The 4:1 Transformer

There are two basic forms of the 4:1 transformer. One is the balun which assumes that the load is grounded (actually or virtually) at its center and the unbalanced-to-unbalanced transformer (unun) which has an input and an output terminal grounded. Furthermore, there are two versions of each: Guanella's and Ruthroff's.

Guanella's 4:1 transformer is shown in Figure 3. It consists of two transmission lines connected in parallel on the low impedance side on the left and in series on the high impedance side on the right. Since each transmission line sees one-half of the load, the maximum high frequency response occurs when the characteristic impedance of the transmission lines is one-half the load, R_L. No standing waves will then change the impedance ratio. When the load is grounded (actually or virtually) at its center, the transformer becomes a balun. When grounded at terminal 2, it becomes an unun. It can even become a 4:1 phase-inverter by grounding terminal 8. In all three cases two cores are used. If the load is floating, a single core can be used successfully.

The low frequency response is treated the same way as for the conventional transformer. As shown in Figure 2-3 of my first book¹, there is no transmission line mode and the transformer (as all transmission line transformers) takes on the characteristics of an auto transformer. One then only has to determine the magnitude of the magnetizing inductance, L_M. This is explained more completely in Chapter 2 of my first book¹.

This transformer, as with most transformers, works equally well in either direction. Furthermore, the Guanella approach adds voltages of equal phases and therefore the high frequency response is determined by the parasitics in the connections and the standing waves (which change the impedance rations) due to characteristic impedances being different than the optimum value. When two cores are used, the top core in Figure 3 has no voltage drop along the transmission line when used as a balun (and hence no inductance) and serves only to hold the transmission line. When used as a unun, the bottom core serves the same mechanical role.

Figure 4 shows Ruthroff's 4:1 unun and balun designs. The top schematic shows his ununs design which uses the bootstrap connection. Thus the load sees an immediate voltage V₁ and a delayed voltage V₂. The optimum characteristic impedance is also ½ R_L. Since a delayed voltage is involved, Ruthroff's 4:1 unun (and balun) has a built-in cutoff mechanism. His 4:1 transformers depend upon transmission lines which are very short compared to a wavelength. Figure 5 shows the high frequency response of his 4:1 unun (and balun) as a function of frequency and different values of the characteristic impedance.

Figure 4B shows Ruthroff's schematic for this 4:1 balun. As noted in the basic building block, this connection is called a phase reversal. Thus the left side of the load sees V₁ immediately and the right side V₂ which is delayed. Thus his balun has the same frequency response as his unun. But his balun cannot operate in the reverse directions as Guanella. Ruthroff's transformers do have the advantage of only requiring one core. Until very recently, Ruthroff's baluns and ununs have been the industry's standards.