High selective dual-band microstrip filter using zero-degree coupled SIR feed lines

A dual-band microstrip filter utilizing zero-degree coupled SIR feed linewith high frequency selectivity is presented. The first two resonant modes of stepped impedance resonators (SIRs) are chosen as the dual-band center frequencies. A pair of folded SIRs are coupled to form a second-order dual-band filter. Meanwhile, the SIRs are surrounded by a pair of zero-degree coupled SIR feed lines which introduce five transmission zeros. Two transmission zeros are realized by the source-load coupling, another two are created by zero-degree feed line; the last one is due to the third harmonic effect of SIR feed line. To demonstrate this technique, a high selective dual-band filter operating at 1.5G and 3.5Gis designed and fabricated with good performance.