A modified star-shaped dual-mode microstrip loop resonator with a triangular patch is proposed to design compact narrow band bandpass filters with transmission zeros. The triangular patch is used as perturbation element to couple degenerated modes. The position of transmission zeros can be tuned by varying the size and location of the perturbation element. The coupling mechanism (inductive or capacitive) and the type of input/output feed (orthogonal or non-orthogonal) determine the type of filter response like symmetric or asymmetric. Three highly selective two-pole narrow band bandpass filters are designed using a modified star-shaped resonator with tunable transmission zeros. First, two filters with non-orthogonal feed lines have an asymmetric response with transmission zeros on one side of the passband, whereas the third filter with orthogonal feed lines exhibits symmetric frequency response with transmission zeros on both sides of the passband. Even and odd mode analysis is applied to the dual mode filters to calculate the position of transmission zeros. Filters are realized using a low loss dielectric substrate, and measured results are in good agreement with the theoretical and simulated ones.
2. Hong, J. S. and M. J. Lancaster, "Microstrip cross-coupled trisection bandpass filters with a symmetric frequency characteristics," IEE Proc. Microwave, Antennas Propag., Vol. 146, No. 1, 84-90, 1999.
3. Wolf, I., "Microstrip bandpass filter using degenerate modes of a microstrip ring resonator," Electron. Lett., Vol. 8, No. 12, 302-303, 1972.
4. Hong, J. S. and M. J. Lancaster, "Bandpass characteristics of new dualmode microstrip square loop resonators," Electron. Lett., Vol. 31, No. 11, 891-892, 1995.
5. Lugo, C. and J. Papapolymerou, "Bandpass filter design using a microstrip triangular loop resonator with dual-mode operation," IEEE Microw. Wireless Compon. Lett., Vol. 15, No. 7, 475-477, 2005.
6. Wu, R. and S. Amari, "New triangular microstrip loop resonators forbandpass dual-mode filter applications," IEEE MTT-S Int. Microwave Symp. Dig., 941-944, 2005.
7. Mansour, R. R., "Design of superconductive multiplexers using single-mode and dual-mode filters," IEEE Trans. Microw. Theory Tech., Vol. 42, No. 7, 1411-1418, 1994.
8. Hong, J. S. and M. J. Lancaster, "Microstrip triangular patch resonator filters," IEEE MTT-S Dig., 331-334, 2000.
9. Hong, J. S. and S. Li, "Theory and experiment of dual-mode microstriptriangular patch resonators and filters," IEEE Trans. Microw. Theory Tech., Vol. 52, No. 4, 1237-1243, 2004.
10. Avinash, K. G. and I. S. Rao, "Design of bandpass filter using star loop dual mode resonator," 2015 IEEE International Conference on Communications and Signal Processing (ICCSP), 0238-0241, Melmaruvathur, 2015.
11. Gorur, A., "Description of coupling between degenerate modes of a dual-mode microstrip loop resonator using a novel perturbation arrangement and its dual-mode bandpass filter applications," IEEE Trans. Microw. Theory Tech., Vol. 52, No. 2, 671-677, 2004.
12. Eryilmaz, G. M., C. Karpuz, and A. Gorur, "Dual-mode microstrip filters with adjustable transmission zeros," IET Microw. Antennas Propag., Vol. 2, No. 8, 839-847, 2008.
13. Rouchaud, F., V. Madrangeas, M. Aubourg, P. Guillon, B. Theron, and M. Maignan, "New classes of microstrip resonators for HTS microwave filters applications," IEEE MTT-S Dig., 1023-1026, 1998.
14. Hsieh, L. H. and K. Chang, "Dual-mode quasi-elliptic-function bandpass filters using ring resonators with enhanced coupling tuning stubs," IEEE Trans. Microw. Theory Tech., Vol. 50, No. 5, 1340-1345, 2002.
15. Wang, J., J. L. Li, J. Ni, S. Zhao, W. Wu, and D. Fang, "Design of miniaturized microstrip dualmode filter with source-load coupling," IEEE Microw. Wireless Compon. Lett., Vol. 20, No. 6, 319-321, 2010.
16. Wang, J., J. Ni, J. B. Pan, L. Ge, and X. Y. Guo, "Design of a miniaturized microstrip dual-mode dual-band bandpass filter," Microwave and Optical Tech. Lett., Vol. 53, No. 11, 1493-1496, 2011.
17. Gorur, A. K., C. Karpuz, A. Ozek, P. O. Ozdemir, and K. Karaca, "A novel compact microstrip dual-mode wideband bandpass filter design using tuning stubs," Microwave and Optical Tech. Lett., Vol. 56, No. 1, 47-49, 2014.
18. Cheng, K. K. M., "Design of dual-mode ring resonators with transmission zeros," Electron. Lett., Vol. 33, No. 16, 1392-1393, 1997.
19. Mao, R. J. and X. H. Tang, "Novel dual-mode bandpass filters using hexagonal resonators," IEEE Trans. Microw. Theory Tech., Vol. 54, No. 9, 3526-3553, 2006.
20. Esfeh, B. K., A. Ismail, R. S. A. Raja Abdullah, H. Adam, and A. R. H. Alahwari, "Compact narrowband bandpass filter using dual-mode octagonal meandered loop resonator for WiMAX application," Progress In Electromagnetics Research B, Vol. 16, 277-290, 2009.