74 S. HUGAR, J. S. BALIGAR, V. DAKULAGI, ET AL., QUASISTATIC RESONATORS BASED TRIPLE-MODE NOTCHED … DOI: 10.13164/re.2023.0074 Quasistatic Resonators Based Triple-Mode Notched Microstrip Bandpass Filter Shobha I. HUGAR 1 , Jambunath S. BALIGAR 1 , Veerendra DAKULAGI 2 , K. M. VANITHA 3 1 Dept. of Electronics and Communication Engineering, Dr. Ambedkar Institute of Technology, Bengaluru, India 2 Dept. of Electronics and Communication Engineering, Guru Nanak Dev Engineering College, Bidar, India 3 Dept. of Electronics and Communication Engineering, M S Ramaiah Institute of Technology, Bengaluru, India shobha_hugar@yahoo.co.in, jbaligar@gmail.com, veerendra.gndec@gmail.com, kmvanitha@msrit.edu Submitted April 22, 2022 / Accepted November 23, 2022 / Online first January 30, 2023 Abstract. This article discusses new approach for design and development of triple-mode notched microstrip band- pass filter based on quasistatic resonators (QR). The pro- posed approach is composed of two quasistatic resonant elements; Horizontal Plane Split Ring Resonator (HP- SRR), Vertical Plane Split Ring Resonator (VP-SRR) and a single Asymmetric Step Impedance Resonator (A-SIR) with parallel coupled feed structure. An additional atten- uation pole realized by VP-SRR in desired passband tunes the dual-mode response to triple mode and enhances the 3dB bandwidth without changing the dimensions of basic the filter cell. The HP-SRR realizes a notch at WiMAX band (IEEE 802.11a lower band) in the desired passband. Further by changing the impedance of VP-SRR and HP- SRR both the location of additional attenuation pole fre- quency and notch band can be controlled. The proposed approach eliminates conventional method of realizing notch in the desired passband using vias and defective ground structures which have practical difficulties in reali- zation and also the proposed approach results in compact notched wideband filter design. Keywords Dual-mode, quasistatic resonators, asymmetric step impedance resonator, split ring resonators 1. Introduction The continuous evolution in wireless communication technology demands advanced microwave components like compact filters. In literature a number of multimode band- pass filters such as dual-mode, triple-mode, quadruple- mode BPFs have been designed to meet the requirements of wireless communication industry. The dual-mode BPFs in [1–3] have been designed using closed loop, meandered dual-mode resonators like octagonal, hexagonal, star shaped resonators. The triangular, square patch resonators with corner cuts [4], [5] have also been proposed to design dual-mode BPFs. However all these reported filters have narrow bandwidth. Further to improve the bandwidth mul- timode resonators, coupled line structures and hairpin res- onators have been proposed in [6–8]. But all these filters have larger footprint. The triple-mode BPFs reported in [9–11] have fabrication difficulties as they use via hole for grounding. Quadraple mode resonators have been used to design wideband filters in [12], [13]. Even through these filters provide wide bandwidth but have less frequency selective passband due to absence of transmission zeros at the passband edges. Besides these wideband BPFs, notched bandpass response filters have also been reported in litera- ture since for some applications there is a need to avoid the interference from existing wireless communication systems such as WiMAX network. In literature these notched-band BPFs have been designed using parallel coupled T shaped SIR, trisection SIRs, parallel coupled trisection resonators with complementary split ring resonator (CSRR) [14–16], defective microstrip structures (DGS) [17–19]. Vias pro- posed in [20] to realize a notch are not practically feasible. Authors Jingbo Liu and Ting Zhang in [21], [22] have also proposed tuning of notch band frequency by varying length and width of via grooves and length of short stubs which has practical difficulties. In this article, we propose a novel approach for design and development of triple-mode notched microstrip band- pass filter based on quasistatic resonators (QR); HP-SRR and VP-SRR. The proposed approach uses a single A-SIR with one step discontinuity as shown in Fig. 1. With proper selection of impedance ratio R and length ratio U for asymmetric stepped impedance resonator (A-SIR), first two resonant modes of the resonator are coupled to attain dual-mode response. Using parallel coupled feed structure, the two resonant modes of the resonator are coupled strongly and transmission zeros at upper and lower pass- band edges are realized to achieve high frequency selective passband. By integrating VP-SRR on low impedance sec- Fig. 1. Asymmetric step impedance resonator (A-SIR).