Design of a Compact Dual-Band Folded-Waveguide Bandpass Filter Using Multilayer Waveguide Resonators Sultan Almorqi, 1 Hussein Shaman, 1 Jia-Sheng Hong, 2 Osama M. Haraz 3,4 1 National Centre for Sensors and Defense Systems Technologies (NCSDST), King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia 2 School of Engineering & Physical Sciences; Sensors, Signals & Systems, Heriot-Watt University, Edinburgh Eh14 4AS, United Kingdom 3 KACST Technology Innovation Center in Radiofrequency and Photonics for the e-Society (RFTONICS), King Saud University, Riyadh 11451, Saudi Arabia 4 Electrical Engineering Department, Faculty of Engineering, Assiut University, Assiut 71515, Egypt Received 16 March 2015; accepted 18 May 2015 ABSTRACT: A novel compact dual-band bandpass filter based on multilayer folded- waveguide (FWG) structure is presented in this article.In this design, slots are used to real- ize direct coupling between adjacent resonators and apertures are adopted to implement cross coupling between non-adjacent resonators.A new technique of external quality factor of FWG resonator and coupling coefficients between different resonators are studied using full-wave simulator. In order to demonstrate the proposed technique, a four-pole dual-band bandpass filter is designed, fabricated and measured using vector network analyzer. Mea- surement results which are well agreed with simulation results are presented. Moreover, four-pole substrate integrated folded waveguide SIFW dual-band bandpass filter, using two layers of slotted folded waveguide resonators, is demonstrated. The proposed filter has a compact size and it is excellent candidate for the application of wireless communication sys- tems. V C 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:780–788, 2015. Keywords: folded-waveguide resonators; microwave filters; dual passband filter; SIFW I. INTRODUCTION Waveguide and cavity resonator structures are widely used in numerous communication technologies such as satellite systems and long range wireless communication applications by virtue of their low-loss and robustness. The increasing demands of wireless communication appli- cations necessitates RF filters operating in multiple sepa- rated frequency bands to allow users to access various services with only single multimode handset or terminal. As wireless systems converge into common devices, it is required to design miniaturized multiband filters that can select more than one passband at the same time. In order to design a dual band bandpass filter with a compact size, low-temperature cofired ceramic (LTCC) technology can be implemented [1, 2]. A synthesis method of a self- equalized dual-band bandpass filter is reported in [3]. In [4], a compact dual band filter is designed using two modified open-loop microstrip resonators. Polynomial approaches and coupling matrices can be applied to design bandpass filters with two passbands separated by one rejection band [5] and bandstop filters with single or dual stopbands [6] using waveguide technology. Many dual-band bandpass filters have also been developed using different methods and structures [7–11]. Even though the size of the so-called folded-waveguide (FWG) quarter- wavelength resonator is only a quarter of the footprint of the conventional TE 101 waveguide resonator, it is still pre- serving the same high-Q property with a low profile. Examples of FWG quarter-wavelength resonator bandpass Correspondence to: O. M. Haraz (oharaz@ksu.edu.sa) DOI: 10.1002/mmce.20916 Published online 1 June 2015 in Wiley Online Library (wileyonlinelibrary.com). V C 2015 Wiley Periodicals, Inc. 780