Wide‐Passband Microstrip Filters With Wide Stopband Based on Half‐Wavelength Resonators Coupled by Short Connected Line

A novel and simple design method of wide‐passband filters with wide stopband based on half‐wavelength ( λ /2) resonators coupled by a short connected line (SCL) is proposed and analysed in this paper. Different from the traditional parallel‐coupled structure filters, SCL is proposed to realize a strong coupling between two adjacent resonators and suppress the parasitic passbands simultaneously. And the eigenmode analysis method and impedance ratio characteristics are both used to reveal the mechanism of wide‐stopband implementation. To further expand the stopband, filters based on step‐impedance resonators (SIRs) with SCL are proposed and analysed in detail. To verify the proposed design method and wide‐stopband filtering structures, three 4‐pole filters with uniform‐impedance resonators (UIRs), SIRs with same impedance ratio, and SIRs with different impedance ratios are designed, fabricated, and measured. The proposed filters can realize a wide stopband up to 5.8 f 0 with rejection level better than 20 dB. Moreover, the proposed filters can easily realize a wide bandwidth.