High-frequency EM scattering by edges in artificially hard and softsurfaces illuminated at oblique incidence

Uniform high-frequency expressions describing the field scattered by edges in anisotropic impedance surfaces illuminated at oblique incidence are provided. The specific anisotropic impedance boundary condition considered here exhibits a vanishing surface impedance along a principal anisotropy axis and an arbitrary one in the orthogonal direction. In certain circumstances, this tensor surface impedance may represent an accurate model for describing the scattering properties of artificially hard and soft surfaces. In order to simplify the analysis but without losing pertinence with real problems, in all canonical configurations we consider a face of the wedge to be perfectly conducting. The anisotropic impedance face is characterized by a tensor surface impedance with the principal anisotropy axes parallel and perpendicular to the edge     

Skew incidence diffraction by an anisotropic impedance half plane with a PEC face and arbitrarily oriented anisotropy axes

High-frequency expressions for the field scattered by a half-plane with a perfectly conducting and an anisotropic impedance face are provided in the format of the uniform geometrical theory of diffraction (UTD), when the half-plane is illuminated by an arbitrarily polarized plane wave obliquely incident on its edge. The loaded face is characterized by a tensor surface impedance with principal anisotropy axes arbitrarily oriented with respect to the edge; a vanishing surface impedance is exhibited in one of the principal directions. This kind of tensor surface impedance can be suitably applied for analyzing the effects on the scattered field of corrugated surfaces or grounded dielectric slabs periodically loaded by metallic strips. This solution extends previous high-frequency formulations valid in those cases in which the direction of corrugations or strips is either parallel or perpendicular to the edge. The analysis is performed by resorting to the Sommerfeld-Maliuzhinets method. To determine the spectral solution, a special function is needed that differs from the standard Maliuzhinets one and was originally introduced to study the electromagnetic scattering by a wedge embedded in a gyroelectric medium.     

Electromagnetic diffraction of an obliquely incident plane wave bya right-angled anisotropic impedance wedge with a perfectly conductingface

The diffraction of an arbitrarily polarized electromagnetic plane wave obliquely incident on the edge of a right-angled anisotropic impedance wedge with a perfectly conducting face is analyzed. The impedance tensor on the loaded face has its principal anisotropy axes along directions parallel and perpendicular to the edge, exhibiting arbitrary surface impedance values in these directions. The proposed solution procedure applies both to the exterior and the interior right-angled wedges. The rigorous spectral solution for the field components parallel to the edge is determined through the application of the Sommerfeld-Maliuzhinets technique. A uniform asymptotic solution is provided in the framework of the uniform geometrical theory of diffraction (UTD). The diffracted field is expressed in a simple closed form involving ratios of trigonometric functions and the UTD transition function. Samples of numerical results are presented to demonstrate the effectiveness of the asymptotic expressions proposed and to show that they contain as limit cases all previous three-dimensional (3-D) solutions for the right-angled impedance wedge with a perfectly conducting face     

Electromagnetic scattering by anisotropic impedance half and fullplanes illuminated at oblique incidence

The three-dimensional electromagnetic (EM) scattering from half and full plane configurations, both characterized by a perfectly conducting and an anisotropic impedance face, is analyzed. The anisotropic impedance boundary condition considered for the loaded face is suitable for modeling corrugated surfaces or strip-loaded grounded dielectric slabs used to realize artificially hard or soft surfaces, with a tensor surface impedance exhibiting a vanishing impedance along the corrugations or strips and a diverging impedance in the orthogonal direction. Previous rigorous solutions, valid when the vanishing impedance direction is either parallel or perpendicular to the edge, are generalized here to the case in which the direction of vanishing impedance is arbitrarily oriented     

Diffraction of high-frequency electromagnetic waves by curvedstrips

The authors describe the construction of a high-frequency solution in the geometrical theory of diffraction (GTD) format for the generation of edge-diffracted space rays and edge-excited surface waves by an electromagnetic wave normally incident on the edge of a curved impedance strip. The transformation coefficients necessary for the analysis of the diffraction on curved surfaces with equal vanishing face impedances are tabulated in a form appropriate for numerical applications. The procedure for evaluating the diffracted field excited by the edge of a cylindrically curved strip, which can be associated with a reflector antenna or an aperture on a curved surface, is presented for both perfectly and nonperfectly conducting cases     

Ground Anisotropy and Extremely Low Frequency (ELF) Transmitting Antennas

The surface impedance concept is used to examine the effect of ground anisotropy on the radiated field and power dissipation of a horizontal ELF transmitting array. The principal result, for a simple homogeneous but anisotropic ground, is that even though the ground skews the antenna pattern, any attempt to reorient the array to steer the pattern peak towards the receiver incurs an increase in power dissipation. In fact, the power dissipation is minimized by steering the pattern peak even further away from the receiver.     

