Published in IET Microwaves, Antennas & Propagation Received on 30th November 2009 Revised on 8th April 2010 doi: 10.1049/iet-map.2009.0590 In Special Issue on Microwave Metamaterials: Application to Devices, Circuits and Antennas ISSN 1751-8725 Miniature antennas based on printed coupled lines emulating anisotropy N. Apaydin 1 E. Irci 1 G. Mumcu 2 K. Sertel 1 J.L. Volakis 1 1 ElectroScience Laboratory, Department of Electrical and Computer Engineering, The Ohio State University, 1320 Kinnear Rd., Columbus, OH 43212, USA 2 Department of Electrical Engineering, 4202 E. Fowler Avenue, Tampa, Florida 33620, USA E-mail: apaydinn@ece.osu.edu Abstract: The author presents small antennas that exploit a new class of slow-wave modes. These modes were originally observed in anisotropic volumetric media and were recently realised using a pair of coupled microstrip transmission lines (TLs) printed on a uniform substrate. The simplicity of the coupled printed lines and slow- group velocity of the photonic crystal modes provided an avenue for designing small conformal antennas. The author reviews the degenerate band edge (DBE) mode antennas and extends their bandwidth by supporting dual resonances. This is done by inserting lumped elements along and between the printed TLs. The resulting coupled double loop (CDL) antenna has 14.7% bandwidth and is as small as l 0 /9.8 × l 0 /9.8 (l 0 /16 thick) in footprint. Alternatively, magnetic photonic crystal (MPC) mode is realised using the same DBE-printed structure and by inserting small ferrite sections within the otherwise uniform substrate. This new MPC antenna was realised on a high-contrast (1 r ¼ 10.2) dielectric section and attained a gain of 3.1 dB (73% efficiency) and 8.1% bandwidth on a footprint as small as l 0 /9.8 × l 0 /10.4 (l 0 /16 thick). 1 Introduction Synthetic material composites such as periodic textures displaying novel electromagnetic properties provide new directions in radio frequency (RF) design [1]. For example, forbidden propagation bands in the electromagnetic band gap (EBG) structures [2] can be exploited as high- impedance ground planes to lower the antenna profile (conformability) and reduce coupling [3]. Likewise, the strong resonances provided by defect modes in EBGs transform small radiators into exceptionally directive antennas [4]. Reactive impedance substrates have also been considered in [5] for antenna miniaturisation. Similarly, negative index metamaterials (NIMs) [6–8] lead to sub- wavelength focusing and greater sensitivity lens systems [7]. More recently, printed circuit realisations of NIMs have led to smaller RF devices such as phase shifters, couplers [8] and antennas [7]. Alternatively to NIMs, our group exploited mode diversity and higher order dispersion (K-v; K: Bloch wave number, v: angular frequency) curves of periodically layered anisotropic (and possibly ferrimagnetic) materials for high gain [9], miniature antenna [10] applications. These anisotropic material arrangements, namely degenerate band edge (DBE) [11] and magnetic photonic crystal (MPC) [12], split the conventional K-v curves of otherwise isotropic EBG structures into two branches. Mode coupling between the branches (via misaligned anisotropy) generates novel slow-group velocity modes to be harnessed by directive and small antennas. Specifically, in contrast to second-order relation at the regular band edges (RBEs) of EBGs, K-v curve behaviour at the DBE mode follows a fourth-order polynomial relation. Likewise, MPCs can exhibit spectral asymmetry [i.e. v(K ) = v(– K )] and support third-order stationary inflection points (SIPs) within their propagation bands. In volumetric layered media, these higher order K-v behaviours result in stronger slow-wave resonances to be harnessed in designs of conformal high-gain antenna apertures [9, 10]. When such dispersion relations are realised on traditional microwave substrates (via printed coupled line emulations [13–15]), they offer additional design flexibilities to enable miniaturisation, higher gain and higher bandwidth [16, 17]. IET Microw. Antennas Propag., 2010, Vol. 4, Iss. 8, pp. 1039–1047 1039 doi: 10.1049/iet-map.2009.0590 & The Institution of Engineering and Technology 2010 www.ietdl.org