Published in IET Microwaves, Antennas & Propagation Received on 28th April 2008 Revised on 10th December 2009 doi: 10.1049/iet-map.2009.0357 ISSN 1751-8725 Design of a new bandpass filter with sharp transition band using multilayer-technique and U-defected ground structure A. Boutejdar 1 A. Batmanov 2 M.H. Awida 3 E.P. Burte 2 A. Omar 1 1 Chair of Microwave and Communication Engineering University of Magdeburg, Universita ¨tsplatz 02, 39106 Magdeburg, Germany 2 Chair of Semiconductor Technology, University of Magdeburg, Universita ¨tsplatz 02, 39106 Magdeburg, Germany 3 EECS Department, University of Tennessee at Knoxville, TN 37996, USA E-mail: Ahmed.Boutejdar@e-technik.uni-magdeburg.de Abstract: A novel compact microstrip bandpass filter with sharp rejection bands is proposed. The proposed filter structure is composed of U-shape microstrip resonators,‘U-strip’, backed by similarly shaped U-slots ‘U-defected ground structure’ (U-DGS). By controlling the electrical coupling between the U-strip and the U-DGS, the bandpass filter’s stopband is optimised for better rejection. The proposed filter has low insertionloss and compact size because of the slow-wave effect. Meanwhile, sharp rejection bands induced by the presence of two transmission zeros on both sides of its passband are achieved. The measured centre frequency, bandwidth and passband insertion loss are 3.6 GHz, 40% and 0.5 dB, respectively. The simulated and measured results show good agreement and validate the proposed approach. 1 Introduction Recently, defected ground structures (DGS) and electromagnetic band gap (EBG) structures have received increased attention because of their use in mobile communication systems that require compact, high- performance filters and couplers. One of the very successful approaches to achieve significant size reduction is to use DGS components that also have the capability of suppressing harmonics. DGS elements can be used in various kinds of components such as low-pass filters and bandpass filters as well as RF phase shifters. DGS, which evolved from EBG, are realised by etching certain pattern in the metallic ground plane of a microstrip line, which perturbs the current distribution causing an increase in the effective inductance and capacitance of the line. Thus, a DGS element is equivalent to a parallel LC circuit. Planar bandpass filters have been extensively studied and exploited as key circuit blocks with operating functions of in-band transmission and out-of-band rejection [1]. To meet the requirements of modern wireless communication, much effort has been made in recent years to develop a variety of compact bandpass filtesr with sharp and deep rejection outside the passband by generating transmission zeros or attenuation poles. Recent advances in high-temperature superconducting circuits and microwave monolithic integrated circuits have additionally stimulated the development of various planar filters, especially narrow- band bandpass filters that play an important role in modern communication and electronic systems [1, 2]. In order to obtain compact three-pole microstrip bandpass filters with two transmission zeros, low stopband performance, low insertion loss in passband and high selectivity, several structures were proposed such as filters using end-coupled slow-wave resonators [3], slow-wave open-loop resonator filters [4, 5] and slow-wave open stub tapped resonator filters [1]. Other techniques employ cross coupling and quasi-elliptic function filters that are able to place the transmission zeros near the cutoff frequencies so that higher selectivity with fewer resonators can be obtained [6]. IET Microw. Antennas Propag., 2010, Vol. 4, Iss. 9, pp. 1415–1420 1415 doi: 10.1049/iet-map.2009.0357 & The Institution of Engineering and Technology 2010 www.ietdl.org