Coupling behaviors of quarter-wavelength impedance transformers for wideband CPW bandpass filters

Quarter-wavelength coplanar waveguide (lambda/4-CPW) impedance transformers with low and high characteristic impedance are characterized via fullwave method of moments short-open calibration (MoM-SOC) approach and further utilized as alternative K- and J-inverter networks with enhanced coupling. Network parameters are obtained and compared with those of two traditional lumped-CPW elements, demonstrating tightened coupling behaviors. The two improved lambda/4 CPW bandpass filters are then constructed and their performances are optimized via cascaded topology. Predicted results show us a few attractive features, i.e., wide passband and first-order harmonic suppression as confirmed by experiment.