Book Excerpt: Sevick’s Transmission Line Transformers, Baluns
This chapter excerpted from Sevick’s Transmission Line Transformers describes the balun as a subset of transmission line transformers with an in-depth treatment of most used types. Schematics and design /build information details are shown and are well described. There will be three parts to this excerpt due to its length. Here is part one. Parts two and three will follow as well as a final article on applications of the balun that I will publish after the three excerpts are published.
Published by Scitech Publishing, an imprint of the IET.
Copyright © 2001, 2014 by Scitech Publishing, Edison, NJ. All rights reserved.
Fifth edition 2014
The book, authored by Raymond A. Mack and Jerry Sevick, has already been reviewed on EDN.
This chapter covers baluns, the subset of transmission line transformers of most interest to antenna builders since most antenna structures have symmetrical feed points. We will look at both Ruthroff-and Guanella-style baluns.
9.2 The 1:1 Balun
The 1:1 balun is well known to radio amateurs and antenna professionals since it is widely used to match coax cables to dipole antennas and to Yagi beams that incorporate matching networks which raise the input impedance to that of the cable. The purpose of the balun is to minimize RF currents on the outer shield of the coax cable which would otherwise distort radiation patterns (particularly the front-to-back ratio of Yagi beams) and also cause problems because of RF penetration into the operator location. The balun accomplishes this by suppressing any induced antenna current on the outer coax conductor due to antenna asymmetry. In other words, a 1:1 balun is intended to operate as a common mode choke whose job is to suppress common mode energy (energy flowing on the outside of the coax shield) and allow differential mode energy to flow unimpeded. Many successful forms of the 1:1 balun have been used. They include: (a) the bazooka which uses ¼ λ decoupling stubs, (b) 10 turns of the coax line with a diameter of 6 to 8 in, (c) ferrite beads over the coax line, and (d) ferrite core or air core Ruthroff designs.
Figure 9-1 presents what is probably the most popular form of the 1:1 balun, the Ruthroff design, and illustrates the toroidal and rod versions. The third winding of the toroidal transformer, shown as an inductor rather than a loaded transmission line, in Sevick’s experience was usually wound on its own part of the toroid. This has the effect of turning the balun from a transmission line transformer to a hybrid using both transmission line and magnetic transformer qualities. A true transmission line transformer would be obtained by using a trifilar winding on the toroid just as we do on the rod. The low frequency model for the 1:1 baluns in Figure 9-1 is shown in Figure 9-2.
Figure 9-1 Schematics show Ruthroff versions of baluns: (a) Toroid version of a balun, where an isolated third winding is used. (b) Rod version using a trifilar winding, where all three wires are closely coupled.
Figure 9-2 Schematic shows the low frequency model of the Ruthroff 1:1 balun.
Ruthroff originally considered the third wire, winding 5–6, as necessary to complete the path for the magnetizing current. In discussions between Sevick and his colleagues, including Ruthroff, they agreed that the third wire is not necessary in the performance of the Ruthroff 1:1 balun in antenna applications. When the reactance of the windings is much greater than RL (at the lowest frequency of interest), then only transmission line currents flow and there is no magnetizing current.
Both Sevick and Ruthroff place the third wire (winding 5–6) at a +Vin/2 potential. This is correct only for the case where all three windings are closely coupled, as in the case of the rod balun or when a toroid is wound with all three windings closely coupled. When all three wires are closely coupled, the outer wires carry only one-half of the current and the center wire carries the full current of the load.
As Sevick observed, if the energy flow is purely by transmission line current, no net field is transferred into the core. This means there is no voltage across the isolated third winding of the toroid design. If there is any imbalance in transmission line mode, a net field will occur in the core and the 5–6 winding will generate a voltage. The voltage at terminals 4 and 5 is a function of the level of imbalance with the limit reached when the voltage is +Vin/2. Notice that the connection is opposite of what we would use for a 1:9 transformer. The voltage at terminal 5 is in phase with the voltage (Vin) and tends to hold terminals 4 and 5 with no potential to ground at balance. Even though the potential at terminal 5 is the same as ground, the load is totally isolated from the ground at the input of the transformer. Any current that flows in or out of terminal 5 will force the voltage to be different from ground.