TABLE OF CONTENTS Chapter 4 dealt with transformer parameters for working at low-impedance levels; transformers which are basically used to match the ubiquitous 50- ? cable to impedances considerably lower than 50 ?. The need, for the author, first arose when trying to match 50 ? to the resonant impedance of a short vertical of 12.5 ?. This required a coiled transmission line with a characteristic impedance of 25 ?, which was easy to obtain on a rod, but not on a toroid. Various techniques for obtaining low-impedance transmission lines were presented. These included low-impedance coaxial cables, striplines and trifilar combinations.
This chapter deals with parameters for high-impedance applications requiring coiled transmission lines with characteristic impedances of 50 ? and greater. As will be shown, it is much more difficult to design broadband transformers which can match 50- ? cable to impedances of 200 ? and greater. This is especially true of the 1:4 Ruthroff transformers, since impedance ratios are obtained by combining fixed voltages and delayed voltages. Because of the phase differences, his transformers are sensitive to transmission line length. The 1:4 balun of Guanella has the advantage of not only combining voltages of equal delays (thus being somewhat insensitive to the length of the transmission line), but also of combining more than two voltages for even higher impedance ratios. This ability of matching into higher impedances is obtained with the same characteristic impedance as his 1:4 balun.
At higher impedance...
