Search

A broadband absorber that utilizes a dispersive metamaterial and covers the entire microwave X-band (8–12 GHz) is proposed in the present study. An ideal absorber attached to the surface of a perfect electric conductor requires the permittivity of the absorbing layer to be anomalously dispersive in the targeted broad frequency band. We show that anomalous dispersion of the permittivity for the X-band can be fitted to a two-pole Lorentz oscillator model and realized with the use of a double-layered, square-loop metamaterial. We explain the connection between the two-pole oscillator model and the double-layered, square-loop metamaterial using an equivalent circuit model and present explicit design rules for the metamaterial. We fabricate a 4-mm-thick metamaterial absorber with flexible silicon rubber, a resistor element, and conductive wire using carbon and silver conductive ink. Our metamaterial absorber achieves a reflectance of less than −20 dB over the entire X-band region.

Large truncated spherical near-field systems with conductive or absorbing floors are typically used in the measurement of the performances of vehicle-installed antennas. The main advantage of conductive floor systems is the ease of accommodation of the vehicle under test, but their performances are affected by the interaction with the reflecting ground floor. Instead, absorbing-based systems emulating free-space conditions minimize the effect of the interaction with the floor, but generally require longer setup times, especially at lower frequencies (70–400 MHz), where bulky absorbers are typically used to improve reflectivity levels. Considering scaled measurements of a vehicle model, the performances of these two typical implementations are analyzed in the 84–1500 MHz range and compared to free-space measurements. Absorbers with different dimensions and reflectivity have been installed in the scaled measurement setup, and measured data have been investigated with proper post-processing to verify the applicability to realistic systems. Figures of merit of interest for automotive applications, like gain and partial radiated powers, have been compared to free-space to evaluate the impact of different scenarios. [ABSTRACT FROM AUTHOR]

In nowadays, two dimensional integral equations are frequently used in various different engineering and scientific studies. In solving these equations using numerical and analytical methods along wide frequency spectrums; the electrical dimensions of the objects and the complexity of their geometric structures are considered as critical computational aspects. This article offers an effective solution to solve some integral equations. Various problems of the theory of diffraction realized by perfectly conductive metal plates are reduced to these integral equations. Thus, a method for solving integral equations is proposed to understand arbitrary values of frequency parameters and strip sizes. Diffraction theory, defines the diffraction processes that occur when waves such as in the form of light, encounter different obstacles in multiples of their wavelengths with dimensions. A certain bending and deflection of waves around these barriers is characterized by diffraction theory. In dealing with diffraction theory, advanced numerical and matrix-based solutions such as the moment method and the T-matrix method are used. On the other hand, the integralbased solutions proposed in this study are used effectively. The integral-based solution obtained from this study is simulated according to the different medium permeability which is calculated with the electrical dimension varying depending on the frequency of the wave, and a graphical representation is provided. It is seen that the normalized current value at the ends of the strip is maximum for two different cases where the medium permeability is 20 and 30. [ABSTRACT FROM AUTHOR]

This paper investigates the behavior of a dipole antenna near the perfect electric conductor, perfect magnetic conductor, and artificial magnetic conductor (AMC). The phase variations due to reflector reflection coefficient and the distance between antenna and reflector are analyzed, and their effect on the antenna radiation patterns are studied for different frequencies. The cause of a null occurrence in the broadside direction of radiation pattern when using the AMC (or HIS) as a reflector is explored. It is shown that this degradation is due to the phase of the reflected waves at different angles from the reflector surface and this phenomenon may occur for any type of reflector. In the next step, a new broadband unit cell is presented. By changing the AMC reflection feature and the distance between the antenna and the ground plane for three different states, the performance of the printed dipole antenna is improved. The maximum gain of the antenna is enhanced by approximately 7 dB, 9 dB, and 9 dB for three different cases of AMC reflectors. Moreover, the 3 dB max‐gain bandwidth of the structures are 33.81%, 29.17% and 20.84%, with profile size of 0.16λ, 0.4λ, and 0.4λ, respectively. All cases of the antenna with AMC structures have been fabricated and the measurement results are presented. [ABSTRACT FROM AUTHOR]

