Effects of metal fences on the scan performance of an infinitedipole array

A theoretical method is presented to investigate the scan characteristics of a dipole array in the presence of thin metal fences along the H-plane. The basic idea of the analysis is to determine the current distribution on the dipole which interacts with the metal fence in the unit cell, so that the active impedance of the element can be computed. This was accomplished by using the moment method and imposing the boundary conditions on the tangential electric fields along the dipole and the metal fence. Since the scan performance in the H-plane is not affected by the fences, the discussion is concentrated on the E-plane behavior. It was found that for a given element spacing and dipole geometry there is an optimum fence height at which the E-plane performance can be most improved. For a typical dipole array geometry the optimum fence height is about 0.4 λ. It was thought that when the fences are to high and the beam is scanned too far from broadside, transverse magnetic (TM) surface waves propagating in the E-plane direction near the corrugated plane would be excited. However, no such sensitive cases were encountered in the study     

The snell and fresnel relationships for electromagnetic waves with the constant linear polarization

General conditions for the existence of plane electromagnetic waves with a single nonzero electric-field component (TE waves) in an anisotropic medium are established. A transformation that makes it possible to derive similar formulas for TM waves (plane electromagnetic waves with a single nonzero magnetic-field component) from the formulas expressing the properties of TE waves is considered. A general numerical algorithm is developed for calculating the amplitudes (with allowance for phases) of the TE waves formed when an external TE wave propagates through a system of plane-parallel plates consisting of media supporting TE waves. Formulas are obtained for the solution of the particular problem on the transmission of a TE wave through a plane interface between two media supporting TE waves. A series of relationships that can be interpreted as generalizations of the Snell and Fresnel relationships for media of the considered type are derived. Formulas for the transmission, reflection, and refraction coefficients are obtained. The existence conditions for TE (TM) surface waves are derived. A numerical method is proposed for computing the spectra of such surface waves formed on the interface between two media. For isotropic media, analytic expressions for the spectra of surface waves are derived. It is shown that the obtained formulas are similar to the known formulas for surface TM waves in dielectrics but, in addition, make it possible to take into account both the permittivities and permeabilities of the media.     

Plane wave reflection from a rectangular-mesh ground screen

The theory of scattering of electromagnetic plane waves of arbitrary incidence and polarization from an infinite rectangular-mesh ground screen is treated. The screen is composed of thin wires of circular cross section, and is parallel to the interface between two homogeneous media. The theoretical results for parallel-wire screens are obtained in the limit of large wire spacing for one dimension of the mesh screen. Results presented for incident parallel polarized plane waves indicate that both the parallel-wire and mesh screens may exhibit a change in reflecting properties as the plane of incidence is varied from the wire axis direction. In addition, it is shown that the parallel-wire screen can produce appreciably higher cross-polarized fields than a square-mesh screen of the same wire spacing.     

High impedance metamaterial surfaces using Hilbert-curve inclusions

A metamaterial surface, composed of a periodic arrangement of Hilbert Curve inclusions above a conducting ground plane, is analyzed numerically and is shown to possess the properties of a high impedance surface by investigating the phase and magnitude of the reflection coefficient, /spl Gamma/, for a plane wave of normal incidence. A parametric study is presented with respect to the iteration order of the Hilbert curve, the surface height above the ground plane, and the separation distance between the neighboring Hilbert elements within the surface array.     

Shorted elliptical patch antennas with reduced surface waves on two frequency bands

In this paper, a study on the surface waves emission of a shorted annular elliptical patch (SAEP) antenna is presented. This radiator is constituted of an annular elliptical patch having the inner border shorted to the ground plane. The surface wave field excited by the SAEP antenna has been evaluated considering an equivalent magnetic current distribution flowing onto the outer boundary of the patch. It will be demonstrated that the dimensions of the outer elliptical contour can be chosen in such a way that the excitation of the TM/sub 0/ surface wave mode is suppressed on two frequencies. This characteristic makes possible the design of SAEP antennas with reduced surface wave (RSW) emissions on two frequency bands. To validate this result both numerical simulations and measurements will be presented. In particular, it will be shown that when the RSW-SAEP is considered the field strength within the substrate undergoes a faster decay resulting in a reduced mutual coupling in dual band array applications. Furthermore, the effective reduction of the surface waves power minimizes the scattering from the ground plane edges leading in smoother radiation patterns.     

