174 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 12, 2013 Novel Miniaturized Artificial Magnetic Conductor R. C. Hadarig, M. E. de Cos, Member, IEEE, and F. Las-Heras, Senior Member, IEEE Abstract—The design of a novel miniaturized artificial mag- netic conductor (AMC) using interdigital capacitors is presented. The operation of the AMC is evaluated using finite element method (FEM) simulations. A prototype is manufactured and characterized based on reflection coefficient phase in an anechoic chamber. The performance of the AMC unit cell for both normal and oblique incidence is also studied. Index Terms—Artificial magnetic conductor (AMC), frequency selective surface (FSS), perfect electric conductor (PEC), periodic boundary conditions (PBC), reflection coefficient. I. INTRODUCTION M ETAMATERIALS are deliberately designed to ex- hibit novel electromagnetic properties not found in nature. Some examples of microwave metamaterials include left-handed media [1], electromagnetic band-gap (EBG) ma- terials [2], as well as bianisotropic media [3] and artificial magnetic conductors (AMCs). An AMC consists of a frequency selective surface (FSS) placed above a perfect electric conductor (PEC) ground plane, with a dielectric material in between [4]. It exhibits a reflec- tion coefficient of 1 at a given frequency (the phase of the reflection coefficient is 0 ), as opposed to a PEC that has a reflection coefficient of 1 [5]. The AMC operation bandwidth is generally considered in the frequency range corresponding to reflection phase variation from to . Modern communication systems require small microwave components, so miniaturization has become increasingly im- portant for applications of AMCs where physical space is constrained [6]. Traditionally, at microwave frequencies, the AMC structure has a unit-cell size of about half to a quarter of a wavelength [7], making the overall AMC prohibitively large if it is used as backing plane for antennas [8]. Designing AMC structures operating at low frequencies is relatively challenging because in terms of its wavelength, the unit cell can still be large. To overcome size limitation, modified cell geometries loaded with lumped capacitors have been discussed [9]. With this method, the resonance frequency relies not only on the physical size of the periodic element, but also on the values of the lumped components. The method’s disadvantage is the difficult fabrication process and cost. Manuscript received January 11, 2013; accepted January 29, 2013. Date of publication February 05, 2013; date of current version March 12, 2013. This work was supported by the Ministerio de Ciencia e Innovación of Spain/FEDER under Projects TEC2011-24492 (ISCAT) and CONSOLIDER-INGENIO CSD2008-00068 (TERASENSE), the Gobierno del Principado de As- turias (PCTI)/FEDER-FSE under Project PC10-06 (FLEXANT), and Grant BP10-039. The authors are with the Electrical Engineering Department, University of Oviedo, 33203 Gijón, Spain (e-mail:rhadarig@tsc.uniovi.es). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2013.2245093 In this letter, a miniaturized AMC using interdigital capac- itors and without via connections is presented. The AMC is designed in the C-band frequency range (more specifically at 4.15 GHz using RO3010 and 6.2 GHz using RO4003C sub- strate), usable in military applications as ground plane for an- tennas, by means of finite element method (FEM) techniques. The results of the simulations are presented and compared to the measured data taken in an anechoic chamber. The letter starts by investigating the reflection phase properties of the pro- posed AMC, and then the unit cell is characterized in terms of transverse electric (TE) and transverse magnetic (TM) polarized waves for both normal and oblique incidence. II. PLANAR AMC DESIGN The resonant nature of an AMC results in frequency depen- dence and generally narrowband operation. The resonance fre- quency is determined by the geometry and dimensions of the elements comprising the design together with the substrate’s thickness and relative dielectric permittivity. When the unit cell is much smaller than the wavelength of operation, the AMC can be modeled as a distributed network giving rise to a reso- nant frequency at which the surface impedance of the AMC tends to be very large while the in-phase reflection bandwidth is proportional to [5]. The presented unit-cell design consists of four rectangular metal pads placed on each corner of the unit cell, two striplines, and respectively two interdigital capacitors connecting each rectangle and another line placed perpendicularly in the middle of the unit cell [see Fig. 1(a)]. The four rectangular pads are re- sponsible for the capacitive behavior (increasing the pad width, the resonance frequency and bandwidth decrease), whereas the other strips provide the inductive behavior (making the strips narrower, the resonance frequency decreases and bandwidth increases). Moreover, the gap between adjacent unit cells introduces capacitive coupling and is a key factor for designing smaller unit cells at lower frequency (decreasing the gap, the resonance frequency decreases and bandwidth becomes narrower). With the purpose of minimizing the unit-cell size, a tradeoff solution regarding and substrate thickness has to be adopted. To accurately identify the electromagnetic properties of the AMC structure, FEM and Bloch–Floquet theory are used to an- alyze its performance. A single cell of the structure with periodic boundary condition (PBC) on its four sides is simulated in order to model an infinite structure [10], [11]. To obtain the AMC re- flection coefficient, a wave port is placed at half a wavelength above the surface, and normal plane waves are launched. The reflection phase of the AMC structure, which is defined as the phase of the reflected electric field normalized to the phase of the incident electric field at the reflecting surface, will be com- pared to that of a PEC plane taken as reference [5]. The unit-cell geometry that exhibits one symmetry plane is shown in Fig. 1(a). For the unit-cell dimensions 1536-1225/$31.00 © 2013 IEEE