This paper reviews the application of electromagnetic bandgap (EBG), artificial magnetic conductor (AMC) and left handed Meta-surface technologies for antenna applications. Examples of AMC configurations in the microwave region include low-profile high-gain planar array antennas without vias, omni-directional H-plane and directive E-plane cylindrical EBG antennas and left handed Meta-surfaces as uniform superstrates based on a finite periodic repetition of a Left-Handed unit cell. Passive periodic arrays of metallic elements or apertures in metallic sheets have been studied for many years as Frequency Selective Surfaces (FSS). The filtering properties of single and multi-layer FSSs for normal and oblique plane wave incidences have been extensively investigated and applied to antenna and waveguiding systems for wireless communications applications. FSS technology has been used in the design of subreflectors and radomes with reduced RCS, leaky-wave antennas, multiband horn antennas, and spatial filters. Since their discovery in the late 1980’s, interest in metamaterials has grown explosively. The analysis and design of metamaterial structures for microwave and millimetre wave frequency has produced very interesting research. The potential take-up of these structures in applications like indoor and outdoor communication systems is primarily due to the control of the amplitudes, frequencies and wave-numbers of propagating and non propagating electromagnetic modes enabled by metamaterials into an extent that was not previously possible. These artificially engineered materials are generically known as photonic bandgap (PBG) materials or photonic crystals. Although "photonic" refers to light, the principle of "bandgap" applies to electromagnetic waves of all wavelengths. More recently metallic arrays printed on metal-backed (grounded) dielectric substrates in presence and the absence of vias have been investigated as Artificial Magnetic Conductors (AMC). These high impedance surfaces, i.e. metamaterials, fully reflect incident waves with a near zero degrees reflection phase and can be utilised as ground planes for antenna applications. AMC surfaces are used to minimise and miniaturise the cost of communication components and systems. A new class of low-profile highly-directive planar antennas has been developed based on EBG superstrates and AMC ground planes. Examples of AMC and EBG technology for planar and cylindrical antennas are presented in this paper. The structures are based on the formation of a resonant cavity between the ground plane and the array (planar), and an array only contained cavity (cylindrical) that act as a partially reflective surfaces (PRS) similar to the concept that was originally proposed in. A plane wave impinges on a screen that is partially reflective and partially transmitting. All the reflected rays propagate laterally and then are redirected toward the partially reflecting screen, so that overall they generate leaky wave propagation predominantly in the broad side direction. At the cavity resonant frequency maximum directivity is obtained. While high gain antenna designs have been produced, the antenna profile, which is determined by the resonance condition of the cavity, has always been close to half wavelength. An example of the use of Meta-surfaces on top of dipole antennas in order to enhance their principal parameters is presented. The main advantages which are derived from the results of this paper are the compactness (less than ë/4) and the high overall efficiency (around 85%). These Meta-surfaces are based Left Handed Materials (LHMs), structures which exhibit negative refraction. These LHMs can be understood as resonators, exhibiting pass band and stop band properties at which the power is transmitted or reflected respectively. Recently, applications of LHMs have shown the benefits of their pass band properties using them as superstrate of planar antennas with the goal of improving their radiation behaviour.

Periodic assemblies of materials have been shown to have unique and useful properties for microwave applications. Examples of these are the bandgap structures, the left handed materials, and other related periodic assemblies. Among them, the magnetic photonic crystals (MPC) and their related "cousins" degenerate band edge (DBE) structures have been shown to lead to significant wave slow down and amplitude increase within a small region. These crystals have therefore been found very attractive for miniature, high sensitivity antennas and possibly miniature microwave devices. However, their anisotropic nature makes their fabrication challenging. Being able to emulate the MPC or DBE properties using printed circuit technology will provide for a significant step in making low cost high performance devices based on MPCs and DBE crystals. In this paper we propose a novel coupled microstrip line circuit which emulates propagation through an anisotropic medium such as the MPC or DBE crystal. The microstrip line model is formed from a pair of coupled and uncoupled lines (one line per field component, Ex and Ey) and is the first (to our knowledge) representation of the propagation within an anisotropic layered medium. Using the standard scattering matrix representation for the three consecutive layer sections forming the DBE unit cell, we calculate the band diagram and show that the periodic microstrip structure supports a DBE for a specific design that can be readily fabricated. It is particularly interesting to show that the small changes in the microstrip circuit can lead to various band diagrams emulating a variety of anisotropic media, thus, allowing for the possibility to examine and introduce new phenomena leading to novel devices for RF applications.

