RESEARCH ARTICLE A novel compact dual-mode SIW filter with wide rejection band and selective response Ahmad Mahan 1 | Seyed Mojtaba Pourjafari 1 | Majid Tayarani 1 | Mohammad Shahidzadeh Mahani 2 1 Electrical Engineering Department, Iran University of Science and Technology, Tehran, Iran 2 Department of Electrical and Computer Engineering, McGill University, Montreal, Québec, Canada Correspondence Ahmad Mahan, Electrical Engineering Department, Iran University of Science and Technology, Tehran, Iran. Email: ahmad_mahan@elec.iust.ac.ir Abstract A compact dual-mode substrate-integrated waveguide (SIW) bandpass filter with wide upper stopband is presented. A cross shaped slot is etched on the top plane of SIW cavity to alter the second mode (TE 201 ) current distribution and move it closer to the first mode (TE 101 ), to enhance the bandwidth flatness. Removing few via near the excitation ports, provides longer path for the current to resonance and give us the transmission zeros (TZ1) in a simple way. Another 2 TZs are used to improve the stopband by suppressing higher order modes resonances. The designed filter is fabricated and tested and a very good result is achieved to confirm the design idea. There is also a good agreement between the measured and Full Wave simulated results. The fabricated sam- ple realizes a fractional bandwidth of 9.1% at 7.5 GHz center frequency and 2 zeros at 12.5 GHz and 15 GHz. KEYWORDS dual-mode filter, substrate-integrated waveguide (SIW), transmission zero and coplanar waveguide (CPW) 1 | INTRODUCTION Substrate-integrated waveguide (SIW) has many benefits such as high quality factor, low cost implementation and easy inte- gration in a planar platform. Many SIW components, such as filters, couplers, antennas, and so on, have been widely stud- ied and used in various microwave systems. 1–3 A dual-mode SIW resonators use 2 modes excited at the same time and causes reduce the number of physical resonators in a SIW filter and therefore improve the area efficiency. Dual mode resonators are realized by perturb- ing the electromagnetic fields of the SIW resonator in a proper way according to the desired exciting mode. Usually, a slot on the top metal layer or a metallic via from the top plate to bottom is used to excite the second mode and obtaining a dual mode operation. 4 Other tech- niques are also reported in the literature such as comple- mentary split ring resonators for implementing dual mode SIW filters, 5 or a cavity corner cut to achieve dual mode performance as in Ref. 1. A dual mode SIW filter using a metallic via in the middle of the structure is presented in Ref. 6. Although being area efficient, dual mode filters usu- ally suffer from higher order resonances which lead to poor out of band rejection. One of the solutions for this shortcoming is to cascade the dual mode filters which in turn increases this region. 8 Using defected ground struc- tures (DGSs) is another solution that is presented in Ref. 11 to improve the stop-band behavior. Although shown effective in improving the stop-band, DGS components pose difficulties on the integration of the system since the area where the ground is defected, cannot touch the any metallic surface. In this article, a single layer dual-mode SIW filter with slot lines perturbation is proposed. The proposed filter uses a new technique to achieve transmission zeros in the stop band. It is shown that the transmission zeros can be designed to suppress the higher order resonances. It is also shown that the pass-band of the filter can be adjusted independent to the transmission zeros which gives the designer great flexibility to shape the overall frequency response. The rest of the article presents the analysis and design of the dual mode filter followed by presentation of the measured results. Comparisons between the measured results and simulations as well as the state of the art are presented in section. Received: 5 July 2018 DOI: 10.1002/mop.31620 Microw Opt Technol Lett. 2018;1–5. wileyonlinelibrary.com/journal/mop © 2018 Wiley Periodicals, Inc. 1