This letter presents a novel quad-band bandstop filter. It is formed by loading a microstrip line with two special resonators. The special resonator can be seen as a coupled-line and shorted stub-loaded half-wavelength microstrip resonator (CSSHMR). The resonator can resonate at four frequencies, which forms the corresponding equivalent shorted points at the microstrip line. Therefore, signals are rejected at these equivalent short-circuited points, which realizes the quad-band bandstop responses. The four resonant frequencies can be separately adjusted in a limited range. The gap coupling between the resonators can be introduced to adjust the performance. A transmission line model is built to analyze the quad-band bandstop filter. A prototype quad-band bandstop filter is designed, fabricated and measured. The measured and simulated results have a good agreement.
2. Chin, K.-S. and C.-K. Lung, "Miniaturized microstrip dual-band bandstop filters using tri-section stepped impedance resonator," Progress In Electromagnetics Research C, Vol. 10, 37-48, 2009.
3. Dhakal, R. and N.-Y. Kim, "A compact dual-band bandstop filter using a circular, folded, symmetric, meandered-line, stepped-impedance resonator," Microw. Opt. Technol. Lett., Vol. 56, No. 10, 2298-2301, 2014.
4. Liu, L., R.-H. Jin, C. Zhang, J.-P. Geng, and X.-L. Liang, "A novel dual-band bandstop filter based on parallel stub-loaded resonator," Microw. Opt. Technol. Lett., Vol. 58, No. 6, 1268-1271, 2016.
5. Cavalcante, G. A., D. R. Minervino, A. G. D’assuncao, V. P. S. Neto, and A. G. D’assuncao, "A compact multiband reject inverted double-E microstrip filter on textile substrate," Microw. Opt. Technol. Lett., Vol. 57, No. 11, 2543-2548, 2015.
6. Chu, Q.-X. and L.-L. Qiu, "Sharp-rejection dual-band bandstop filter based on signal interaction with three paths," Microw. Opt. Technol. Lett., Vol. 57, No. 3, 657-660, 2015.
7. Ning, H., J. Wang, Q. Xiong, and L.-F. Mao, "Design of planar dual and triple narrow band bandstop filters with independently controlled stopbands and improved spurious response," Progress In Electromagnetics Research, Vol. 131, 259-274, 2012.
8. Zhang, X. Y., H.-L. Huang Zhang, and B.-J. Hu, "Novel dual-band bandstop filter with controllable stopband frequencies," Microw. Opt. Technol. Lett., Vol. 54, No. 5, 1203-1206, 2012.
9. Luo, Y. and J. Bornemann, "Open and short U-shaped microstrip resonators for second-order single- or dual-bandstop filter design," Microw. Opt. Technol. Lett., Vol. 59, No. 6, 1362-1365, 2017.
10. Lee, S., S. Oh, W.-S. Yoon, and J. Lee, "A CPW bandstop filter using double hairpin-shaped defected ground structures with a high Q factor," Microw. Opt. Technol. Lett., Vol. 58, No. 6, 1265-1268, 2016.
11. Ai, J., Y. H. Zhang, K. D. Xu, M. K. Shen, and W. T. Joines, "Miniaturized frequency controllable band-stop filter using coupled-line stub-loaded shorted SIR for tri-band application," IEEE Microw. Wireless Compon. Lett., Vol. 27, No. 7, 627-629, 2017.
12. Adhikari, K. K. and N. Y. Kim, "A miniaturized quad-band bandstop filter with high selectivity based on shunt-connected, T-shaped stub-loaded, stepped-impedance resonators," Microw. Opt. Technol. Lett., Vol. 57, No. 5, 1129-1132, 2015.
13. Karpuz, C., A. Gorur, A. K. Gorur, and A. Ozek, "A novel compact quad-band microstrip bandstop filter design using open-circuited stubs," IEEE MTT-S Int. Microwave Symp. Digest, 1-3, 2013.