Abstract: Endfire tapered slot antennas are suitable for many integrated circuit applications, imaging and phased arrays. We report on an investigation of single elements of such antennas, including slots which are exponentially tapered (Vivaldi), linearly tapered (LTSA) and constant width (CWSA). For antennas of all types, a good general agreement is obtained for curves of beamwidth versus length, normalized to wavelength, when one compares the data with that for traveling-wave antennas published by Zucker. An important condition for this agreement is that the effective dielectric thickness, defined in the text, is in a certain optimum range. This condition is qualitatively explained in terms of the theory for traveling-wave antennas.

Abstract: Two novel configurations for increasing the impedance bandwidth of the microstrip patch antennas are described. One of these configurations uses two additional resonators which are gap-coupled to the nonradiating edges of a rectangular patch, whereas in the second case, four additional resonators are gap-coupled to the four edges of a rectangular patch. Green's function approach and segmentation method are used for analysis. The experimental results are in reasonable agreement with analysis and impedance bandwidths of 480 MHz and 815 MHz are obtained for the three resonators and five resonators configurations, respectively (in S -band with substrate \epsilon_{r} = 2.55 and thickness = 0.318 cm).

Abstract: A numerical method and experimental technique for microwave imaging of inhomogenous bodies is presented. This method is based on the interpretation of the diffraction phenomena and leads to tomographic reconstruction of the body under investigation. Various numerical examples are given on spatial impulse response, recognition of dielectric rods, inhomogeneous bodies, and simulated human arm. Different experimental results on dielectric rods and isolated animal organs are also given.

Abstract: A previously derived theory is applied to a microstrip antenna with a reactive load to produce a dual-band radiator. A model consisting of a rectangular patch radiator loaded with a variable length short-circuited coaxial stub was investigated experimentally. Comparisons of theoretical predictions and experimental data are made for the impedance and resonant frequencies as a function of the position of the load, the length of the stub, and the characteristic impedance of the stub.

Abstract: A uniform high-frequency solution is presented for the diffraction by a wedge with impedance faces illuminated by a plane wave perpendicularly incident on its edge. Arbitrary uniform isotropic impedance boundary conditions may be imposed on the faces of the wedge, and both the transverse electric (TE) and transverse magnetic (TM) cases are considered. This solution is formulated in terms of a diffraction coefficient which has the same structure as that of the uniform geometrical theory of diffraction (UTD) for a perfectly conducting wedge. Its extension to the present case is achieved by introducing suitable multiplying factors, which have been derived from an asymptotic evaluation of the exact solution given by Maliuzhinets. When the field point is located on the surface near the edge, a more accurate asymptotic evaluation is employed to obtain a high-frequency expression for the diffracted field, which is suitable for several specific applications. The formulation described in this paper may provide a useful, rigorous basis to search for a more numerically efficient but yet accurate approximation.

Abstract: An approach to the analysis of microstrip antennas on cylindrical bodies is presented. The printed radiator is replaced by as assumed surface current distribution, and the fields are solved taking into account the presence of the dielectric layer and the metallic cylinder. Calculation takes place in the Fourier domain. The far field, calculated asymptotically from this solution, is used to get the radiation patterns of the wraparound antenna for any dielectric and the half-wavelength patch for \epsilon_{r} = 1 .

Abstract: Three new configurations for increasing the impedance bandwidth of the microstrip patch antennas are described. In these configurations, additional resonators are directly coupled through short sections of microstrip line to the radiating edges, nonradiating edges, and all the four edges of the rectangular patch antennas, respectively. Green's function approach and segmentation method are used for the analysis. The experimental results are in reasonable agreement with the analysis and impedance bandwidths of 548 MHz, 605 MHz, and 810 MHz are obtained for these three configurations, respectively in S -band (substrate thickness = 0.318 cm and \epsilon_{r} = 2.55 ). The variation in the radiation pattern over this impedance bandwidth is discussed.

Abstract: The dc conductivity and complex dielectric constant at frequencies of 7.50, 9, 11, 30, and 40 GHz of 16 samples of sea water brine in equilibrium with sea ice with freezing temperatures ranging from - 2.8\deg C to -25.0\deg C have been measured. The data is analyzed to yield parameters occurring in a Debye relaxation equation so that the dielectric constant of brine may be calculated throughout the microwave region of the electromagnetic spectrum.

Abstract: Michaeli recently derived a set of equivalent edge currents for scattering directions not on the Keller cone. Some years ago, Mitzner developed his incremental length diffraction coefficient (ILDC) for the same purpose. It is shown that Michaeli's results relate to Mitzner's for arbitrary directions in identically the way Keller's relate to Ufimtsev's for directions on the Keller cone.

