Balun bandpass filters

In this chapter, a new coupled line balun topology is presented by analyzing the standing wave pattern around the open-circuited half-wavelength transmission line. Then, a class of planar-type balun bandpass filters with single-/dual-band and wideband responses is introduced based on this balun topology. This method to design balun filters is simple and effective with widely used multi-mode resonators. To further extend the application of the design approach, a balun diplexer is briefly introduced by using stub-loaded dual-mode resonators. What is more, some other planar balun bandpass filters based on different transition structures are also discussed in the end. All these designed circuits are verified by both simulation and experiment, and measured results indicate that the balun filters exhibit not only good balance performance, but also show high frequency selectivity. In a nutshell, the design method in this chapter provides us some new concepts for the implementation of balun bandpass filters.