Design optimization of microstrip square-ring band-pass filter with quasi-elliptic function

Resonant processes in dual-mode microstrip square-ring band-pass filter fed through L-shaped coupling arms have been studied using the finite difference time domain (FDTD) simulations. The performed field analysis has revealed that coupling arms act as additional half-wavelength resonators coupled with the ring through both electric and magnetic fields. New filter designs with horseshoe resonators instead of coupling arms have been developed, which provide either pure magnetic or electric coupling at the resonant frequency. The obtained results demonstrate the possibility to essentially improve filter characteristics. Simulation results have been verified by the measurements of fabricated prototypes.