In this paper, a coplanar waveguide (CPW) fed pot‐shaped electronic textile (E‐textile) antenna aided by rectangular ring ground plane has been proposed. The textile patch in pot form is designed by cutting triangular slots on the non‐radiating edges of the rectangular patch, which yields a broadband of 6.45 GHz. The measured antenna impedance bandwidth is from 2.15 to 8.6 GHz with a peak gain of 10.7 dB on fabricated human tissue muscle equivalent phantom model. The proposed antenna exhibits a broadside radiation pattern from 2.15 GHz to less than 6 GHz and conical radiation pattern from 6 GHz to 8.6 GHz covering wide impedance matching bandwidth (S11 ≤ −10 dB) of 120% for dual‐mode off and on‐body communication. In CPW fed technique, the back radiation from the antenna to on body is reduced by using a perfect electric conductor. For wearable applications, considering human safety concern the specific absorption rate levels are maintained, that is less than 1.6 W/kg limit over the operating bandwidth according to IEEE C95.1‐1999 standard averaged over 1 g of tissue model. [ABSTRACT FROM AUTHOR]

This study presents a planar single‐layer metasurface, with tri‐band 10 dB radar cross‐section (RCS) reduction under normal incidence for both polarisations, by employing optimised multi‐element phase cancellation. The designed artificial magnetic conductor (AMC) unit cell is composed of an upper metal structure, a dielectric layer, and a bottom metal ground. When the dielectric constant and thickness of the substrate are fixed, regardless of any upper structure, the reflection coefficients of AMC are the same as the perfect electric conductor (PEC) at the short‐circuit frequencies. Taking into account the multiple reduction bands that avoid short‐circuit frequencies, a RCS reduction surface with a large frequency ratio is achieved. The optimised planar metasurface obtains 10 dB RCS reduction in the tri‐band frequency (3.81–7.33, 19.16–22.83, and 35.63–38.37 GHz) with bandwidths of 63, 17, and 7.4%, respectively. Moreover, the bistatic scattering patterns between the proposed surface and the equal‐sized PEC are investigated. The theoretically predicted, simulated, and measured results are in good agreement and provide a new strategy for achieving multi‐band RCS reduction. [ABSTRACT FROM AUTHOR]

An ultra‐wideband (UWB) array is presented for UHF‐S band (0.19–2.3 GHz) communications with an emphasis on dual‐linear polarisation and wide‐angle scanning. The array achieves 12:1 impedance bandwidth for a voltage standing wave ratio (VSWR) <3 at broadside with low‐angle scanning off broadside to 0°, 45°, 60° in the E‐plane (VSWR < 3, 4, 3.1) and 0°, 45°, 60° (VSWR < 3, 3.8, 5.5) in the H‐plane. A 7 × 11 prototype was fabricated and tested for verification. In addition, finite effects with respect to mounting UWB arrays in airborne platforms are discussed. Specifically, the effect of radomes, perfect electric conductor shielding walls and resistive walls on the gain of the designed tightly coupled dipole antenna was studied, showing that losses can approach 2 dB. [ABSTRACT FROM AUTHOR]

The generalized Debye source representation of time-harmonic electromagnetic fields yields well-conditioned second-kind integral equations for a variety of boundary value problems, including the problems of scattering from perfect electric conductors and dielectric bodies. Furthermore, these representations, and resulting integral equations, are fully stable in the static limit as ω → 0 in multiply connected geometries. In this paper, we present the first high-order accurate solver based on this representation for bodies of revolution. The resulting solver uses a Nyström discretization of a one-dimensional generating curve and high-order integral equation methods for applying and inverting surface differentials. The accuracy and speed of the solvers are demonstrated in several numerical examples. • Efficient solver for wideband electromagnetic scattering from bodies of revolution. • Thorough discussion of the evaluation of modal Green's functions. • Computation of high-fidelity radar cross sections. • Numerical examples demonstrating that high-order accuracy is basically required for high-frequency calculations. [ABSTRACT FROM AUTHOR]

