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

This 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.