In this paper, a microstrip lowpass filter with -3 dB cutoff frequency of 1.8 GHz composed of two resonators with different polygon patches and six symmetric suppressing cells is presented. To design the proposed filter, the impact of each microstrip transmission line on the scattering parameters of the employed resonators is separately determined by extracting the equations of the insertion loss (S21) and return loss (S11) on the basis of their equivalent LC circuit. The designed filter is fabricated and measured, and a good agreement between the results of simulation and measurement is obtained. In the whole stopband region, a return loss better than -0.35 dB and an acceptable suppression level of -22 dB from 1.87 to 19.75 GHz are achieved. Furthermore, a flat insertion loss in the passband and an acceptable return loss (-19.32 dB) in this band can verify desired in-band performance. The designed lowpass filter has a high figure of merit about 36969.34.
2. Luo, S., L. Zhu, and S. Sun, "Stopband-expanded low-pass filters using microstrip coupled-line hairpin units," IEEE Microw. Wireless Compon. Lett., Vol. 18, No. 8, 506-508, Aug. 2008.
3. Wei, X. B., P. Wang, M. Q. Liu, and Y. Shi, "Compact wide-stopband lowpass filter using stepped impedance hairpin resonator with radial stubs," IEE Electron. Lett., Vol. 47, No. 15, 862-863, Jul. 2011.
4. Li, L., Z. F. Li, and J. F. Mao, "Compact lowpass filters with sharpand expanded stopband using stepped impedance hairpin units," IEEE Microw. Wireless Compon. Lett., Vol. 20, No. 6, 31-312, Jun. 2010.
5. Velidi, V. K. and S. Sanyal, "Sharp roll-off lowpass filter with wide stopband using stub-loaded coupled-line hairpin unit," IEEE Microw. Wireless Compon. Lett., Vol. 21, No. 6, 30-303, Jun. 2011.
6. Mandal, M. K., P. Mondal, S. Sanyal, and A. Chakrabarty, "Low insertion-loss, sharp rejection and compact microstriplowpass filter," IEEE Microw. Wirel. Compon. Lett., Vol. 16, No. 11, 600-602, 2006.
7. Gomez-Garcia, R., M. A. Sanchez-Soriano, M. Sanchez Renedo, G. Torregrosa-Penalva, and E. Bronchalo, "Extended-stopband microstrip lowpass filter using rat-race directional coupler," Electron. Lett., Vol. 49, No. 4, 272-274, 2013.
8. Wang, C. J. and C. H. Lin, "Compact lowpass filter with sharp transition knee by utilizing a quasi-π-slot resonator and open stubs," IET Microw. Antennas Propag., Vol. 4, No. 4, 512-517, 2010.
9. Wang, J., H. Cui, and G. Zhang, "Design of compact microstrip lowpass filter with ultra-wide stopband," IEE Electron. Lett., Vol. 48, No. 14, 854-856, Jul. 2012.
10. Wei, F., L. Chen, X.-W. Shi, Q.-L. Huang, and X.-H. Wang, "Compact lowpass filter with wide stop-band using coupled-line hairpin unit," Electron. Lett., Vol. 46, No. 1, 88-90, 2010.
11. Ma, K. and K. S. Yeo, "New ultra-wide stopband low-pass filter using transformed radial stubs," IEEE Trans. Microw. Theory. Tech., Vol. 59, No. 3, 604-611, Mar. 2011.
12. Abdipour, A., Ar. Abdipour, and S. Lotfi, "A lowpass filter with sharp roll-off and high relative stopband bandwidth using asymmetric high-low impedance patches," Radioengineering, Vol. 24, No. 3, 712-716, 2015.
13. Nouritabar, A. R., A. Abdipour, and Ar. Abdipour, "A design of low-pass filter with wide stopband and sharp roll-off rate using series LC tanks resonator," Applied Computational Electromagnetics Society (ACES) Journal, Vol. 31, No. 11, 1343-1350, 2016.
14. Abdipour, Ar. and A. Abdipour, "Compact microstrip lowpass filter with an ultra-wide stopband and sharp transition band using T-shaped and polygon resonators," Progress In Electromagnetics Research C, Vol. 74, 51-61, 2017.
15. Hong, J.-S. and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, John Wiley & Sons, Inc., 2001.