actual circumference of the proposed spiral-loop antenna is signif- icantly reduced by 33%. This CPS-fed CP antenna will be very useful for exploring a new class of printed CP array using the CPS feeding technique, as in [3, 5]. REFERENCES 1. H. Morishita and K. Hirasawa, Wideband circularly polarized loop antenna, Proc IEEE AP-S Int Antennas Propagat Symp Dig 2 (1994), 1286 –1289. 2. H. Morishita, K. Hirasawa, and T. Nagao, Circularly polarised wire antenna with a dual rhombic loop, IEE Proc Microwave Antennas Propagat 145 (1998), 219 –224. 3. B. Strassner and K. Chang, 5.8-GHz circularly polarized dual-rhombic- loop travelling-wave rectifying antenna for low power-density wireless power transmission applications, IEEE Trans Microwave Theory Tech 51 (2003), 1548 –1553. 4. S. Makoto, H. Kazuhiro, and S. Shi, Two rectangular loops fed in series for broadband circular polarization and impedance matching, IEEE Trans Antennas Propagat 52 (2004), 551–554. 5. L. Zhu and K. Wu, Model-based characterization of CPS-fed printed dipole for innovative design of uniplanar integrated antenna, IEEE Microwave Guided Wave Lett 9 (1999), 342–344. © 2005 Wiley Periodicals, Inc. APPLICATION OF COMPLEMENTARY SPLIT-RING RESONATORS TO THE DESIGN OF COMPACT NARROW BAND-PASS STRUCTURES IN MICROSTRIP TECHNOLOGY Jordi Bonache, 1 Ferran Martı ´n, 1 Francisco Falcone, 2 Juan D. Baena, 3 Txema Lopetegi, 2 Joan Garcı ´a-Garcı ´a, 1 Miguel A.G. Laso, 2 Ignacio Gil, 1 Antonio Marcotegui, 4 Ricardo Marque ´ s, 3 and Mario Sorolla 2 1 Department d’Enginyeria Electro ` nica Universitat Auto ` noma de Barcelona 08193 Bellaterra (Barcelona), Spain 2 Departamento de Ingenierı ´a Ele ´ ctrica y Electro ´ nica Universidad Pu ´ blica de Navarra Campus Arrosadı ´a E-31006 Pamplona, Spain 3 Departamento de Electro ´ nica y Electromagnetismo Universidad de Sevilla 41012 Sevilla, Spain 4 CONATEL s.l. Sancho Ramı ´rez, 1-3 31008 Pamplona (Navarra), Spain Received 28 February 2005 ABSTRACT: In this paper, a compact and narrowband microstrip band-pass structure based on complementary split-ring resonators (CS- RRs) etched in the back metal level (ground plane) is presented. Specifi- cally, the structure is a two-stage CSRR-based device, where a series gap is etched in the output CSRR stage and two shunt stubs are added in the input cell. By these means we obtain a narrow and quite symmet- ric band-pass structure. A prototype device with 2% fractional band- width has been designed and fabricated for operation at the S-band. The dimensions of the device are as small as 14.6  11 mm, while high- frequency selectivity is achieved at both band edges due to the presence of two transmission zeros. To demonstrate the possibility to control the bandwidth over a narrow band, a wider (10% bandwidth) filter has been also designed and fabricated. These structures can be of interest for application in narrow band-pass filters where miniaturization and compatibility with planar technologies are key issues. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 508 –512, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21031 Key words: complementary SRRs; duality; microstrip technology; mi- crowave filters 1. INTRODUCTION Ever since the first experimental verification of left-handedness [1], the number of works devoted to artificially fabricating mate- rials (metamaterials) able to exhibit the unique electromagnetic properties predicted by Veselago [2] in the late 1960s has dramat- ically increased. These properties (namely, the reversal of Snell’s law, the Doppler effect, and Cherenkov radiation) are closely related to backward-wave propagation, which is in turn due to the simultaneous negative values of dielectric permittivity  and mag- netic permeability  in the medium [2]. Among these left-handed materials (LHMs), also called double-negative (DNG) media, those based on split-ring resonators (SRRs) have attracted great interest. Proposed by Pendry [3], SRRs are subwavelength reso- nators that consist of a pair of concentric metal rings on top of a dielectric slab with splits etched in opposite sides (see Fig. 1). Arranged periodically, these constituent particles cause the struc- ture to behave as an anisotropic effective medium with negative permeability in the vicinity of resonance [1]. Anisotropy comes from the fact that the magnetic field vector of incident radiation Figure 4 Two sets of predicted and measured radiation patterns on the y–z plane 508 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 46, No. 5, September 5 2005