Sergei A. Tretyakov

Simple and Accurate Analytical Model of Planar Grids and High-Impedance Surfaces Comprising Metal Strips or Patches

Simple analytical formulas are introduced for the grid impedance of electrically dense arrays of square patches and for the surface impedance of high-impedance surfaces based on the dense arrays of metal strips or square patches over ground planes. Emphasis is on the oblique-incidence excitation. The approach is based on the known analytical models for strip grids combined with the approximate Babinet principle for planar grids located at a dielectric interface. Analytical expressions for the surface impedance and reflection coefficient resulting from our analysis are thoroughly verified by full-wave simulations and compared with available data in open literature for particular cases. The results can be used in the design of various antennas and microwave or millimeter wave devices which use artificial impedance surfaces and artificial magnetic conductors (reflect-array antennas, tunable phase shifters, etc.), as well as for the derivation of accurate higher-order impedance boundary conditions for artificial (high-) impedance surfaces. As an example, the propagation properties of surface waves along the high-impedance surfaces are studied.

Magnetodielectric Substrates in Antenna Miniaturization: Potential and Limitations

We discuss patch antenna miniaturization using magnetodielectric substrates. Recent results found in the literature reveal that with passive substrates advantages over conventional dielectric substrates can only be achieved if natural magnetic inclusions are embedded into the substrate. This observation is revised and the physical background is clarified. We present a detailed discussion concerning magnetic materials available in the microwave regime and containing natural magnetic constituents. The effects of magnetic dispersion and loss are studied: constraints on the microwave permeability are used to estimate the effect of magnetic substrates on the achievable impedance bandwidth. Microwave composites filled with thin ferromagnetic films are considered as a prospective antenna substrate. We calculate the impedance bandwidth of a lambda/2-patch antenna loaded with the proposed substrate, and challenge the results against those obtained with conventional dielectric substrates. The results are verified using full-wave simulations, and it is shown that the radiation quality factor is strongly minimized with the proposed substrate even in the presence of realistic losses. Estimates for the radiation efficiency are given as a function of the magnetic loss factor

A Thin Electromagnetic Absorber for Wide Incidence Angles and Both Polarizations

A design for planar electromagnetic absorbers is presented. The performance of this absorber is maintained over a wide incidence angles and for both TE and TM polarization. The absorber is composed of a high-impedance surface comprising an array of patches over a grounded lossy dielectric slab perforated with metallic vias (wire medium). The main contribution of the paper is to demonstrate and practically use the presence of an additional resonance when the plasma frequency of the wire medium in the dielectric substrate is close to the original resonance of the high-impedance surface. The presence of the vias between FSS and the ground plane is discussed both for the case of a high-permittivity absorber and for a low permittivity one, through the derivation of simple and efficient analytical expressions, specifically derived for the problem at hand. We show that the presence of the vias influences the oblique incidence TM absorption, and when properly designed, the insertion of the vias into the absorber structure results in a bandwidth enlargement and higher absorption performance.

Backward-wave regime and negative refraction in chiral composites

Possibilities to realize a negative refraction in chiral composites in dual-phase mixtures of chiral and dipole particles are studied. It is shown that because of a strong resonant interaction between chiral particles (helices) and dipoles, there is a stop band in the frequency area where the backward-wave regime is expected. The negative refraction can occur near the resonant frequency of chiral particles. Resonant chiral composites may offer a root to realization of negative-refraction effect and superlenses in the optical region.

Analytical antenna model for chiral scatterers: comparison with numerical and experimental data

An analytical model for polarizability dyadics of small chiral conductive particles in free space or those embedded in a lossy material is presented and discussed. Chiral particles are modeled by a wire loop connected to two straight wire elements. The electromagnetic analysis is based on the replacement of the particles by two connected antennas representing the wire and loop portions. Analytical expressions for polarizabilities are given. For electrically small particles, a lumped-element equivalent circuit can be constructed and the polarizabilities can be expressed in terms of equivalent circuit parameters. It is shown that the wire-and-loop antenna model for scatterers satisfies the reciprocity condition and other basic physical requirements. Approximate analytical expressions are compared with numerical simulations and with the experimental data on reflection from single chiral particles, and the results are seen to be in good agreement. The model can be used in analytical modeling of chiral and omega composite materials.

Transmission-Line Networks Cloaking Objects From Electromagnetic Fields

We consider a novel method of cloaking objects from the surrounding electromagnetic fields in the microwave region. The method is based on transmission-line networks that simulate the wave propagation in the medium surrounding the cloaked object. The electromagnetic fields from the surrounding medium are coupled into the transmission-line network that guides the waves through the cloak thus leaving the cloaked object undetected. The cloaked object can be an array or interconnected mesh of small inclusions that fit inside the transmission-line network.

On artificial magnetodielectric loading for improving the impedance bandwidth properties of microstrip antennas

The effect of artificial magnetodielectric substrates on the impedance bandwidth properties of microstrip antennas is discussed. We review the results found in the literature and then focus on practically realizable artificial magnetic media operating in the microwave regime. Next, a realistic dispersive behavior of a practically realizable artificial substrate is embedded into the model. It is shown that frequency dispersion of the substrate plays a very important role in the impedance bandwidth characteristics of the loaded antenna. The impedance bandwidths of reduced size patch antennas loaded with dispersive magnetodielectric substrates and high-permittivity substrates are compared. It is shown that unlike substrates with dispersion-free permeability, practically realizable artificial substrates with dispersive magnetic permeability are not advantageous in antenna miniaturization. This conclusion is experimentally validated.

Dispersion and Reflection Properties of Artificial Media Formed By Regular Lattices of Ideally Conducting Wires

An analytical theory of electromagnetic waves in artificial media formed by a rectangular lattice of thin ideally conducting cylinders using the local field approach is developed. As a result, the transcendental dispersion equation is obtained in closed form and solved numerically. Typical dispersion curves are calculated. Using these results, the reflection problem from an interface between a half space of wire medium and free space is solved for plane-wave excitation. In the low-frequency approximation a simple analytical formula for the frequency dependent effective dielectric permittivity is established.

Modeling of isotropic backward-wave materials composed of resonant spheres

A possible realization of isotropic artificial backward-wave materials is theoretically analyzed. An improved mixing rule for the effective permittivity of a composite material consisting of two sets of resonant dielectric spheres in a homogeneous background is presented. The equations are validated using the Mie theory and numerical simulations. The effect of a statistical distribution of sphere sizes on the increase of losses in the operating frequency band is discussed and some examples are shown.

Electromagnetic cloaking with metamaterials

Electromagnetic cloaking has aroused increasing interest in the scientific community, especially amongst researchers who are developing so-called metamaterials - artificial composites having exotic electromagnetic properties. In this paper we review the basic principles of metamaterials, especially those for cloaking applications, and describe the recent developments in the field of electromagnetic cloaking. Attention is given also to the recently proposed cloaking technique which is based on networks of transmission lines.