Scattering by an infinite wedge with tensor impedance boundaryconditions-a moment method/physical optics solution for thecurrents

Plane wave scattering by an infinite, two-dimensional wedge whose faces are characterized by impedance tensors is discussed. A combination of the moment method (MM) and physical optics (PO) is used to obtain a solution for the equivalent electric currents. The currents near the edge on each face are expanded with a set of basis functions consisting of pulse functions, defined on a meshed region, plus a function spanning the whole face. The currents outside the meshed region are taken to be the sum of physical optics currents, taken to be known, plus the whole-face basis function current. Expressing the equivalent magnetic currents in terms of the electric currents through the impedance tensors, the expansion coefficients for the electric current expansion are determined through an MM solution of the magnetic field integral equation. Sample results for wedges with isotropic and anisotropic face impedances are presented     

On the diffraction of an electromagnetic skew incident wave by a non perfectly conducting wedge

We study the diffraction by a wedge of an electromagnetic plane wave with skew incidence on the edge, when boundary conditions give us two equations by face with combined electric and magnetic fields. The problem is reduced principally to a non linear scalar functional equation with one unknown. As an example of application, the solution for a wedge with arbitrary angle and relative impedance unity (the most usual model for absorbing material) is given.     

Diffraction by a wedge with variable-impedance faces

The scattering from a wedge with nonuniform impedance faces illuminated by a plane wave, perpendicularly incident on its edge, is analyzed. The solution technique is in the framework of perturbative methods; it applies to surface impedances of the wedge faces having the form of a constant plus a small amplitude perturbation which exhibits an exponential dependence on the distance from the edge in a plane transverse to the edge. This is of remarkable importance for applications as it allows the modeling of the actual behavior of the equivalent surface impedance in the special case of wedges coated with dielectric slabs. Uniform asymptotic expressions for the fields are obtained in the context of the uniform geometrical theory of diffraction (UTD)     

Simultaneous Edge‐on to Face‐on Reorientation and 1D Alignment of Small π‐Conjugated Molecules Using Room‐Temperature Mechanical Rubbing

In this study, room‐temperature mechanical rubbing is used to control the 3D orientation of small π‐conjugated molecular systems in solution‐processed polycrystalline thin films without using any alignment substrate. High absorption dichroic ratio and significant anisotropy in charge carrier mobilities (up to 130) measured in transistor configuration are obtained in rubbed organic films based on the ambipolar quinoidal quaterthiophene (QQT(CN)4). Moreover, a solvent vapor annealing treatment of the rubbed film is found to improve the optical and charge transport anisotropy due to an increased crystallinity. X‐ray diffraction and atomic force microscopy measurements demonstrate that rubbing does not only lead to an excellent 1D orientation of the QQT(CN)4 molecules over large areas but also modifies the orientation of the crystals, moving molecules from an edge‐on to a face‐on configuration. The reasons why a mechanical alignment technique can be used at room temperature for such a polycrystalline film are rationalized, by the plastic characteristics of the QQT(CN)4 layer and the role of the flexible alkyl side chains in the molecular packing. This nearly complete conversion from edge‐on to face‐on orientation by mechanical treatment in polycrystalline small‐molecule‐based thin films opens perspectives in terms of fundamental research and practical applications in organic optoelectronics.     

A model for edge scattering from dichroic surfaces

This paper discusses experimental and theoretical results for edge scattering from dichroic (frequency selective) surfaces. Measurements presented for half planes show the nature of the scattering from the edge for both reflection and transmission frequencies. An impedance boundary condition approach is used to calculate the diffraction from dichroic edges. In the analysis the dichroic surface is replaced with an equivalent surface impedance and the total scattered field calculated. Measurements support the theory demonstrating the usefulness of the approach. The method is also useful for calculating diffraction from dielectric edges.     

High-frequency approximations for electromagnetic field near a face of an impedance wedge

We employ the exact solution given by G.D. Maliuzhinets (see Sov. Phys. Doklady, vol.3, p.752-5, 1958) for the canonical problem of diffraction of a plane wave by an arbitrarily angled impedance wedge to derive asymptotic approximations to the field components in a region contiguous to a face of the wedge. The asymptotic solution accounts for terms of order (k/spl rho/)/sup -3/2/ (k is the wave number and /spl rho/ is the distance from the edge), is uniform with respect to observation and illumination aspects and includes the case of grazing illumination of a wedge face, which is known to be particularly difficult for high-frequency analysis (Uflmtsev's singularity).     

Backscattering by loaded and unloaded dihedral corners

An accurate mathematical model for the backscattering from a loaded dihedral corner has been developed. Such a model employs a generalization of physical optics (PO) to loaded surfaces which takes into account the lighting of each face by the rays diffracted by the edge of the other one. The inclusion of the currents due to such diffracted fields in the PO current distribution has been found relevant in order to improve the accuracy of the solution. Moreover the solution is uniform with respect to the loading impedance; as it coincides with the previously obtained solution for the perfectly conducting case, when the loading impedance approaches zero.     

