In a previous article I wrote about the use of computer models to predict the performance of 1:1 current baluns.1 This article describes the modeling, design, construction, and application of 4:1 and 9:1 Guanella baluns that provide high common model rejection and useful impedance transformations for amateur communication.
The Basic Guanella Balun
The Swiss inventor Gustav Guanella described these devices in a 1944 article. Guanella’s baluns consisted of two or more connected 1:1 current baluns. Wound with a short transmission line (TRL), the ideal 1:1 balun is lossless and it works equally well at all frequencies. In each 1:1 balun, RF signals propagate without loss or delay along the wound TRL. Figure 1 shows that the 4:1 Guanella balun consists of two 1:1 devices whose inputs are connected in parallel and outputs connected in series. This series connection is possible because the outputs are isolated from each other by the choking effect of the windings. A 9:1 Guanella balun consists of three 1:1 current baluns connected this way. Figure 1 also shows the currents and voltages in the 4:1 Guanella balun. When a voltage is applied across the input terminals, half of the input current flows into each 1:1 balun and out through the load . Theloadvoltageis2V. When a signal is applied to the input terminals by a source with an internal impedance of R (typically 50 ohms), the load power and the source power are equal when = 4R. With this load, the input impedance to the balun is equal to , and the input