McCART: Monte Carlo Code for Atmospheric Radiative Transfer

McCART is a numerical procedure to solve the radiative transfer equation for light propagation through the atmosphere from visible to near-infrared wavelengths. The procedure has been developed to study the effect of the atmosphere in the remote sensing of the Earth, using aerospace imaging spectrometers. The simulation is run for a reference layered plane nonabsorbing atmosphere and a plane ground with uniform reflectance. For a given distribution of ground reflectance and for a specific profile of scattering and absorption properties of the atmosphere, the spectral response of the sensor is obtained in a short time from the results of the Monte Carlo simulation by using scaling relationships and symmetry properties. The procedure also includes an accurate analysis of the adjacency and trapping effects due to multiple scattering of photons coming from neighboring pixels. McCART can generate synthetic images of the Earth's surface for arbitrary viewing conditions. The results can be used to establish the limits of applicability of approximate algorithms for the processing and analysis of hyperspectral images acquired by imaging spectrometers. In addition, the algorithm can be used to develop procedures for atmospheric correction for the accurate retrieval of the spectral ground reflectance.     

Dispersive delay at gigahertz frequencies using magnetostatic waves

Magnetostatic waves are intrinsically suited for use in microwave dispersive delay lines, because they are readily generated at high frequencies and their velocity is frequency dependent. Here we review progress toward meeting the systems requirement that the delay time be linear in frequency. Magnetostatic waves can approach this condition if a ground plane is nearby or if two or more ferrite layers are present; with forward volume waves one may also use a dispersive reflecting array, as is done with surface acoustic waves. The use of a properly shaped ground plane gives the lowest phase errors so far measured (±16° over 0.6 GHz); multiple ferrite layers show promise of allowing the greatest delay.     

Surface Waves Due to Geometrical Discontinuities over Ground

The “ground wave” measured with the receiving antenna is actually inhomogeneous lateral wave, guided by the homogeneous plane wave which propagates along the surface of the ground and they have strong effect in communication systems. In this study, we examine surface waves due to geometrical discontinuities over the ground. This is done by simulating the problem as a diffraction phenomena. The boundary value problem leads to vector Wiener-Hopf equation which is reduced to scalar form and solved by standard techniques. The asymptotic expressions for the diffraction are obtained by evaluating the field integrals asymptotically. Surface waves and related currents are analyzed by using Wiener-Hopf technique in detail. Some numerical examples are also presented.     

Scattering from multiple objects buried beneath two-dimensional random rough surface using the steepest descent fast multipole method

Generalized formulations are presented to analyze the electric field scattered from multiple penetrable shallow objects buried beneath two-dimensional random rough surfaces. These objects could have different materials, shapes, or orientations. In addition, their separation distance may range from a fraction of a wavelength to several wavelengths. The fast algorithm, steepest descent fast multipole method (SDFMM), is used to compute the unknown electric and magnetic surface currents on the rough ground surface and on the buried objects. Parametric investigations are presented to study the effect of the objects proximity, orientations, materials, shapes, the incident waves polarization, and the ground roughness on the scattered fields. A significant interference is observed between the objects when they are separated by less than one free space wavelength. Even when the clutter due to the rough ground is removed, the return from the second object, can be dominating causing a possible false alarm in detecting the target. The results show that the distortion in target signature significantly increases with the increase of both the proximity to a clutter item and the ground roughness.     