Scattering by an imperfect right-angled wedge

A solution is obtained for the problem of a plane electromagnetic wave at skew (oblique) incidence on a right-angled wedge one of whose faces is imperfectly conducting. An exact integral expression for the total field is derived, and the geometrical optics and edge diffracted fields are obtained. These are used to produce a uniform solution in the uniform asymptotic theory (UAT) format. Plots of the edge diffracted and total fields are presented to show the effect of the impedance of the wedge face.     

RCS analysis and reduction for lossy dihedral corner reflectors

The radar cross-section patterns of lossy dihedral corner reflectors are calculated using a uniform geometrical theory of diffraction for impedance surfaces. All terms of up to third order reflections and diffractions are considered for patterns in the principal plane. The surface waves are included whenever they exist for reactive surface impedances. The dihedral corner reflectors examined have right, obtuse, and acute interior angles, and patterns over the entire 360° azimuthal plane are calculated. The surface impedances can be different on the four faces of the dihedral corner reflector; however, the surface impedance must be uniform over each face. Computed cross sections are compared with a moment method technique for a dielectric/ferrite absorber coating on a metallic corner reflector. The analysis of the dihedral corner reflector is important because it demonstrates many of the important scattering contributors of complex targets including both interior and exterior wedge diffraction, half-plane diffraction, and dominant multiple reflections and diffractions     

Asymptotic analysis of edge-excited currents on a convex face of aperfectly conducting wedge under overlapping penumbra region conditions

The edge-excited surface currents on a convex face of a perfectly conducting curved wedge are investigated in the asymptotic high-frequency limit for the case where the penumbra regions of the edge and surface diffractions overlap. The edge of the wedge is assumed straight, and the incident electromagnetic wave locally plane and normal to the edge. Both polarizations are considered. The surface field induced by the edge diffraction is synthesized in the spirit of the spectral theory of diffraction (STD): the solution for the edge-diffracted field is interpreted as a spectrum of inhomogeneous plane waves, and the surface field excited by each spectral plane wave is obtained by analytical continuation of the Fock (1965) functions into complex space. The main purpose of this work is to prove the reciprocity of a solution deduced previously for the problem of line source radiation from the wedge in question. As a by-product, useful identities for an incomplete Airy function and an Airy-Fresnel integral are developed     

Anisotropic Structure and Charge Transport in Highly Strain‐Aligned Regioregular Poly(3‐hexylthiophene)

A novel method of strain‐aligning polymer films is introduced and applied to regioregular poly(3‐hexylthiophene) (P3HT), showing several important features of charge transport. The polymer backbone is shown to align in the direction of applied strain resulting in a large charge‐mobility anisotropy, where the in‐plane mobility increases in the applied strain direction and decreases in the perpendicular direction. In the aligned film, the hole mobility is successfully represented by a two‐dimensional tensor, suggesting that charge transport parallel to the polymer backbone within a P3HT crystal is strongly favored over the other crystallographic directions. Hole mobility parallel to the backbone is shown to be high for a mixture of plane‐on and edge‐on packing configurations, as the strain alignment is found to induce a significant face‐on orientation of the originally highly edge‐on oriented crystalline regions of the film. This alignment approach can achieve an optical dichroic ratio of 4.8 and a charge‐mobility anisotropy of 9, providing a simple and effective method to investigate charge‐transport mechanisms in polymer semiconductors.     

Waveguiding and reflective properties of plane boundaries with an anisotropic chiral impedance

The properties of circularly polarized surface waves propagating along a plane boundary with an anisotropic chiral impedance are investigated. Boundaries with rectangular and radial-ring structures of anisotropy are considered. The problem of the normal incidence of plane circularly polarized waves on such boundaries is solved.     

Electromagnetic scattering by a right angled anisotropic impedancewedge

The 3D electromagnetic scattering of plane waves from a right angled impedance wedge, with one anisotropic face, is addressed. An exact integral representation for the total field is obtained by resorting to the Maliuzhinets method. The integral representation for the field is defined along the Sommerfeld integration path, so that it can be asymptotically evaluated by standard procedures. The particular kind of anisotropic impedance considered for one of the faces of the wedge is suitable for modelling corrugated surfaces or strip loaded grounded dielectric slabs, with corrugations or strips perpendicular to the edge     

Moment-method solution of the center-fed microstrip disk antennainvoking feed and edge current singularities

The problem of a microstrip disk antenna excited by a probe at its center is studied using a moment-method approach for the evaluation of the surface current on the disk. The novelty of the work is a careful treatment of the singular effects of the probe and of the disk edge on the surface current on the disk. This is done by including in a proper way an attachment-mode current and an edge current in the surface current on the disk. The inclusion of these two currents accelerates the convergence of the moment-method solution and enables the input impedance to be obtained in a broad frequency range, including the zero-frequency limit. The influence of the probe on the input impedance is taken into account in a correct and accurate way. A comparison between measured and computed results is given, and good agreement is obtained