Ultra-Wideband Single and Dual Baluns Based on Substrate Integrated Coaxial Line Technology

In this paper, ultra-wideband (UWB) Marchand baluns using substrate integrated coaxial line (SICL) technology are first proposed. SICL is a shielded planar coaxial line, which is very suitable for wideband and high-speed applications due to its broad unimodal operation band, low insertion loss, very small dispersion, and easy integration with other planar circuits. In order to achieve wider bandwidth and better VSWR performance of the SICL balun, a pair of quarter-wave impedance transformers are inserted into the balanced line. The impedance characteristics of the proposed SICL baluns are analyzed, and the parameters that determine the balun bandwidth are discussed. Then, curves of the balun resistance, reactance, and the SICL characteristic impedance are presented. According to these curves, the design parameters of the proposed SICL baluns can easily be determined. To verify these ideas, an UWB SICL single balun and an UWB SICL dual balun are designed, fabricated, and measured. Both full-wave simulated and measured results are presented, and good agreement between them is observed. The proposed SICL baluns have a simple structure and good performance, which are suitable for applications in UWB communication systems.