Scan blindness free phased array design using PBG materials

Scan blindness occurs for phased arrays when propagation constants of Floquet modes (space harmonics) coincide with those of surface waves supported by the array structure. In this paper, we studied the possibility of using photonic band-gap (PBG) substrate to eliminate scan blindness. A specially designed printed PBG substrate can suppress surface wave propagation inside its bandgap range, therefore it can be used to eliminate scan blindness. In this paper, we presented a method of moments (MoM) analysis of the scan properties of dipole arrays on PBG substrates with ominidirectional bandgap(s). We found that scan blindness is completely eliminated. The elimination of scan blindness makes PBG materials very attractive in phased array design.     

Novel Design of a High-gain and Wideband Fabry-Pérot Cavity Antenna Using a Tapered AMC Substrate

A high-gain and wideband electromagnetic bandgap (EBG) resonator antenna with a tapered artificial magnetic conductor (AMC) ground plane is presented. The proposed EBG resonator antenna is comprised of a frequency selective surface (FSS) superstrate with a strip dipole array and an AMC ground plane with tapered rectangular patches. The realized gain and the bandwidth of the antenna can be improved simultaneously by using the tapered AMC where the phase difference of the reflected waves from the patches with different length is within 180° and the destructive interference among them can be considerably reduced. The maximum gain is increased about 2∼3 dB and the bandwidth is improved about 2.5 times compared to when the uniform AMC is used.     

Salisbury screen with reduced angular sensitivity

A new Salisbury screen absorber design is presented, which yields a significantly reduced sensitivity to angle of incidence. This is achieved by using an artificial magnetic ground plane, which is designed to scatter the reflected signals in a conjugate phase relationship with respect to the change in the electrical spacing between the ground plane and the resistive screen. Numerical and experimental results in the range 8?12 GHz are used to demonstrate that scan compensation can be obtained over the angular range 0?40° thus removing the shift in the resonant frequency that occurs in conventional Salisbury screen absorbers when operated off normal incidence.     

Reduction of mutual coupling using perfectly conducting fences

A theoretical and experimental study of the use of thin perfectly conducting fences to reduce the mutual coupling of two parallel-plane waveguides radiating through a ground plane is described. The two cases which are studied areE-plane coupling of parallel planes with transverse electromagnetic mode propagation andH-plane coupling of parallel planes with the fundamental transverse electric mode propagation. The analysis presented accounts for a number of higher order modes in the waveguide apertures and uses a Fourier series approximation to the fence currents. Close agreement with experimental results is demonstrated.     

General reflection rule for electromagnetic multipole fields on aplane interface

The general rule for reflection of a vector 2'-pole field on a plane interface between two media of different dielectric properties is established starting from the expansion of the spherical multipole field as a linear combination of inhomogeneous vector plane waves. In fact, by considering vector multipole fields that satisfy the radiation condition at infinity we are able to define a matrix that effects their reflection on the plane interface. Such a matrix can also be used to reflect a superposition of many 2'-pole fields and so can be useful to describe the effect of a plane surface near to a specified source or to a scattering particle     

Surface Waves on Symmetrical Three-Layer Sandwiches (Correspondence)

The theory of surface waves on plane dielectric slabs has been presented by Plummer and Hansen. Additional numerical results are shown in Figs. 1 and 2 for the lowest order TM and TE modes that can exist on a grounded dielectric slab. The slab has thickness d, and a relative dielectric constant of 4. It is separated by an air gap of thickness a from the ground plane. c/v represents the ratio of the velocity of light in free space and the phase velocity of the surface wave. By image theory, these modes (TM/sub 0/ and TE/sub 1/) can also exist on a symmetrical, three-layer, air-core sandwich to which the given numerical data also apply.     

