Chapter 7: Unbalanced-to-Unbalanced Transformer Designs with Impedance Ratios Less Than 1:4

Sec 7.1: Introduction

Little information is available on the characterization and practical design of transmission line transformers with impedance ratios of less than 1:4. Investigations reported in the literature have only treated the bifilar winding and its application to obtain impedance ratios of 1:1, 1:4, 1:9 and 1:16. But many applications can be found for efficient, broadband transformers with impedance ratios of 1:1.5, 1:2 and 1:3. Some examples include the matching of 50- ? coaxial cable to: (A) vertical antennas, inverted Ls and slopers (all over good ground systems), (B) a junction point of two 50- ? coaxial cables, (C) 75- ? coaxial cable and (D) shunt-fed towers performing as vertical antennas.

Early work by the author has shown that the tapped-Ruthroff 1:4 transformer can yield low-impedance designs under certain conditions. Their designs and limitations are included in this chapter. More recent work by the author has shown that, by far, a better technique is to use higher-order windings with the Ruthroff "bootstrap" method. This technique leads to further interesting applications such as broadband, multi-match transformers and compound arrangements leading to a new class of baluns: baluns which are capable of matching 50- ? cable directly to the low-impedance, balanced inputs of Yagi antennas and to the higher-impedance, balanced inputs of quad antennas.

The operation of these single-core, higher-order (trifilar, quadrifilar, and so on) winding transformers can be explained, briefly, by the 1:2.25 unun shown in Fig 7-1. The high-frequency schematic of Fig 7-1A assumes that...