Design of microwave baluns with flexible structures

In this letter, two types (Type-I and Type-II) of microwave baluns with generalized structures are presented. Different from the conventional transmission-line-based baluns where λ/2 transmission lines or λ/4 microstrip lines are used, the proposed baluns are constructed by transmission lines with arbitrary electrical lengths. Both of the two types of baluns are designed based on a symmetric four-port structure with one of the ports open-ended. Even-odd mode method is applied to analyze the performance of these baluns. Specifically, for the Type-I design, a symmetrical impedance transformer is employed to realize the desired flexible balun function. For the Type-II design, an asymmetric impedance transforming network is applied, and a compact size is achieved. Analytical design equations have been derived for both designs. Based on these equations, two different baluns (Type-I and Type-II) have been designed, fabricated and measured. The measurement results match well with the theoretical analysis, which practically proves the design theory.