Abstract: Input impedance and bandwidth of an annular ring microstrip antenna have been determined by modeling the antenna as a section of radial line loaded with wall admittances. The effect of mutual coupling between the radiating apertures has been taken into account. The theoretically calculated values of input impedance for TM 12 -mode are compared with measurements. The agreement is good. Higher order modes have been found to be present in the vicinity of TM 12 -mode. The reactance due to these modes can be utilized to increase the voltage standing-wave ratio (VSWR) bandwidth of the antenna.

Abstract: The problem of scattering from a resistive strip grating is formulated in the spectral domain. Results of numerical calculations of the reflection coefficient are presented for perfectly conducting strips and strips with resistivities up to 750\Omega /square.

Abstract: Integral equation formulations are presented for characterizing the electromagnetic (EM) scattering interaction for nonmetallic surfaced bodies. Three different boundary conditions are considered for the surfaces: namely, the impedance (Leontovich), the resistive sheet, and its dual, the magnetically conducting sheet boundary. The integral equation formulations presented for a general geometry are specialized for bodies of revolution and solved with the method of moments (MM). The current expansion functions, which are chosen, result in a symmetric system of equations. This system is expressed in terms of two Galerkin matrix operators that have special properties. The solutions of the integral equation for the impedance boundary at internal resonances of the associated perfectly conducting scatterer are examined. The results are compared with the Mie solution for impedance-coated spheres and with the MM solutions of the electric, magnetic, and combined field formulations for impedance-coated bodies.

Abstract: To simulate a thin layer of material whose permittivity and permeability both differ from the values for the surrounding medium, a combination resistive and conductive sheet is defined and its properties described.

Abstract: An analysis is described for determining the current induced by a known excitation on a conducting cylinder located near the planar interface between two semi-infinite, homogeneous half-spaces of different electromagnetic properties. The perfectly conducting cylinder of general cross section is of infinite extent and the excitation is transverse magnetic to the cylinder axis. An integral equations for the induced current is derived and a numerical method for solving it is developed. The kernel of the integral equation contains a term corresponding to the usual open-space Green's function plus a term proportional to a Sommerfeld-type integral in two dimensions. Various forms of the Sommerfeld-type integral are given and the choices of form amenable to efficient evaluation are discussed. For a flat strip, a circular cylinder, and a rectangular cylinder, data are presented and discussed for selected parameters. Data are presented for cylinders above and below the interface as well as for a cylinder resting on the interface.

Abstract: The directivity of a reflector antenna deteriorates as the feed moves away from the focal point for beam scanning. This deterioration can be substantially reduced if a cluster feed instead of a single feed is used to control a beam. A closed-form solution is presented for the cluster excitation to achieve the optimum directivity. For an offset 108 \lambda parabolic reflector scanning 10 beamwidths, the optimum-directivity achieved by a 19-element (seven-element) cluster is 12 dB (8 dB) higher than that of a single element. Comparison of the optimum-directivity design and the popular conjugate field matching design is made. When the cluster spacing d is greater than 1 \lambda , it is found that the optimum directivity is higher than that of conjugate field matching (CFM) scheme by an insignificant amount, although the excitations of two designs can be quite different. For d , the optimum design may exhibit the supergain phenomenon, namely, extremely high directivities achieved by an oscillatory cluster excitation.

Abstract: The singularity expansion method (SEM) represents transient scattering by superposition of damped oscillatory fields corresponding to the complex resonant frequencies of the scatterer. The series of these global wave fields, which encompass the scattering object as a whole, is slowly convergent at early observation times and even deficient at very early times when portions of the object are as yet unexcited. Thus, the resonance series representation must generally be augmented by an entire function in the complex frequency domain. The choice of the entire function is relatively arbitrary but affects the excitation coefficients, called coupling coefficients, of individual resonances and also the "turn-on" and "switch-on" times of the SEM series. Moreover, it contains essential (intrinsic) and nonessential (removable) portions which have been subjected to various interpretations. By formulating the transient problem in terms of traveling (progressing) incident, reflected and diffracted wavefronts, these constructs in the SEM can be interpreted in a precise and physical manner. Furthermore, the analysis clarifies the evolution of resonances as collective summations of multiple wavefront fields which are caused by successive reflections or diffractions at the surfaces and scattering centers comprising the object. By combining wavefronts and resonances self-consistently, one may construct a hybrid field that avoids the difficulties at early times in the SEM formulation. The systematic exploration of the interplay between wavefronts and resonances is facilitated through use of a flow diagram, as introduced in system theory. These concepts are developed in broad generality and are illustrated for two-dimensional scattering by various special configurations.

