IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 23, NO. 5, MAY 2013 243 Design and Implementation of a Triple-Mode Planar Filter Xi-Cheng Zhu, Student Member, IEEE, Wei Hong, Fellow, IEEE, Ke Wu, Fellow, IEEE, Hong-Jun Tang, Member, IEEE, Zhang-Cheng Hao, Member, IEEE, Ji-Xin Chen, Member, IEEE, and Peng Chu, Student Member, IEEE Abstract—A novel planar triple-mode resonator is proposed and implemented by placing an additional metallic via at the center of a circular dual-mode substrate integrated waveguide (SIW) cavity resonator. The resonant frequency of the dominant mode in the SIW cavity increases remarkably and become close to the resonant frequencies of the -similar mode. Three asymmetric band- pass filters utilizing this resonator are designed and fabricated. The measured results are in good agreement with the simulation results. Index Terms—Degenerated modes, dual-mode resonator, sub- strate integrated waveguide (SIW) cavity, triple-mode filter. I. INTRODUCTION M ULTIMODE resonators and filters have been inten- sively investigated for their attractive characteristics including high performance in response selectivity, inherent size reduction, etc. [1]–[3]. The low cost, high Q-factor, compact size and ease of manufacture make the substrate integrated waveguide (SIW) technology attractive for planar filter design, especially in millimeter-wave frequency band. To the best of our knowledge, few planar triple-mode filters based on SIW technology have been reported. In [1], a microstrip square-loop dual-mode resonator was placed in a SIW cavity that provided the third resonance, where multilayer process is required. A triple-mode resonator with a complementary split ring resonator (CSRR) etched on the top surface of a dual-mode SIW resonator was re- ported in [4]. Nevertheless, the slots on the SIW cavity surface usually result in the increase of radiation loss and the reduction of the Q-factor. Besides, a planar triple-mode resonator uti- lizing the high-order resonances of SIW cavity was presented in [5]. Compared with the dominant or second-order resonant modes, this SIW cavity resonated at higher-order modes can be operated at higher frequency but with larger size. Manuscript received January 09, 2013; revised February 25, 2013; accepted March 05, 2013. Date of publication March 29, 2013; date of current version May 06, 2013. This work was supported in part by the National 973 project of China 2010CB327400 and in part by the National Nature Science Foundation of China (NSFC) under Grant 60921063. X.-C. Zhu, W. Hong, H.-J. Tang, Z.-C. Hao, J.-X. Chen, and P. Chu are with the State Key Laboratory of Millimeter Waves, School of Information Science and Engineering, Southeast University, Nanjing 210096, P. R. China (e-mail: xczhu@emfield.org; weihong@seu.edu.cn). K. Wu is with the Poly-Grames Research Center, Department of Electrical Engineering, Ecole Polytechnique, Montreal, QC, Canada H3C3A7 (e-mail: ke.wu@ieee.org). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LMWC.2013.2253313 Fig. 1. Topology of proposed triple-mode SIW cavity resonator. In this letter, a triple-mode SIW resonator is proposed by placing an additional via at the center of a circular dual-mode SIW cavity resonator. The resonant frequency of the dominant mode increases and become close to that of the two degener- ated modes, i.e., -similar mode. The size of this triple- mode cavity resonator remains almost the same as the circular dual-mode SIW resonator. Three filters based on this novel SIW triple-mode resonator were fabricated, measured and analyzed. II. DESIGN AND SIMULATION A. Triple-Mode SIW Cavity Resonator At the center of the circular SIW cavity, the electric field strength of the dominant mode reaches its maximum, and that of the mode reaches its minimum. Then, a triple-mode SIW cavity resonator is created by adding a metallic via at the center of a circular dual-mode SIW cavity resonator. Fig. 1 shows the layout of the resulting structure, which can be treated as a very flat coaxial resonator. The electric field distributions of the dominant mode and two -similar degenerated modes are shown in Fig. 2(a), Fig. 2(b), and Fig. 2(c), respectively. And the electric field of these three modes are normal to the substrate surface, while that of TEM mode in the traditional coaxial res- onator is parallel to the substrate surface. To avoid the resonator working at the TEM mode, the following requirement should be satisfied: (1) where is the thickness of substrate material. And the substrate thickness is usually much smaller than the equivalent width and length of the SIW cavity. As an example, considering a triple-mode SIW cavity res- onator with and fabri- cated on Rogers 5880 with 0.254 mm thickness, its resonant frequencies versus are shown in Fig. 3. These resonant fre- quencies are obtained by using the HFSS software. The elec- tromagnetic field of the dominant mode is greatly changed and 1531-1309/$31.00 © 2013 IEEE