一种新型频率选择表面及其在天线雷达散射截面减缩中的应用

为了减缩天线的雷达散射截面,该文提出一种新型带阻频率选择表面结构(FSS)。对于入射电磁波,该结构与传统方环结构相比具有窄阻带和宽通带特性。因此该结构能代替天线的传统金属地板从而减缩天线的结构项雷达散射截面(RCS)。仿真与测试结果表明该结构作为天线地板可很大程度的减缩天线的雷达散射截面。     

Subsurface Sensing of Buried Objects Under a Randomly Rough Surface Using Scattered Electromagnetic Field Data

This paper proposes a new inverse method for microwave-based subsurface sensing of lossy dielectric objects embedded in a dispersive lossy ground with an unknown rough surface. An iterative inversion algorithm is employed to reconstruct the geometry and dielectric properties of the half-space ground as well as that of the buried object. B-splines are used to model the shape of the object as well as the height of the rough surface. In both cases, the control points for the spline function represent the unknowns to be recovered. A single-pole rational transfer function is used to capture the dispersive nature of the background. Here, the coefficients in the numerator and denominator are the unknowns. The approach presented in this paper is based on the state-of-the-art semianalytic mode matching forward model, which is a fast and efficient algorithm to determine the scattered electromagnetic fields. Numerical experiments involving two-dimensional geometries and TM incident plane waves demonstrate the accuracy and reliability of this inverse method     

Theoretical considerations in the optimization of surface waves on a planar structure

The problem of optimum excitation of surface waves on a grounded dielectric slab by means of slots in the ground plane is considered. By adopting a two-dimensional (2-D) model, analysis lead to closed forms for the power launched as surface waves and power leaked as radiation. Input admittance of a single slot source and mutual admittance between two slots are derived and utilized to design a three element Yagi array of slots to achieve a prescribed ratio of forward to backward surface wave power. As a development of the 2-D model, we allow finite extent of slot excitation by assuming a Gaussian E-field distribution across the slot. The effect of the Gaussian width on the excited surface wave power is studied. The analysis is relevant to the study of surface waves on printed circuits. Specifically, it applies to the implementation of power combiners based on quasioptical slab beam that have been recently introduced in the literature for use in the millimeter wave band.     

Surface waves on a lossy planar ferrite slab

The parallel-polarized surface waves which propagate on a planar ferrite slab are considered. The ferrite material is isotropic and homogeneous, the slab rests on a perfectly conducting ground plane, and the ambient medium is free space. Only one mode can propagate on a thin lossless slab, but more and more of the higher order modes begin to propagate when one increases the thickness or the loss tangent of the slab. As the thickness and the loss are increased, the lowest order mode takes on the properties of the Zenneck wave. At the same time, the higher order modes take on the properties of a homogeneous plane wave propagating in the grazing direction in the slab. Among the most interesting properties of a surface wave are the attenuation constant and the phase velocity. Contour maps are included to show how these quantities depend on the thickness, permittivity, permeability and loss tangents of the ferrite slab. Additional insight is provided by graphs showing the root trajectories, cutoff boundaries and field distributions of the four lowest order surface-wave modes.     

Coupling of vias in electronic packaging and printed circuit board structures with finite ground plane

A full wave method is presented for modeling and analyzing multiple interactions among vertical vias in densely packaged integrated circuits and printed circuit board with ground plane of finite extent. In such structures, the TEM mode in the planar structure is excited and can propagate and cause interaction of waves among vias. Reflections will also occur at the edges of the finite ground plane. The electromagnetic analysis methodology is an extension of the previous methodology in analyzing multiple scattering among vias for infinite ground plane . The analysis is based upon the cylindrical wave mode expansion of the magnetic field Green's function, the Foldy-Lax multiple scattering formalism and utilizing the resonator modes of a circular cavity. The circular resonator modes are transformed into cylindrical waves onto the cylindrical via structures. Numerical results illustrate the physics of the underlying resonance scattering problems. We consider the cases of a) two coupled active vias of differential mode and b) two coupled vias of common mode. Results are also illustrated for ground plane resonance and the effects of shorting vias on such resonance. The effects of off-centering and the presence of idle vias are also illustrated.