A Digest by Dr. Jerry Sevick, W2FMI

Designs with Ratios Other Than 1:1 and 4:1

Although the transmission line transformer has been in existence for over five decades, it's only in the last few years where the importance of Guanella's designs have been recognized and where designs with rations other than n2:1 (where n is 1, 2, 3, etc.) have become available. Transformers with rations of 1.5:1, 2:1 and 6:1 (or ratios very close to these values) are now readily available 1,2.

One technique for obtaining these new ratios incorporates windings greater than two in the bootstrap connection. Figure 6 shows a trifilar winding on a toroid that yields a natural impedance ration of 2.25:1 or a tapped ration very close to 2:1. Furthermore, by placing winding 5-6 between the other two windings, the effective characteristic impedance is lowered and the transformer works better in stepping down from 50 ohms. Figure 7 shows the tapped trifilar step-up design (like Figure 6) at three different impedance levels. note the extremely high efficiency and that the highest response occurs at about the 100:50 ohm level. By using a quadrifilar winding, a broadband natural ration of 1.78:1 results. Even more interesting, a quintufilar windings yields a very broadband ration of 1.56:1. This ration obviously satisfies the 1.5:1 requirement in most cases. It should be noted that these higher-order windings allow shorter lengths of transmission lines and thus have a smaller ratio of delayed voltages to direct voltages. Since more transmission lines are used, the low frequency responses can still equal that of Ruthroff's popular 4:1 unun with longer transmission lines.

With these very efficient and broadband ununs with rations very close to 1.5:1 and 2:1, it is now possible to put them in series with 1:1 and 4:1 baluns and achieve results which were not available before.

A version of Guanella's equal delay is possible for a 2.25:1 and 6.25:1 balun (and unun). The first one uses a 1:1 balun in series-parallel with a 4:1 balun. The other uses a 4:1 balun in series-parallel with another reversed 4:1 balun5.

For higher ratios, Guanella's approach can be used for 9:1 and 16:1 baluns (and ununs). Figure 8 shows the high frequency and low frequency models for his 9:1 transformer. His 16:1 transformer would obviously use four transmission lines. Ruthroff's "bootstrap" connection can also be used to obtain ununs with rations of 9:1 and 16:1. A 9:1 ration can be obtained simply by connecting, in Figure 6, the input to terminal 3 instead of terminal 5. By using a fourth winding above winding 5-6, a 16:1 ration is obtained. In fact, by using multiple inputs and outputs and various taps on the winding, broadband multimatch ununs now become available. Although the Guanella approach yields a much higher frequency capability, it is considerably more complex and not as versatile as Ruthroff's approach.

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