16. J.P. Kim and W.S. Park, Analysis and network modeling of an aperture-coupled microstrip patch antenna, IEEE Trans Antennas Propagat AP-49 (2001), 849–854. 17. Taconic, Petersburgh, NY 12138, USA. Available at: http://www. taconic-add.com. 18. CST Microwave Studio. Computer Simulation Technology, Darmstadt, Germany, 2015. Available at: https://www.cst.com. V C 2017 Wiley Periodicals, Inc. COMPACT AND TUNABLE MICROSTRIP TRI-BAND BANDSTOP FILTER INCORPORATING OPEN-STUBS LOADED STEPPED-IMPEDANCE-RESONATOR Gyan Raj Koirala and Nam-Young Kim KwangWoon University, Seoul, Korea; Corresponding author: nykim@kw.ac.kr Received 19 September 2016 ABSTRACT: This article proposes a compact microstrip tri-band band- stop filter (TBBSF) operating at 2.57, 4.62, and 7.92 GHz. The proposed design composed of open-stubs loaded in a meandered stepped- impedance-resonator (SIR). Three different open-stubs are loaded in a SIR, among which first open-stub is responsible for realizing the second resonance frequency, whereas additional two identical open-stubs are capable of tuning the third resonance frequency. This allows the possi- bility of individual tuning of the resonance frequencies by adjusting the electrical length of the respective resonators. Based on the design con- cept, an experimental circuit is designed and fabricated, offering a sim- ple design methodology and a very compact circuit size of 0.08 k g 3 0.08 k g , which is more than 15 % smaller than the previously reported literatures. The simulation results are verified from the measurement and are found in good agreement. V C 2017 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:815–818, 2017; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.30394 Key words: bandstop filter; compact; open-stubs; stepped-impedance- resonator; tri-band 1. INTRODUCTION Microstrip bandstop filter (BSF) has long been captivated as an indispensable part of microwave circuits and systems such as amplifiers, mixers, featuring as a compact size, low cost, and high performance device for the effective suppression of spuri- ous as well as harmonic signals. The design of multiband BSF is garnered by the advent of multiband operative single trans- ceiver radio frequency (RF) communication systems such as in WiMax technology, which operates at non-harmonic frequency bands of 2.4/3.5/5.2 GHz [1–3]. Dual- [3–7], tri- [1,3,4,8,9], and quad-band [9] bandstop filters are proposed on several litera- tures as the possible candidates for multi-frequency operative BSFs. Basically, microstrip filters are designed using Hilbert- fork network [1], stepped-impedance-resonator (SIR) [9], defected microstrip structure (DMS) [3,8], defected ground structure (DGS) [4] techniques to name few. Nevertheless, real- izing a high performance device with a compact size is still challenging. This article deals with the realization of a very compact and high performance microstrip tri-band bandstop filter (TBBSF) operating at 2.57/4.65/7.92 GHz resonance frequencies using three open-stubs embedded in a conventional two-stage SIR structure. Meandered SIR is employed to maintain the compact size, whereas the addition of the first open-stub is responsible for realizing second resonance frequency and two additional open-stubs explore the extra degree of freedom in individually controlling the resonance frequencies by simply varying the electrical lengths of the resonators effective for each frequency band. The design idea is analyzed and is verified from the mea- sured results obtained after the fabrication, which is in concor- dance with the simulation counterpart. 2. DESIGN, LAYOUT, AND ANALYSIS The design of the proposed open-stub-loaded SIR (OSLSIR) is carried out in three major steps. First, employing meandered SIR model to estimate the first, f 1 , and third, f 3 , resonance fre- quencies. Second, tapping the low impedance section of the SIR with an open-stub to realize the second resonance frequency, f 2 , and finally, identify some effective methods for tuning the third resonance frequency, in particular. Starting with the design of SIR, a conventional SIR having impedances Z 1 with the electrical length, h 1 , and Z 2 with electri- cal length, h 2 , corresponding to the high and low impedance sections, respectively, is studied as shown in Figure 1(a). The input impedance, Z in , of the SIR thus can be expressed as Z in 5jZ 1 ðtan h 1 tan h 2 2R z Þ ðR z tan h 1 1tan h 2 Þ (1) In the above equation (1), R z 5 Z 2 /Z 1 is the ratio of low to high impedance sections of the SIR. At resonance condition, Z in 5 0 such that R z 5tan h 1 tan h 2 (2) Based on the above condition, a SIR is designed having a mean- dered Z 2 as shown in Figure 1(b). The parametric values of the designed SIR are selected as Z 1 5 66.98 X, Z 2 5 90.52 X corre- sponding to the electrical lengths of h 1 5 21.488, h 2 5 73.228, respectively. Following this configuration, the first and third res- onance frequencies operating at f 1 5 2.75 GHz and f 3 5 8.10 GHz are obtained having fractional bandwidths of 33.54 % and 16.22 %, respectively. All the simulations were carried out in a full-wave EM simulation software, SONNET. In the second step, a meandered open-stub resonator, L 3 , having impedance, Z 3 5 112.83 X and electrical length, h 3 5 48.558, is embedded to the previously designed SIR, which is shown in Figure 2. The main objective of adding the open- stub is to realize the second resonance frequency, f 2 , which can be closely estimated by the following equation Figure 1 Simplified design model for a SIR. (a) Conventional two- stage SIR having input impedance, Z in , (b) Designed SIR model having meandered high impedance section, Z 2 DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 59, No. 4, April 2017 815