Overview

The broadband transmission line transformer transmits the energy from input to output by a transmission line mode instead of by flux linkages as in the case of the conventional transformer. When properly designed, they can have much greater bandwidths and efficiencies than their conventional counterparts. There are no sets of rules stating that the lengths of the transmission lines should be a quarter wave, or as some claim, less than an eighth wave long. As will be shown, their lengths are dependent upon a host of factors such as the impedance level of the load, the efficiency requirement, low and high frequency limits, and type of design (explained later). Although they now can provide many new rations and impedance levels ^1,2, the conventional transformer still offers DC isolation and more practical ratios.

The first presentation on the concept of the transmission line transformer was in 1944 by the classic paper of Guanella's ³. He proposed coiling a transmission line such that the the reactance formed is much greater than the input impedance of a matched line, then only transmission line currents are allowed to flow. Ruthroff, who published the only other classic paper⁴ in 1959, called this the Basic Building Block which is shown in Figure 1.

Ruthroff said by grounding terminal 5, the transformer becomes a 1:1 balun with terminal 4 at +V₂/2 and terminal 2 at -V₂/2. By grounding terminal 4, he said terminal 2 becomes -V₂ thus resulting in a phase reversal transformer. I have added two others. By grounding terminal 2, the transformer becomes a delay line. Since there is no voltage drop along the length of the transmission line, inductance and a core for the winding (if used) play no roles. And finally, by connecting terminal 3 to terminal 2, the whole output of the transmission line is lifted by a voltage +V₂. This I call the "Bootstrap" connection. As will be seen, all forms of the basic building block are used in the many designs that now have become available.

Ruthroff took a different approach from Guanella on the 1:1 balun. Instead of grounding (actually or virtually) the center of the load, R_L, he introduced a third winding as shown in Figure 2. The top diagram is the actual circuit he proposed. The two top windings form a voltage divider which immediately puts terminals 4 and 5 at +V₁/2. Then a voltage traveling down the line brings terminal 2 at -V₂/2. His balun added a direct voltage to a delayed voltage. But at higher frequencies, the reactance of the grounded coil on the top of Figure 2A becomes so great that the load impedance takes control and the balun takes on the characteristic of a Guanella 1:1 balun. The bottom circuit in Figure 2 has been used by many until very recently. Since the winding is trifilar in nature winding 5-6 never drops out of the picture at the high frequency end and the balun thus becomes more sensitive to unbalanced, mismatched loads and to the delay of the transmitted energy. Guanella's balun has become known as a "current" balun and Ruthroff's balun (Figure 2B) as a "voltage" balun.