This chapter presents the basics of impedance matching transformer. The discussion focuses on the basic building blocks and magnetic transformer design.

The transmission line transformers Guanella presented in 1944 were high impedance open wire transmission lines wound on insulating forms. Coiling the transmission line made the transformer physically smaller and reduced common mode current flow. Probably the largest commercial use of his design was the 75:300 Ω balun used in the VHF tuner of TV receivers, especially by RCA, in the 1950s and 1960s. Guanella's transformers are an example of equal delay transformers. The delay through all of the component transmission lines is the same. The transformers operate by adding or subtracting voltages and currents in phase. His investigation was directed toward developing a broadband transformer for matching the balanced output of a 100 W push-pull vacuum tube amplifier to the unbalanced load of a coaxial cable. The objective was to match a balanced impedance of 960 Ω to an unbalanced impedance of 60 Ω (16:1) from 100 to 200 MHz. His experimental data, with a 53 Ω resistor as a load, showed a deviation of less than 10% from the theoretical value over this frequency range. Guanella accomplished this by incorporating four 240 Ω transmission lines in a parallel-series arrangement resulting in a high impedance, 16:1 balun. Sevick (and probably others) had overlooked his technique of summing in-phase voltages at the high impedance side of the transformer, as is evidenced by the scarcity of information on Gunaella's designs in the literature. This chapter presents the analysis of his transformers and explains the advantages of his technique with transformers for high power and high impedance applications.

We saw in Chapters 3 and 4 that the optimum characteristic impedance for a stepup or step-down transformer is the geometric mean between the source impedance and the load impedance. This requires us to use impedances that, in general, are nonstandard or hard to find commercially.

This chapter presents many low-ratio designs that use bifilar and higher order windings. Various kinds of conductors, such as rectangular line and coax cable, are also investigated. The presentation on transformers with more than two windings is unique and should find many uses. Also, a novel concept is presented for rearranging windings for optimally matching 50 ohms to higher or lower impedances.

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 Ruthroffand Guanella-style baluns.

There are two distinct paths of development for equal delay transformers, each of which can be characterized as achieving a more generalized form of transformer derived from Guanella's earlier research. Although others have explored parallel development, this chapter describes the work of W. A. Lewis at Collins Radio, which was further developed by Blocksome at Collins, and independent work by McClure at RCA.

This chapter is concerned with the practical considerations of transformer construction: how to (1) select the proper ferrites; (2) wind rod and toroidal transformers; (3) construct low impedance coax cable; and (4) handle and take care of ferrite transformers. The techniques described in the following sections evolved over many years of winding hundreds of transformers (it took Sevick about three attempts to arrive at a final design). Certainly many other techniques can do the job as well (or even better), but the ones reviewed worked well for Sevick.

Several transmission line transformers are described which have bandwidth ratios as high as 20,000:1 in the frequency range of a few tens of kilocycles to over a thousand megacycles. Experimental data are presented on both transformers and hybrid circuits. Typical applications are: interstage transformers for broad-band amplifiers; baluns for driving balanced antennas and broad-band oscilloscopes; and hybrids for use in pulse reflectometers, balanced modulators, etc.