Abstract: The radiation from a dipole in the presence of a grounded general gyromagnetic-electric (gyrotropic) layer is investigated. The use of matrix methods in conjunction with Fourier transformation techniques greatly facilitates the formulation of the boundary-value problem, reducing the algebraic complexity to a minimum, and provides a closed-form representation of the electromagnetic field over the anisotropic region. Numerical results in plots, related to the radiation pattern of the structure, are also included for various cases.

Abstract: The computation of low frequency scattering of electromagnetic fields by solid/hollow dielectric or conducting cylinders using the boundary element method (BEM) is considered. A general computer program has been developed for both transverse electric and magnetic cases. Numerical examples are given for conducting circular cylinders, and solid and hollow dielectric cylinders. The computational accuracy is checked by comparing the results with the analytic solution or computing an error defined from the optical theorem. In addition some problems at an interior resonance of the scatterer are discussed. The method can be directly applied to more complicated geometries.

Abstract: One of the objectives of this paper is to discuss the mathematical requirements that the expansion functions must satisfy in the method of moments (MM) solution of an operator equation. A simple differential equation is solved to demonstrate these requirements. The second objective is to study the numerical stability of point matching method, Galerkin's method, and the method of least squares. Pocklington's integral equation is considered and numerical results are presented to illustrate the effect of various choices of weighting functions on the rate of convergence. Finally, it is shown that certain choices of expansion and weighting functions yield numerically acceptable results even though they are not admissible from a strictly mathematical point of view. The reason for this paradox is outlined.

Abstract: The resolving capability of underground imaging radar employing a multifrequency holographic approach is characterized in terms of controlling parameters such as the synthetic aperture length, soil conductivity and dielectric constant, and antenna beamwidth. The propagation velocity of electromagnetic wave in soil, which varies from soil to soil, is an essential parameter for reconstructing object images using the holographic approach. Hence, a method of estimating the propagation velocity and also the depth of an object from a pulse-echo image based on the minimum squared error is proposed. The result of underground object image reconstruction from real pulse-echo data is reported to demonstrate high estimation accuracy of the propagation velocity and fine resolution of the reconstructed image.

Abstract: Assessment of the radiation characteristics of certain types of radome covered antennas poses the problem of transmission of amplitude tapered large-aperture fields through a curved dielectric layer. Modeling the amplitude taper by a Gaussian, the incident illumination can be generated by a source at a complex coordinate location. The radome problem is then addressed by tracing complex ray fields from the complex source point through the complex extension of the radome configuration to the real location of the observer. No integrations over equivalent apertures are required here. The complex ray tracing has been performed for conventional complex geometric trajectories as well as more accurate trajectories with lateral shifts at the radome interfaces, and takes into account multiple reflections inside the radome. As previously for real rays, the multiple internally reflected complex rays can be combined into a "collective ray" that is weighted with a curvature and (or) taper corrected slab transmission coefficient. Numerical calculations for various two-dimensional circular cylindrical and wedge-tapered prototypes and various beam illuminations demonstrate the feasibility of the complex ray method, the efficiency of the collective ray concept, and the adequacy of the ray model without lateral shifts for the radome application.

Abstract: A solution for scattering from thin dielectric disks has been obtained by approximating the currents induced inside the disk with the currents which would exist inside a dielectric slab of the same thickness, orientation and dielectric properties. This procedure yields an electrostatic approximation when the disk thickness T is small compared to the wavelength of the incident radiation and yields a conventional physical optics approximation when the dimension A , characteristic of the geometrical cross section of the disk, is large compared to wavelength. When the ratio A/T is sufficiently large one or the other of these approximations applies, regardless of the frequency of the incident radiation. Consequently, the solution provides a conventional approximation for the scattered fields at all kA . As a check on this conclusion, a comparison has been made between measurements of the radar cross section of thin dielectric disks and the cross sections predicted using this theory. Agreement was found for thin disks with both large and small values of kA .

Abstract: The normalized backscattering width of an infinitely long, dielectric coated circular cylinder is obtained via a high frequency ray solution. The ray solution provides a physical picture of the scattering process in terms of a geometrical optics ray and two surface waves. It is shown that the surface wave resonance phenomena in the backscattering fields of the coated cylinder can be predicted in terms of the Regge poles of the coated cylinder. The numerical results for the backscattering widths of the cylinder obtained via the high frequency ray solution show excellent agreement with the eigenfunction results. The trajectories of the Regge poles associated with the coated circular cylinder are also presented.

