Novel compact band-pass filters with horse-shoe microstrip resonators

Novel filter designs consisting of stepped impedance microstrip resonators of a horse-shoe type, which are positioned parallel or in series and are coupled either by magnetic or electric fields, respectively, are presented. Both parallel and series designs provide for deep attenuation poles in both upper and lower stop-bands of the filters, at no cross-coupling or bypassing. FDTD field analysis and calculations of equivalent circuits are used to explain the transmission zero appearance. The possibilities to control the transmission zero frequencies and the pass band parameters are demonstrated. The results of simulations are confirmed by the prototype measurements of fabricated prototypes.