Microstrip spiral-coupled scheme bandpass filters with mutual electric and magnetic coupling

42nd European Microwave Conference (EuMC 2012) being held as Part of 15th European Microwave WeekThis paper presents miniaturized microstrip bandpass filters based on the quarter-wavelength (λ/4) spiral-coupled scheme (SCS). By introducing the mutual electric and magnetic coupling between two SCS, two resonant modes can be well adjusted to form a passband and four transmission zeros can be generated to obtain a good out-of-band rejection. Without modifying the initial design parameters, another transmission zero with strong attenuation can be further introduced by loading a parallel-coupled line section along the symmetrical plane, thus largely widening the upper stopband. To verify our proposal, two filters are finally implemented and fabricated, which exhibit the following attractive features: low insertion loss, good passband selectivity, wide stopband with a high rejection level, and compact size. © 2012 EUROPEAN MICROWAVE ASSOC.published_or_final_versio