This paper presents an approach to the design of novel diplexers with pairs of transmission zeros for each output port. A rotationally symmetric structure with shorted circuited stubs is proposed to achieve diplexer operation, and the diplexer exhibits a good filtering performance with a pair of transmission zeros allocated at two sides of the passbands as well. The analysis of the rotationally symmetric structure bandpass filter is also presented in this paper in detail. Moreover, the interdigital coupled-lines are introduced to improve the out-of-band performance of the proposed rotationally symmetric diplexer. In order to verify the proposed structure, first a diplexer with rotationally structure operating at 2.4/2.73 GHz with simulated insertion loss of 1.217/0.930 dB is designed and simulated, then a diplexer with interdigital coupled-lines operating at 5/5.8 GHz with measured insertion loss of 1.524/1.524 dB is designed, simulated and measured to improve the passband selectivity, and the measured results are in good agreement with simulated ones.
2. Shi, L., H.-J. Sun, W.-W. Dong, and X. Lv, "A dual-band multi-function carborne hybrid antenna for satellite communication relay system," Progress In Electromagnetics Research, Vol. 95, 329-340, 2009.
3. Behera, S. and K. J. Vinoy, "Microstrip square ring antenna for dual-band operation," Progress In Electromagnetics Research, Vol. 93, 41-56, 2009.
4. Tilanthe, P., P. C. Sharma, and T. K. Bandopadhyay, "A monopole microstrip antenna with enhanced dual band rejection for UWB application," Progress In Electromagnetics Research B, Vol. 38, 315-331, 2012.
5. Wang, X.-H., B.-Z. Wang, and K. J. Chen, "Compact broadband dual-band bandpass filters using slotted ground structures," Progress In Electromagnetics Research, Vol. 82, 151-166, 2008.
6. Chen, Z.-X., X.-W. Dai, and C.-H. Liang, "Novel dual-mode dual-band bandpass filter using double square-loop structure," Progress In Electromagnetics Research, Vol. 77, 409-416, 2007.
7. Patin, J. M., N. R. Labadie, and S. K. Sharma, "Investigations on an H-fractal wideband microstrip filter with multi-passbands and a tuned notch band," Progress In Electromagnetics Research B, Vol. 22, 285-303, 2010.
8. Chen, M., Y.-C. Lin, and M.-H. Ho, "Quasi-lumped design of bandpass filter using combined CPW and microstrip," Progress In Electromagnetics Research Letters, Vol. 9, 59-66, 2009.
9. Wu, Y., Y. Liu, and S. Li, "A compact pi-structure dual band transformer," Progress In Electromagnetics Research, Vol. 88, 121-134, 2008.
10. Castaldi, G., V. Fiumara, and I. Gallina, "An exact synthesis method for dual-band chebyshev impedance transformers," Progress In Electromagnetics Research, Vol. 88, 305-319, 2008.
11. Strassner, B. and K. Chang, "Wide-band low-loss high isolation microstrip periodic-stub diplexer for multiple-frequency applications," IEEE Trans. Microw. Theory Tech., Vol. 49, No. 10, 1818-1820, 2001.
12. Wang, R., J. Xu, M.-Y. Wang, and Y.-L. Dong, "Synthesis of microwave resonator diplexers using linear frequency transformation and optimization," Progress In Electromagnetics Research, Vol. 124, 441-455, 2012.
13. Chen, C.-F., T.-Y. Huang, C.-P. Chou, and R.-B.Wu, "Microstrip diplexers design with common resonator sections for compact size, but high isolation," IEEE Trans. Microw. Theory Tech., Vol. 54, No. 5, May 2006.
14. An, J., G.-M. Wang, C.-X. Zhang, and P. Zhang, "Diplexer using composite right-/left-handed transmission line," Electronics Letters, Vol. 44, No. 11, May 22, 2008.
15. Yang, R.-Y., C.-M. Hsiung, C.-Y. Hung, and C.-C. Lin, "Design of a high band isolation diplexer for GPS and WLAN system using modified stepped-impedance resonators," Progress In Electromagnetics Research, Vol. 107, 101-114, 2010.
16. Huang, C.-Y., M.-H. Weng, and C.-S. Ye, "A high band isolation and wide stopband diplexer using dual-mode stepped-impedance resonators," Progress In Electromagnetics Research, Vol. 100, 299-308, 2010.
17. Shi, J., J.-X. Chen, and Z.-H. Bao, "Diplexers based on microstrip line resonators with loaded elements," Progress In Electromagnetics Research, Vol. 115, 423-439, 2011.
18. Wong, S. W., K.Wang, Z.-N. Chen, and Q.-X. Chu, "Rotationally symmetric coupled-lines bandpass filter with two transmission zeros," Progress In Electromagnetics Research, Vol. 135, 641-656, 2013.
19. Gomez-Garcia, R. and J. I. Alonso, "Design of sharp-rejection and low-loss wide-band planar filters using signal-interference techniques," IEEE Microwave Wireless Compon. Lett., Vol. 15, No. 8, 530-532, Aug. 2005.
20. Zysman, G. I. and A. K. Johnson, "Coupled transmission line networks in an inhomogeneous dielectric medium," IEEE Trans. Microw. Theory Tech., Vol. 17, No. 10, 753-759, Oct. 1969.
21. Schwindt, R. and C. Nguyen, "Spectral domain analysis of three symmetric coupled lines and application to a new bandpass filter," IEEE Trans. Microw. Theory Tech., Vol. 42, No. 7, 1183-1189, Jul. 1994.
22. Tripathi, V. K., "On the analysis of symmetrical three-line microstrip circuits," IEEE Trans. Microw. Theory Tech., Vol. 25, No. 9, 726-729, Sept. 1977.
23. Hong, J. S. and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, Wiley, New York, 2001.