Composite right/left-handed metamaterial antennas for dual-band and dual-mode applications are presented. In particular, the fundamental backward wave of an anisotropic composite right/left-handed metamaterial is used to realize dual-band antennas. By using the infinite wavelength and backward wave of the composite right/left-handed metamaterial, dual-mode antennas for radiation selectivity are realized.

This paper presents the theoretical and experimental characterization of a multilayered volumetric negative-refractive-index transmission-line (NRI-TL) metamaterial [1]. An equivalent transmission-line model provides expressions for the effective material parameters of the medium and yields its dispersion and transmission properties. These approximate results are supported by finite-element full-wave simulations illustrating the phase-restoration and transmission properties of a volumetric NRI-TL slab lens with refractive index -1 and matched to free space, for a plane-wave incident over a wide range of angles. Related experimental results for a fabricated prototype at X-band will also be presented.

[1] A.K. Iyer and G.V. Eleftheriades, "A volumetric layered transmission-line metamaterial exhibiting a negative refractive index," Journal of the Optical Society of America (JOSA-B), vol. 23, no. 3, pp. 553-570, March 2006.

In this contribution, we present a possible realization of a miniaturized circular patch antenna with metamaterial loading. We have already shown in [1] how it is possible to excite a resonant mode in a sub-wavelength patch by loading it with materials with oppositely-signed real parts of permittivities and/or permeabilities. When the material parameters are judiciously chosen, in principle the patch resonance does not depend on its total size, but only on the filling factor of the two employed materials. An analogous concept had already been proposed in [2] for the design of miniaturized 1D cavities and, then, extended in order to squeeze the dimensions of other microwave devices, such as resonators, scatterers, absorbers, antennas, waveguides, etc. A review of some efforts in this field may be found in [3]. As to the application of this concept to resonant sub-wavelength rectangular and circular patches loaded with metamaterials, we have shown that, although in both geometries we are able to keep the resonant dimensions of the patch considerably smaller than the operating wavelength, in the rectangular geometry, however, the antenna does not radiate properly, since the field distribution of the fundamental mode excited in this geometry gives rise to out of phase radiating contributions from the two patch edges, which, when their distance gets too small, cancels out most of the radiation [1]. On the other hand, the circular geometry can effectively provide more degrees of freedom in the design, due to the various different orders of the resonant modes that can be supported by this geometry [1]. In such a case, a judicious choice of the geometry and of the filling materials can indeed select the proper angular variation for the resonant modes, again independently of the total size of the patch. In this way, the current distribution on a circular sub-wavelength patch can be properly adjusted to provide an efficient radiation. In this talk, we will provide further insights into the physical mechanisms behind the resonance and radiation features of such sub-wavelength circular patches and we will present some full-wave simulations confirming our theoretical investigations. We will also present some preliminary numerical results concerning the practical implementation of the required metamaterials through properly designed resonant inclusions to be embedded underneath the patch. References [1] A. Alù, F. Bilotti, N. Engheta, and L. Vegni, "Radiation Properties of Sub-Wavelength Resonant Patch Antennas Filled with a Pair of DPS, DNG, and/or SNG Metamaterial Blocks," Dig. of USNC/CNC/URSI National Radio Science Meeting, Washington, DC, USA, p. 113, July 3-8, 2005; an extended version has been submitted to IEEE Trans. Antennas Propagat, and is currently under review. [2] N. Engheta, "An Idea for Thin Subwavelength Cavity Resonators Using Metamaterials with Negative Permittivity and Permeability," IEEE Antennas and Wireless Propagation Letters, Vol. 1, No. 1, pp. 10-13, 2002. [3] F. Bilotti, "Application of metamaterials for miniaturized components," Metamaterials for Industry, Short Course for Industries and SMEs, Jouy-en-Josas, France, 28-30 November 2005

