Ali Eren Culhaoglu

Mono- and Bistatic Scattering Reduction by a Metamaterial Low Reflection Coating

Significant reduction of mono- and bistatic scattering cross sections of an electrically large metallic cube through the application of an electrically thin metamaterial low reflection coating operating in the Ka-band (30-40 GHz) is described. A coating composed of a metallic array of sub-wavelength sized capacitively loaded strip inclusions printed on top of a thin, slightly lossy and grounded substrate has been designed and fabricated to experimentally evaluate the performance of the coating in scattering reduction. The scattering reduction is estimated by comparing the mono- and bistatic scattering patterns of the coated and uncoated cube. The measurement results are compared with full wave and physical theory of diffraction simulations, and the possibility of accurately modeling the low reflection coatings with impedance boundary conditions is pointed out.

Experimental Characterization of a Broadband Transmission-Line Cloak in Free Space

The cloaking efficiency of a finite-size cylindrical transmission-line cloak operating in the X-band is verified with bistatic free space measurements. The cloak is designed and optimized with numerical full-wave simulations. The reduction of the total scattering width of a metal object, enabled by the cloak, is clearly observed from the bistatic free space measurements. The numerical and experimental results are compared resulting in good agreement with each other.

Experimental study of anti-resonant behavior of material parameters in periodic and aperiodic composite materials

In the literature on composite materials with extreme or unusual electromagnetic properties (so-called metamaterials), it is common to retrieve the effective material parameters from the scattering parameters (complex reflection and transmission coefficients) of the metamaterial layer. Most often, this retrieval is done by using the well-known Nicolson-Ross-Weir method or its modifications. It is commonly known that one of the two effective parameters of many metamaterials retrieved in this way (either effective permittivity or permeability) exhibits the so-called anti-resonance, which is a nonphysical feature. This effect has been understood as a manifestation of spatial dispersion in the sample. Many previous works have either claimed or assumed that the anti-resonance is an effect caused by the periodicity of the composite material. To our knowledge, this claim was never supported by comparisons of periodical and random samples. In some other works, it has been theoretically shown that the anti-resonance of the retrieved material parameters can be avoided, e.g., by the introduction of additional surface parameters into the retrieval procedure. If it is so, the anti-resonance has nothing to do with the internal periodicity of the bulk medium. In the present work, we numerically and experimentally investigate the question if the internal periodicity of metamaterial layers is essential for the appearance of anti-resonances in extracted material parameters. We show that it is not so. Therefore, our result is in favor of the theory which treats the anti-resonance as a deficiency of the Nicolson-Ross-Weir retrieval procedure applied to spatially dispersive media.

Bistatic scattering characterization of a three-dimensional broadband cloaking structure

Here we present the results of full experimental characterization of broadband cloaking of a finite-sized metallic cylinder at X-band. The cloaking effect is characterized by measuring the bistatic scattering patterns of uncloaked and cloaked objects in free space and then comparing these with each other. The results of the measurements demonstrate a broadband cloaking effect and are in good agreement with numerical predictions.

Imaging by a double negative metamaterial slab excited with an arbitrarily oriented dipole

The imaging capability of nonideal double negative metamaterial slabs excited by arbitrarily oriented electric or magnetic dipoles is investigated. A dipole oriented parallel to the interface of the slab excites transverse electric and transverse magnetic modes simultaneously, which are transmitted unequally by the slab if either the slabs relative permittivity (er) or permeability (μr) or both are perturbed from the ideal case, i.e., er=μr=−1 for a slab in vacuum. Approximations in the electrostatic limit, valid for modes with large wave numbers, fail to predict this phenomenon, and more accurate analysis of the field distribution in the vicinity of the image plane behind a nonideal slab becomes important. By expressing the fields in terms of a one‒dimensional Hankel transform in the spectral domain and using an adaptive Gauss‒Kronrod quadrature, a tool for evaluating the spatial field intensity in the vicinity of the image plane behind the slab is developed. The calculated field intensities reveal that a double negative slab does not focus in the three‒dimensional space and is only capable of two‒dimensional imaging.

A free space measurement approach for dielectric material characterization

The free space method allows nondestructive dielectric characterization of materials in a wide range of frequencies and temperatures, which is basic to several applications, as the development of radomes for aerospace proposals. This paper comprises the free space method from an initial procedure, the TRL calibration, to the permittivity calculation by the NRW method, including the software implemented and its practical validation with specific materials.

Impedance modeling of electromagnetic scattering from canonical bodies with metamaterial absorbers and chessboard-patterned deflectors

This paper summarizes the studies on impedance modeling of electromagnetic scattering from electrically large structures with metamaterial absorbers and chessboard-patterned deflectors performed at the DLR in the recent years. In particular, analytical, numerical and experimental results for mono- and bistatic scattering from metal plates, cubes and cylinders coated with periodic arrays of subwavelength-sized metal inclusions printed on top of a thin dielectric layer are presented.

Design and free-space measurements of a simple electromagnetic cloak for conducting cylindrical objects

In this work, we demonstrate experimentally with bistatic microwave measurements the effective cloaking of a long conducting (metallic) cylinder. The device that enables this cloaking effect is – unlike most known cloaks – a very simple structure composed of two conventional dielectric materials. Both materials are isotropic and homogeneous and have the relative permittivity larger than unity. The device design is carried out with an analytical model and is validated numerically. The cloaking effect of the realized cloak is characterized by measuring the bistatic scattering patterns of uncloaked and cloaked objects in free space and then comparing these with each other.

Metamaterial slab excited by an arbitrarily oriented dipole

A double negative metamaterial slab with material parameters perturbed from the ideal case єr = μr = −1 and excited by an arbitrarily oriented Hertzian dipole is considered. An arbitrarily oriented dipole excites both TE and TM modes, and depending on the degree of perturbation of the material constants the modes are transmitted unequally through the slab. Approximations in the electrostatic limit fail to predict this phenomenon. A tool for evaluating the field intensities in the focal region is developed and the imaging characteristics of the slab are investigated in the spectral domain. Differences between two types of excitation are pointed out and the effect of perturbations in the slab material on the field distribution in the focal plane is discussed.