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 5.8f₀ with rejection level better than 20 dB. Moreover, the proposed filters can easily realize a wide bandwidth.