Advances in technology will continue to require smaller circuitry to further reduce the overall size of current communication and sensor systems. One approach to achieve this reduction in size is to develop small, efficient and multi-band structures that will fit into restricted areas while simultaneously accommodating the desired frequencies and high data rates. The conventional design of antennas in high density systems is a difficult task due to the large coupling that exists between the individual radiating elements in the system when they are in close proximity to each other. This coupling changes both the field distributions and input impedances of each radiating element. Thus, it is a common practice to place array elements one-half wavelength away from each other to minimize this coupling.

An electrically small dipole antenna in free space is defined by the constraint that ka ≤0.5, where k is the wave number and a is the radius of the sphere which minimally surrounds the entire antenna. Recent research work on efficient metamaterial-based (MTM-based) electrically small antennas have shown that it is theoretically possible to obtain highly efficient radiating systems that are on the order of χwavelength/83 is size [1], [2], e.g. about 6.6 times smaller than the electrically small antenna constraint. Analytically derived properties of the individual MTM-based antennas will be presented. These results are verified numerically with ANSOFT’s High Frequency Structure Simulator (HFSS). The MTM-based antenna properties are further exploited to obtain multi-band enhanced radiating and reduced coupling/radiating properties. These results are based on a hybrid GA-MATLAB® optimization model that has been integrated with the analytical model [3]. Dispersion properties of these MTM media are included in all the analytical models. We present our investigations on the possibility of using these MTM-based antennas in an array configuration that includes a number of elements and yet is electrically small. The near- and far-field radiation characteristics of such an ultra-dense antenna array will be discussed. We will demonstrate a variety of radiating and coupling properties of these MTM-based antennas when they are incorporated into such an ultra-dense array. The directivity and bandwidth characteristics of these systems will be discussed.

[1] R. W. Ziolkowski and A. Erentok, "Metamaterial-based efficient electrically small antennas," to appear in IEEE Trans. Antennas Propagat., 2006.

[2] R. W. Ziolkowski and A. Erentok, "At and beyond the Chu limit: passive and active broad bandwidth metamaterial-based efficient electrically small antennas,"submitted to IEE Proceeding, December 2005.

[3] A. Erentok, and R. W. Ziolkowski, "A hybrid optimization method to analyze metamaterial-based electrically small antennas," submitted to the IEEE Trans. Antennas Propagat., March 2006.

Growing interest to metamaterials and their unusual properties in the course of last several years, stimulated invention and design of numerous microwave devices, based on transmission-line representation of metamaterials [1, 2]. In particular, compact and broadband phase shifters [3, 4, 5], power dividers [6] and antenna baluns [7] have been reported, and lots of varieties of similar devices designed. In spite of these developments, there is still room for enhancement.

We report an advanced phase compensation principle, which employs combined backward-forward transmission lines having similar frequency dispersion. Various applications of this principle to the design of microwave devices allow for an exceptionally low dispersion in a wide frequency range while keeping the structure very compact and simple compared to conventional solutions. To illustrate the idea, we present the performance of (i) phase shifters, (ii) power dividers and (iii) baluns (complete antenna feeders) built on the reported principle.

Corresponding phase shifters are characterized with negligible frequency dependence in a wide frequency range. We show that for an ideal performance, phase deviation can be less than 1° within a 20% bandwidth while excellent impedance matching is retained. The power dividers offer a performance, comparable with conventional Wilkinson dividers, occupying, however, two times smaller area or even less. Such size reduction is especially relevant in view of miniaturization trend in modern technology. For antenna feeders, the reported principle opens an excellent opportunity to combine a power divider and an adjacent balun into one simple structure, also leading to essential size reduction while keeping characteristics, comparable with conventional assemblies.

We support theoretical estimates by microwave circuit simulations and direct measurements, showing that the novel devices can be easily implemented with simple electronic components.