Abstract: The objective is to show from a mathematical standpoint that there are certain rules that must be followed in the choice of weighting functions used in the method of moments (MM). It is shown that for a particular problem it is the operator that dictates the method (Galerkin's method or another method such as the method of least squares) to be applied, and it is not computational considerations only. For example, it is shown that in solving Hallen's and Pocklington's equation by the method of moments, it is unnatural to choose the weighting functions which are zero at the ends of the domain of the solution. The deficiency of certain weighting functions is presented based on mathematical reasoning, and a numerical example is given to illustrate the effect of the choice of the weighting functions on the rate of the convergence of the solution.

Abstract: A wide-band, electrically small, disk-loaded antenna, comprising of a disk, 0.26 wavelength (at midband) in diameter, located 0.097 wavelength above a ground plane, has been designed and tested. A unique experimental procedure was used to determine the parameters of the impedance matching network, which consists of a conductive biconical center post and two structural side posts located in the space under the disk. The resulting antenna has a maximum voltage standing-wave ratio (VSWR) of 2:1 over a frequency bandwidth ratio of approximately 2:1. A second model, designed using the same technique, has a maximum VSWR of 3:1 over a frequency bandwidth ratio of 3:1. This antenna was compared to a multi-element disk-loaded antenna (with the same size profile) designed by Dr. Georg Goubau. This multi-element antenna also has a maximum VSWR of 2:1 over a frequency band of approximately 2:1. The comparison shows that the simple disk-loaded antenna, with fixed double tuning, achieves the same low VSWR as the multi-element disk-loaded antenna with fixed triple tuning. Therefore, an increase in bandwidth could be achieved in the simple disk-loaded antenna by applying higher order tuning.

Abstract: The first experimental validation is reported of the finite-difference time-domain (FD-TD) method for modeling the monostatic radar cross section (RCS) of three-dimensional conducting structures. The structures modeled and tested span up to nine free-space wavelengths ( k_{0}s =57 ). This represents a thirty-fold increase in electrical size over the previous analytically validated case of FD-TD modeling of radar cross section. It appears that the cases studied represent the largest detailed three-dimensional numerical scattering models of any type ever verified wherein a uniformly fine spatial resolution and the ability to treat nonmetallic composition is incorporated in the model. It was found that FD-TD provided a high modeling accuracy of 1 dB (with respect to the measurements) over at least a 40-dB dynamic range of radar cross section values for the nine-wavelength size objects, which exhibited such scattering physics as edge and corner diffraction, corner reflection (double bounce), and cavity penetration.

Abstract: Standard Chebyshev and Taylor reduced sidelobe synthesis techniques ignore mutual coupling, and so can lead to pattern errors where the resulting array pattern departs significantly from the desired pattern. Two methods for correcting this error are described and examples of their application to eight-element dipole arrays are presented. One approach uses characteristic modes, while the other method employs a simpler array mode and point matching. The techniques can also be applied to synthesis of nonuniform arrays.

Abstract: Analytic solutions of the integral equations for the current induced on a narrow conducting strip are obtained by two methods. One method follows from an expansion of the strip current in a series of Chebyshev polynomials while the other is based on a special power series expansion. In the former, Fourier-type coefficients, depending on integrals of the excitation, are derived while in the latter the coefficients depend upon derivatives of the excitation. From knowledge of the strip current simple expressions for the far-zone scattered field are derived. Explicit results are given for plane wave excitation. The solution methods apply to the equations for the narrow slot in a planar conducting screen.

Abstract: Reflector antennas with mesh surfaces are used extensively in many satellite and ground antenna systems. A strip-aperture modeling of commonly used mesh surfaces is presented which provides considerable versatility in characterizing the mesh cells. The mesh transmission coefficients are constructed using a Floquet-modal expansion in conjuction with two dominant aperture modes. To account for the mesh local coordinates, the Eulerian angle transformation is invoked to obtain the total induced current on the curved reflector surface. General formulas are presented to show how the solid surface induced current is modified due to the transmission through the mesh. The effects of a variety of mesh configurations on both the co-polar and cross-polar patterns of reflector antennas are studied by numerically evaluating the vector diffraction integral using the Jacobi-Bessel expansion. For some special cases, a comparison is made with the results of the commonly used wire-grid formulation. Many of the numerical data are tailored to the dimensions of a conceptually designed mesh deployable offset reflector of the land mobile satellite system (LMSS).