Two compact dual-band bandpass filters (BPFs) with closely spaced passbands are presented in this paper. Each of the filters consists of a stub loaded resonator, to which shorted lines are coupled. The ratio of the center frequencies of two passbands can be easily adjusted from 1.2 to 1.1 by changing the gap of the coupled line. In addition, seven transmission zeros (TZs) can be yielded to obtain high passband selectivity and enhance the out of band performances. As an example, two filters are designed, fabricated and measured. Both filters exhibit the merits of high passband selectivity, very low center frequency ratio, and wide stopband suppression.
2. Liu, S., J. Xu, and Z. T. Xu, "Compact dual-band bandpass filters using complementary split-ring resonators with closely spaced passbands," Electronics Letters, Vol. 52, No. 15, 1312-1314, 2016.
3. Eun, J. W. and J. H. Lee, "A microstrip dual-band bandpass filter using feed line with SIR," IEICE Electronics Express, Vol. 14, No. 4, 1-7, 2017.
4. Denis, B., K. J. Song, and F. Zhang, "Compact dual-band bandpass filter using open stub-loaded stepped impedance resonator with cross-slots," International Journal of Microwave and Wireless Technologies, Vol. 9, No. 2, 269-274, 2016.
5. Belyaev, B. A., A. M. Serzhantov, and V. V. Tyurnev, "A miniature dual-band filter based on microstrip dual-mode resonators," Technical Physics Letters, Vol. 38, No. 9, 839-842, 2012.
6. Chen, D., L. Zhu, and C. H. Cheng, "A novel dual-band bandpass filter with closely spaced passbands," IEEE Microwave and Wireless Components Letters, Vol. 24, No. 1, 38-40, 2014.
7. Wu, B., C. H. Liang, P. Y. Qin, and Q. Li, "Compact dual-band filter using defected stepped impedance resonator," IEEE Microwave and Wireless Components Letters, Vol. 18, No. 10, 674-676, 2008.
8. Song, K. J., F. Zhang, and Y. Fan, "Miniaturized dual-band bandpass filter with good frequency selectivity using SIR and DGS," AEU-International Journal of Electronics and Communications, Vol. 68, No. 5, 384-387, 2014.
9. Sun, S. J., T. Su, K. Deng, B. Wu, and C. H. Liang, "Compact microstrip dual-band bandpass filter using a novel stub-loaded quad-mode resonator," IEEE Microwave and Wireless Components Letters, Vol. 23, No. 9, 465-467, 2014.
10. Liu, H. W., B. P. Ren, X. H. Guan, J. H. Lei, and S. Li, "Compact dual-band bandpass filter using quadruple-mode square ring loaded resonator (SRLR)," IEEE Microwave and Wireless Components Letters, Vol. 23, No. 4, 181-183, 2014.
11. Xu, J., W. Wu, and C. Miao, "Compact and sharp skirts microstrip dual-mode dual-band bandpass filter using a single quadruple-mode resonator (QMR)," IEEE Transactions on Microwave Theory and Techniques, Vol. 61, No. 3, 1104-1113, 2013.
12. Matthaei, G., L. Young, and E. M. T. Jones, Microwave Filters Impedance-Matching Networks, and Coupling Structures, McGraw-Hill, New York, 1964.