High selectivity dual-band dual-mode microstrip BPF with multi-zeros

In this paper, a high selectivity dual-band dualmode microstrip bandpass filter (BPF) with multi-zeros is proposed utilizing short-stub-loaded folded SIRs and Y-shaped source-load coupling structure. The folded SIR supports two non-degenerated modes, thus achieving two independently controlled passbands with adjustable bandwidths by means of coupling two such SIRs. Due to the main path signal counteraction and the folded arms of the SIRs, five transmission zeros can be generated near the passband edges and in the upper-stopband to improve frequency selectivity. Zero Tz1 is created at the lower-stopband by the loaded short stub to improve stopband rejection of lower frequency. Three zeros are created in the upper-stopband owning to the special feed structure. To verify the principle above, one filter prototype with altogether nine transmission zeros is simulated, fabricated and measured for experimental verification of the predicted results.