References:

[1] G. V. Eleftheriades, "Enabling RF/microwave devices using negative-refractive-index transmission-line metamaterials," Radio Science Bulletin, 312:57, 2005.
[2] C. Caloz and T. Itoh, "Metamaterials for high-frequency electronics," Proc. IEEE, 93(10):1744, 2005.
[3] M. A. Antoniades and G. V. Eleftheriades, "Compact Linear Lead/Lag Metamaterial Phase Shifters for Broadband Applications," IEEE Antenn. Wireless Propag. Lett., 2:103, 2003.
[4] I. Vendik, O. Vendik, and S. Zubko, "Composite right/left-handed transmission line phase shifter," Proc. EPFL Latsis Symposium 2005, p. 107 (2005).
[5] H. Kim, A. B. Kozyrev, A. Karbassi, and D. W. van der Weide, "Linear tunable phase shifter using a left-handed transmission-line," IEEE Microwave Wireless Compon. Lett., 15(5):366, 2005.
[6] M. A. Antoniades and G. V. Eleftheriades, "A Broadband Series Power Divider Using Zero-Degree Metamaterial Phase-Shifting Lines," IEEE Microwave Wireless Compon. Lett., 15(11):800, 2005.
[7] M. A. Antoniades and G. V. Eleftheriades, "A Broadband Wilkinson Balun Using Microstrip Metamaterial Lines," IEEE Antenn. Wireless Propag. Lett., 4:209, 2005.

The needs for mobility lead to the use of increasingly complex base stations. To address the need of multi-band and multi-beams antenna capacities (GSM, DCS and UMTS), we have tried to replace the complex classical feeders systems (RF components, Butler matrix, …) by the association of an omnidirectional probe with a cylindrical reconfigurable Electromagnetic Band Gap. The main advantage of this new kind of antenna is to simplify the beam steering by the biasing of PIN diodes inserted along the metallic wire that composed the EBG material. The diodes control the size of the composite wire: long metallic wire when the diodes are ON and the EBG material is reflector, discontinuous wires when the diodes are OFF and the EBG material is transparent. So azimuth angle, the beamwidth and the number of beams are managed by the two states of the EBG material in the different angular sectors of the cylindrical lattice thanks to the biasing of the diodes with a "simple DC voltage".

The measurements of the adaptive antenna breadboard showed the capabilities to manage the beam(s) in the required direction(s) with the adequate beamwidth(s) thanks to the reconfigurable EBG material including PIN diodes. Up to 1.7 GHz, these diodes have “good” performances and we can steer the beam(s) in the azimuth plane with controlling the aperture(s) of the EBG material and match the antenna by small modification in the ON/OFF wire distribution. But beyond 1.7 GHz, we can no longer monitor the radiation. This phenomena is partly related to the reverse capacitance of the PIN diodes that is not enough small to simulate an open circuit when the diodes are OFF. The design of this antenna has been made without taking into account the parasitic elements of the PIN diodes in its two states.

So in order to avoid this problem in next designs and studies of reconfigurable material, we have introduce in our code SR3D the capabilities to taking into account the equivalent RF circuits of the diodes thanks to RLC lumped components. This implementation will reduce the dubiousness of the simulations due to the integration of "simple" electronic components and their parasitic elements that modify the behavior and the performances of the radiating structures.

In a first step, we have simulated several "heoretical PIN diodes" to estimate the minimal capacitance of the OFF diode that allow to steer the radiation up to 2.2 GHz and evaluate the losses in the lumped components for a single 60° aperture in the EBG material. Several capacitances have been tested and compared to the experiments, the inductance, capacitance of the package and resistance of the junction have been taking into account. These simulations show that a capacitance about 0.1 pF, for the reverse diodes, is the upper limit to manage the radiation and to have losses compatible with the use of the antenna.

If we compare the experiments and the simulations, we can estimate the capacitance of the reverse diodes about 0.3 pF to obtain the same behavior of the directivity, the gain curve is different for the lower frequencies (< 1.0 GHz).

After a parametric study of the RLC equivalent circuits of the diodes, we will present an optimization of the cylindrical reconfigurable EBG material in order to obtain a beam steering in all the frequency bands with the lowest losses. This optimization is applied on the diodes characteristic or on a new distribution of the wire in the EBG material.

