Miniaturized dual mode microwave filter

Dual mode filters have been widely used in wireless communications systems due to their advantageous of being small size, planar and low loss. Most of the dual mode filters are realized based upon dual mode ring or patch resonator such that the two orthogonal resonant modes occur, yielding a dual mode response. However the electrical length needed to establish the resonant condition is still considerably large. These resonators are unacceptable for the design of high performance multi-pole filter. In this paper, a novel stepped impedance dual mode resonator is introduced where a significant size reduction is achieved. In contrast to dual mode ring resonator which has an electrical length of 360°, the filter gives a circuit size reduction of 67% and a very wideband harmonic suppression due to its shorter electrical length of the resonator. Therefore the proposed dual mode resonator is very suitable for high-order filter realization. Theoretical analysis together with experimental prototypes is presented in this paper.