IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 12, DECEMBER 2009 3395 Applications of Open Split Ring Resonators and Open Complementary Split Ring Resonators to the Synthesis of Artiﬁcial Transmission Lines and Microwave Passive Components Miguel Durán-Sindreu, Student Member, IEEE, Adolfo Vélez, Francisco Aznar, Member, IEEE, Gerard Sisó, Student Member, IEEE, Jordi Bonache, Member, IEEE, and Ferran Martín, Senior Member, IEEE Abstract—This paper is focused on the application of open split ring resonators (OSRRs) and their dual counterparts, open complementary split ring resonators (OCSRRs), to the synthesis of composite right/left-handed transmission lines, that is, artiﬁcial lines exhibiting backward wave propagation at low frequencies and forward wave propagation at high frequencies. Due to the small dimensions of these resonators, the resulting lines are very compact. Several artiﬁcial lines, with different electrical charac- teristics and topologies, are reported as illustrative examples. It is shown that these artiﬁcial lines can be applied to the synthesis of dual-band components and bandpass ﬁlters, and two prototype device examples are designed and fabricated in coplanar wave- guide technology: a dual-band impedance inverter applied to a dual-band power divider, and an order-3 wide-band bandpass ﬁlter. Finally, it is also demonstrated that OSRRs and OCSRRs can be combined for the synthesis of band pass ﬁlters in microstrip technology. Since OSRRs and OCSRRs are described by means of series and shunt resonant tanks, respectively, and they are electrically small, their potential to the design of semi lumped planar microwave devices is very high. Index Terms—Artiﬁcial transmission lines, dual-band compo- nents, metamaterials, microwave ﬁlters, open complementary split ring resonators (OCSRRs), open split rings resonators (OSRRs). I. INTRODUCTION A RTIFICIAL transmission lines based on metamaterial concepts, that is, metamaterial transmission lines, have been a subject of growing interest in recent years. Such lines are artiﬁcial structures consisting on a host propagating medium loaded with reactive elements. Special efforts have been ded- icated to the synthesis of artiﬁcial lines exhibiting backward wave transmission at low frequencies and forward wave prop- agation at high frequencies. These composite right/left-handed lines have been implemented in microstrip [1], [2], coplanar Manuscript received May 13, 2009; revised September 07, 2009. First published November 13, 2009; current version published December 09, 2009. This work was supported in part by Spain-MEC under Project TEC2007-68013-C02-02 METAINNOVA, inpart by the Catalan Government (CIDEM/COPCA) through CIMITEC and under Project VALTEC08-1-0009 COMPATIBLE, and in part by the CONSOLIDER-INGENIO 2010 program (Spain-MCI) under Project CSD2008-00066. The authors are with GEMMA/CIMITEC (Departament d’Enginyeria Elec- trònica), Universitat Autònoma de Barcelona. 08193 Bellaterra (Barcelona), Spain (e-mail: ferran.martin@uab.es). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TMTT.2009.2033867 waveguide (CPW) [3], [4], low-temperature co-ﬁred ceramic (LTCC) [5], and monolithic microwave integrated circuit (MMIC) [6] technologies, among others, and two main ap- proaches have been considered for their fabrication: 1) the capacitor and inductor (CL)-loaded approach, where the host line is loaded with series capacitances and shunt inductances (in practice implemented by means of lumped or semi lumped planar components) [1], [3], [5], [6]; and 2) the resonant-type approach [2], [4], where the line is either loaded with split ring resonators (SRRs) [7] and shunt connected inductive elements [4], or with complementary split ring resonators (CSRRs) [8] and series capacitances [2], [9]. Resonant-type artiﬁcial lines exhibit a transmission zero to the left of the backward wave transmission band, which is re- lated to the presence of the resonant elements (SRRs or CSRRs) coupled to the line. This transmission zero has been used for the design of compact ﬁlters with severe requirements in terms of frequency selectivity [10]. However, in many other applications such transmission zero represents a drawback since it limits the operative bandwidth. In CL-loaded lines, the loading elements and the line elements combined provide a series connected se- ries resonator and a shunt connected parallel resonator to the unit cell of the line, and the transmission zero is located at the origin. In a very recent paper [11], it was demonstrated that sim- ilar characteristics as those of CL-loaded lines can be achieved by loading a CPW with electrically small open resonators, that is, open split rings resonators (OSRRs) [12], and open com- plementary split ring resonators (OCSRRs) [13]. As long as these open resonators are electrically small, the line parasitics do not play a fundamental role, and the unit cell structure can be roughly (although not accurately, as will be discussed later) described by that circuit model of CL-loaded lines. As compared to [11], in this paper we will show improve- ments in the proposed structures and we will clearly demon- strate that the circuit models describing the novel structures provide an accurate description of the main phenomenology. Moreover, it will be shown that these structures can ﬁnd many applications in planar microwave device design. Speciﬁcally, it will be shown that these lines can be applied to the design of compact wide-band bandpass ﬁlters, and dual-band compo- nents. Although this paper is mainly focused on devices imple- mented in CPW technology, it will be also shown that OSRRs and OCSRRs can be combined for the design of microwave 0018-9480/$26.00 © 2009 IEEE Authorized licensed use limited to: Universitat Autonoma De Barcelona. Downloaded on December 15, 2009 at 11:33 from IEEE Xplore. Restrictions apply.