Short- and open-stub loaded spiral resonator and its application in planar microstrip filters

A novel short- and open-stub loaded spiral resonator (SOSL-SR) is presented and analysed by the odd- and even-mode method to obtain three controllable resonant modes. Therefore, a triple-mode bandpass filter (BPF) has been designed with good frequency selectivity based on the proposed SOSL-SR. Then, by dividing these three resonant modes into two passbands or stopbands, a dual-band BPF and a dual-band bandstop filter using the proposed SOSL-SR have been demonstrated. Because of the spiral structure, the sizes of these three designed microstrip filters are reduced significantly. Good agreement between simulated and measured results validates the design concept.