To evaluate the shape of an Ag tip with regard to tip-enhanced Raman scattering (TERS) signal, the enhanced electromagnetic (EM) field and scattering spectrum, arising from surface plasmon resonance at the apex of the tip, were calculated using a finite-difference time domain (FDTD) method. In the calculated forward scattering spectra from the smooth Ag tip, the band appeared within the visible region, similar to the experimental results and calculation for a corrugated Ag cone. In the FDTD calculation of TERS, the Ag tip acting as a monopole antenna was adopted by insertion of a perfect electric conductor between the root of the tip and a top boundary surface of the calculation space. As a result, the EM field was only enhanced at the apex. The shape dependence i.e. the EM field calculated at the apex with various curvatures on the different tapered tips, obtained using the monopole antenna model, was different from that simulated using a conventional dipole antenna model. [ABSTRACT FROM AUTHOR]

Resonances, i.e., extrema of the eigenvalues of characteristic modes for closed perfectly conducting objects are investigated. The characteristic modal solutions based on the electric, magnetic, and combined field integral operators (EFIO, MFIO, and CFIO) are studied and compared with analytical solutions for a sphere. All these formulations are found to capture both external (radiating) and internal (cavity) resonances predicted by the analytical expressions. At the internal resonances, the eigenvalues obtained with the EFIO- and MFIO-based approaches are not correct, and the corresponding modes are nonunique. These solutions also exhibit a strong duality between the electric (TM) and magnetic (TE) type modes. A connection is found between the external and internal resonances and the condition numbers of the matrices. The modal expansion of the CFIO-based solution is correct, even though it also experiences the nonuniqueness of the EFIO- and MFIO-based solutions. [ABSTRACT FROM PUBLISHER]

A full-wave analytical method using the addition theorems and Hertzian potential functions are used to compute the radar cross section of a sphere coated by several layers composed of common materials and metamaterials. The minimization and maximization of radar cross section of a perfectly electric conductor sphere with such coatings are realized in a frequency band-width and in a wide interval of angles. One of the novelities of this contribution is, taking into dispersion relations of physically realizable metamaterials. So that the optimization procedure for RCS reduction is applied due to the coefficients describing dispersion characteristics. The method of least square is used for the design of a class of radar absorbing materials. The minimization of the error functions are performed by the combination of genetic algorithm and conjugate gradient method. It is shown that the proposed method of computation of radar cross section and its extremization effectively leads to the design of dispersive and isotropic metamaterials for the realization of radar absorbing materials. [ABSTRACT FROM AUTHOR]

Aerospace structures can be approximately modeled as a combination of canonical structures such as cylinder, cone and ellipsoid. Thus the RCS estimation of such canonical structures is of prime interest. Furthermore metamaterials possess peculiar electromagnetic properties which can be useful in modifying the RCS of structures. This paper is aimed at calculating the RCS of an infinitely long PEC circular cylinder coated with one or two layers of metamaterial. The incident and scattered fields of coated cylinder are expressed in terms of series summation of Bessel and Hankel functions. The unknown coefficients of summation are obtained by applying appropriate boundary conditions. The computations are carried out for both principal polarizations. The computed results are validated against the numericalbased method of moments. Further, the variation of RCS of the metamaterial coated PEC cylinder with material parameters, frequency, aspect angle and polarization is analyzed. [ABSTRACT FROM AUTHOR]

This paper employs the spectral domain approach for full wave analysis of metal strip grating on grounded dielectric slab and microstrips shielded with either Perfect Electric Conductor (PEC) or Perfect Magnetic Conductor (PMC) walls. The modal relations between these structures are revealed by exploring their symmetries. It is derived analytically and validated numerically that all the even and odd modes of the latter two (when they are mirror symmetric) find their correspondence in the modes of metal strip grating on grounded dielectric slab when the phase shift between adjacent two unit cells is 0 or n. Extension to non-symmetric case is also made. Several factors, including frequency, grating period, slab thickness and strip width, are further investigated for their impacts on the effective permittivity of the dominant mode of PEC/PMC shielded microstrips. It is found that the PMC shielded microstrip generally has a larger wave number than the PEC shielded microstrip. [ABSTRACT FROM AUTHOR]

