A compact coupled-line hairpin resonator is proposed and analyzed for designing a novel bandpass filter (BPF) in this letter. Compared with conventional stepped-impedance and stub-loaded resonator, the proposed resonator can produce three transmission zeros, which can be applied to achieve a wide and highly-attenuated upper stopband. To validate this attractive feature, a microstrip BPF is designed and fabricated with the center frequency at 2.4 GHz and a fractional bandwidth of 6%. Measured frequencies responses show a wider upper stopband up to 6.9 GHz (2.9f0) with insertion loss higher than 20 dB.
2. Fan, J.-W., C.-H. Liang, and X.-W. Dai, "Design of cross-coupled dual-band filter with equal-length split-ring resonators," Progress In Electromagnetics Research, Vol. 75, 285-293, 2007.
3. Wang, J.-P., B.-Z. Wang, and W. Shao, "A novel partly shielded finite ground CPW low pass filter," Journal of Electromagnetic Waves and Applications, Vol. 19, No. 5, 689-696, 2005.
4. Wang, Y. X., B.-Z. Wang, and J. P. Wang, "A compact square-loop dual-mode bandpass filter with wide stop-band," Progress In Electromagnetics Research, Vol. 77, 67-73, 2007.
5. Wang, Y. X., "Microstrip cross-coupled tri-section stepped-impedance bandpass filter with wide stop-band performance," Journal of Electromagnetic Waves and Applications, Vol. 23, No. 2-3, 289-296, 2009.
6. Makimot, M. and S. Yamashita, "Bandpass filters using parallel coupled stripline stepped impedance resonators," IEEE Trans. on Microw. Theory and Tech., Vol. 28, No. 12, 1413-1417, 1980.
7. Wang, Y. X., B. Z. Wang, and J. P. Wang, "The design of coupled resonator bandpass filter with wide stop-band," IEEE Microw. Wirel. Compon. Lett., Vol. 18, No. 4, 251-253, 2008.
8. Kim, I. K., N. Kingsley, M. Morton, R. Bairavasubramanian, J. Papapolymerou, M. M. Tentzeris, and G. Y. Jong, "Fractal-shaped microstrip coupled-line bandpass filters for suppression of second harmonic," IEEE Trans. on Microw. Theory and Tech., Vol. 53, No. 9, 2943-2948, 2005.
9. Zhang, S. B. and L. Zhu, "Compact and high-selectivity microstrip bandpass filters using tri-/quad-mode stub-loaded resonators," IEEE Microw. Wirel. Compon. Lett., Vol. 21, No. 10, 522-524, 2011.
10. Luo, S., L. Zhu, and S. Sun, "Stopband-expanded low-pass filters using microstrip coupled-line hairpin units," IEEE Microw. Wirel. Compon. Lett., Vol. 18, No. 8, 506-508, 2008.
11. Hong, J.-S. and M. J. Lancaster, Microstrip Filter for RF/Microwave Application, Wiley, New York, 2001.
12. Cheong, P., S.-W. Fok, and K.-W. Tam, "Miniaturized parallel coupled-line bandpass filter with spurious-response suppression," IEEE Trans. on Microw. Theory and Tech., Vol. 53, No. 5, 1810-1816, 2005.