Design of a Wideband Balun Using Parallel Strips

Abstract

A wideband balun designed by exact synthesis is presented in this letter. The designed balun has a symmetrical four-port structure with one port open-ended. It consists of a wideband impedance transformer and a broadband phase inverter, which is realized by connecting bottom and top layer parallel strip through two via holes. To design the wideband impedance transformer, an <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$S$</tex></formula> -plane highpass prototype has been exactly synthesized with third-order Chebyshev response. The proposed design procedure is very flexible, and it can be easily scaled for other frequency bands. A wideband parallel strip balun operating from 0.72 to 2.05 GHz is fabricated and tested to verify the design concepts. The measured results match well with the design theory.