Abstract: The scattering of electromagnetic waves by a perfect electromagnetic conductor (PEMC) cylinder coated with a homogeneous plasma anisotropic material is studied in this paper. Both of the transverse electric and the transverse magnetic polarizations of the incident waves have been analyzed and formulated. The presented analysis and formulations are general for any perfect conductor cylinder (PEC, PMC, or PEMC) with general isotropic/anisotropic material coatings that include plasma and metamaterials. The co-polarized and the cross-polarized components of the scattered fields are computed for different cases of the anisotropic plasma coated PEMC cylinders and for an anisotropic plasma column. Bistatic echo widths for the cases of PEMC, PEC (perfect electric conductor) and PMC (perfect magnetic conductor) cores have been computed and compared. The behavior of the monostatic echo width with the variation of the admittance parameter for the co-polarized and the cross polarized fields is also investigated. The comparisons of the computed results of the presented formulations with the published results of some special cases confirm the accuracy of the presented analysis. [Copyright &y& Elsevier]

Magnetic induction tomography (MIT) is an imaging technique based on the measurement of the magnetic field perturbation due to eddy currents induced in conducting objects exposed to an external magnetic excitation field. In MIT, current-carrying coils are used to induce eddy currents in the object and the induced voltages are sensed with the receiving coils. When the driving frequency is significantly high relative to the frequency range in which MIT normally operates, metallic targets with high conductivity between the coils can be treated as perfect electric conductors (PEC) with negligible errors. In this scenario, the penetration depth of the magnetic field into the target is extremely small and the traditional versions of the finite element method (FEM) are not efficient for the calculation of the sensitivity and the forward problem due to the requirement for large number of elements to reach an acceptable computational precision. Other versions of FEMs (such as Hp-FEM), which have higher discretization efficiency and more advanced elements to satisfy the requirement, are exceptions. Nevertheless, the discretization regions for all FEMs have to extend beyond the region that contains the conducting object and volumetric elements are generally required for 3D problems. In contrast, the boundary element method (BEM) based on integral formulations becomes an effective way to analyze this kind of scattering problem since meshes are only required on the surface of the object. By point collocation, the boundary integral equations can be transformed into linear equations. Numerical methods are used to solve the linear equations and the solution of the original integral equations can be obtained. In this paper, we compute four typical sensitivity maps between the coil pairs in high-frequency MIT system due to a PEC perturbation. The magnetic scalar potential is used to improve the efficiency. Five PEC objects of different shapes are used in the simulation. The results have been compared with the experimental results and that obtained from the H · H formulations. We can know that the sensitivity maps derived by BEM are in good agreement with that from the experiment and theoretical solution. Overall, BEM is an effective way to calculate the sensitivity distributions of a high-frequency MIT system. [ABSTRACT FROM AUTHOR]

The Finite-Difference Time-Domain (FDTD) method is used to study the frequency dependence of the transmission through a plasmonic screen with slots of sizes and spacing that were chosen using the spatially shifted beam approach to achieve subwavelength focusing. It is shown that the electric field beamwidth at the focal plane exhibits a resonance-like behavior in the vicinity of the frequency for which the meta-screen was designed to work. At this frequency range, the power flow path is shown to exhibit circulation around the slotted screen. Results are also compared with those given in [1] for a similar device working at a lower frequency range where the metallic screen behaves as a perfect electric conductor. [ABSTRACT FROM PUBLISHER]

High frequency electromagnetic field expressions are analyzed for the focal region of a circular reflector coated with chiral medium. Maslov's method has been used for the study of focal region fields because geometrical optics is an approximate method and fails at the focal points. The effects of chirality parameter, thickness of chiral coating and permittivity of the medium on the focused fields are observed. It is concluded from the results that focused field strength increases with increase in chirality parameter, relative permittivity and thickness of the chiral coating. [ABSTRACT FROM AUTHOR]