> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—The radiation properties of split ring resonators (SRRs) at their second resonance frequency are studied for the first time in this work. In particular, the electric and magnetic dipole moments of the edge-coupled SRR (EC-SRR) are calculated analytically under the assumption of strong coupling between the internal and external rings. Based on these results, the radiation resistance and the radiation efficiency are obtained theoretically. Electromagnetic simulations of the structure reveal that there is very good agreement with the theoretical predictions, pointing out the validity of the proposed analysis. As a proof of concept, an SRR antenna prototype is designed and fabricated. Experimental data are in good agreement with the theoretical and simulated results, and demonstrate the validity of the SRR working at its second resonance frequency as a radiating element. Index Terms— Split ring resonators (SRRs), planar antennas, radiation efficiency, metamaterials. I. INTRODUCTION PLIT ring resonators (SRRs), first proposed by Pendry et al. [1] as an evolution of the structures proposed by Schelkunoff [2] and Hardy [3], have been widely used for the implementation of metamaterial-based or inspired microwave devices in the last years [4]. Due to their small electrical size and negative (and very high) magnetic polarizability above the fundamental resonance, SRRs can be used for the implementation of one-dimensional effective media metamaterials [5], including metamaterial transmission lines. Negative permeability (or mu-negative −MNG) [6], left- handed (LH) [7] and composite right-/left-handed (CRLH) [8] transmission lines have been implemented by loading a host line with SRRs (and with other additional elements for LH and CRLH lines). These artificial lines and other artificial lines based on modifications of the SRR topology (including the complementary counterpart, i.e., the CSRR [9]) have been applied to improve the performance and/or the size of microwave components and to implement new functionalities. This work has been supported by MINECO-Spain (projects TEC2010- 17512 METATRANSFER, CONSOLIDER EMET CSD2008-00066, and TEC2013-40600-R COM-SEN-RFID), by FEDER funds, and by Generalitat de Catalunya (project 2014SGR-157). Ferran Martín has been awarded with an ICREA Academia Award. The authors are with GEMMA/CIMITEC, Departament d’Enginyeria Electrònica, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain (e- mail: Jordi.Bonache@uab.es). Compact wideband and ultra-wideband (UWB) filters [10], power dividers [11] and microwave sensors [12]-[13] among other devices, have been implemented by means of SRR- or CSRR-loaded lines. The control of the phase constant and characteristic impedance (a unique feature of metamaterial transmission lines) in SRR- and CSRR-loaded lines has also been successfully applied to the design of leaky-wave antennas with broadside and backward-to-forward radiation capabilities [14]-[16]. Moreover, metamaterial structures and metamaterial-inspired resonators have been widely applied to electrically small antenna design, in order to reduce dimensions [17]-[18], obtain multiband and multi-frequency operation [19]-[21], high radiation efficiency [22], and achieve tuning capability [23]. In many of the works mentioned above, SRRs were used in order to achieve the improvements in the antenna response. The resonance modes of SRRs have been extensively studied in [24], and the values of their resonance frequencies were analytically predicted in [25]. Nevertheless, since the particle is normally designed to work around its first (fundamental) resonance, all the theoretical studies have been focused on the properties of the SRR at that frequency. For instance, the first-order terms of the polarizability tensor of the particle have been quantified in [4],[26],[27],[28] by using the quasi-static analysis at the fundamental resonance. From such analysis, it follows that the SRR acts as a current loop at the fundamental resonance, thus suggesting the possibility of using it as a radiating element. However, since the particle size is in the order of 0.1 free-space wavelengths, and the radiation resistance of a current loop depends on the fourth power of the radius in terms of wavelengths, poor radiation efficiencies and narrow bandwidths are expected. For these reasons, the SRR has been rarely used as stand-alone radiating element at the fundamental resonance [29]. However, as it will be demonstrated in this work, the second resonance of the SRR exhibits interesting antenna properties in terms of radiation resistance and efficiency. In this paper, an analytical study of the radiation resistance associated to the second resonance of the SRR is carried out, providing equations that can be used in the design process to obtain high radiation efficiency and good impedance matching. Given that the radius of the particle at the second resonance is typically in the order of 0.1 free-space wavelength (although it is electrically larger than the radius at the first resonance), the radiator can be treated as an electrically small antenna, according to commonly given definition (kr < 1 [30], where k On the Radiation Properties of Split Ring Resonators (SRRs) at the Second Resonance Simone Zuffanelli, Gerard Zamora, Member, IEEE, Pau Aguilà, Ferran Paredes, Ferran Martín, Fellow, IEEE, and Jordi Bonache, Member, IEEE S