Composite Right and Left Handed (CRLH) metamaterials have been applied to antennas and microwave circuits designs recently [1] [2]. The CRLH ring antenna proposed in [1] possesses dual band property with the same current distribution, which is desired in wireless communications. This antenna radiates with linear polarization. In this paper, a dual band circularly polarized antenna based on the CRLH ring structure is designed, which is depicted in Fig. 1. Two perpendicular modes are excited and a quadrature hybrid feeding system [2] is designed to provide 90° phase shift at both frequency bands. Moreover, this new design allows for reversed phase shifts at the two resonant frequencies. Thus, the antenna has right handed circular polarization at one frequency and left handed circular polarization at the other frequency. The simulated axial ratios at both resonances are plotted in Fig.2.

References [1] C. Caloz, and T. Itoh, Electromagnetic Metamaterials, Transmission Line Theory and Microwave Applications. New York: Wiley, 2006, ch. 6. [2] I.-H. Lin, M. DeVincentis, C. Caloz, and T. Itoh, "Arbitrary Dual-Band Components Using Composite Right/Left- Handed Transmission Lines," IEEE Trans. Microwave Theory Techniques, vol. 52, pp.1142-1149, April, 2004.

Fig. 1. Antenna geometry.

Fig. 2. Axial ratios at two resonant frequencies.

Our investigation of two-dimensional (2D) and three-dimensional (3D) arrays of magnetodielectric spheres is motivated in part by the recent theoretical demonstration by Holloway et al. that a doubly negative (DNG) material can be formed by embedding an array of spherical particles in a background matrix. [1] The work described here builds on and extends our earlier investigations of traveling waves on linear periodic arrays of acoustic monopoles [2], electric dipoles [3], and magnetodielectric (penetrable) spheres [4,5], using a spherical-wave source scattering-matrix formulation.

The general class of problems we consider is as follows. We have a periodic array of identical elements each characterized by a scattering coefficient that relates the field scattered from the element to the field incident on the element. It is assumed that only the lowest order multipole fields are significant in analyzing scattering from the array elements. The spacing of the elements in the direction parallel to the array axis is denoted by d and in the direction(s) normal to the array axis by h. Our interest is in lossless traveling waves that can be supported by the array in the direction parallel to the array axis. The focus of our attention is the kd -- βd equation (or diagram) that relates the traveling wave electrical separation distance βd of the array elements in the direction parallel to the array axis to the corresponding free-space electrical separation distance kd.

For the arrays we consider, an initial form of the kd -- βd equation is obtained very simply by assuming a traveling wave excitation of the array and summing the electromagnetic fields incident on a reference element from all the other elements of the array. Unfortunately these summations converge so slowly as to make the initial forms of the kd -- βd equations useless for calculation purposes. Accordingly we convert the slowly convergent summations to rapidly convergent forms by using either the Poisson summation formula or two different methods based on Floquet mode expansions, combined with expressions for efficiently summing Schlomilch series.

In the paper we show plots of the kd -- βd diagrams for 2D and 3D infinite periodic arrays of different magnetodielectric spheres. We demonstrate that backward traveling waves with group velocity and phase velocity in opposite directions characteristic of DNG materials can be supported by 2D and 3D arrays of penetrable spheres, and that these arrays can be characterized by negative effective relative permittivity and permeability.

REFERENCES
[1] C.L. Holloway et al., IEEE Trans. Antennas Propagat., 51, pp. 2596-2603, 2003.
[2] A.D. Yaghjian, IEEE Trans. Antennas Propagat., 50, pp. 1050-1064, 2002.
[3] R.A. Shore and A.D. Yaghjian, AFRL Technical Report, AFRL-SN-HS-TR-2004-045, 2004. (PDF file available on request).
[4] R.A. Shore and A.D. Yaghjian, Electronics Letters, 41, pp. 578-580, 2005.
[5] R.A. Shore and A.D. Yaghjian, IEICE Trans. Commun., E88-B, pp. 2346-2352, 2005.