidth caused by looser coupling can be a big asset.

To study the effect of loop location, I would merely keep changing the area of the loop in a different location until it would again couple equally to the field present at that location in the cavity. And when I did I would obtain performance equal to any other location. This includes both insertion loss and bandwidth. So my final conclusion was, that as long as the total amount of coupling between the loops and the cavity is made to be the same by adjusting loop area, shape, material and orientation, the position of the loop within the cavity, loop position does not matter, at least within wide practical limits.

Loop Grounding:

Armed then with knowing how tolerant loops are, it then came to me as no surprise that it also does not matter much how you ground a loop. That's probably why I again saw many variations in commercial and amateur designs. The three common configurations are shown in Figure 7-1.

Figure 7-1: Loop grounding: end, side, to connector

As we have learned, only the area of the loop matters. For example, in the left cavity of Figure 7-1, a section of the loop is actually a part of the cavity's outer wall. The area contained by the wall and the remainder of the loop performs the coupling. My personal favorite for loop grounding is the right configuration. If you ground the loop to the body of the connector that feeds the loop, the loop and the connector can be made easily removable and rotatable. The convenience of this method makes it a very common grounding configuration.

All this leads to a Golden Rule. Nothing about loops is critical. All you need do is to alter the area and the orientation of the loop until it correctly couples to the magnetic field and you will obtain the same results for a wide range of cavity locations. I like a little analogy here. Imagine you are pushing a child on a swing. Where on the ropes should you push? Actually anywhere is fine. If you push at the bottom, you only need to push gently, over a long distance. If you push nearer to the top, you'll have to push harder but over a shorter distance. This is a perfect analogy to the size, placement and orientation of the loops in a cavity.

Feel free to experiment with the loops in the sample cavity, it is excellent instruction. They are only mounted on the ends, on the baking pans, for simplicity and to permit easy rotation, should that be desired. A side-mount position would work well too. It would, however, not likely be as convenient for use in a complete duplexer.

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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