Kuniaki Yoshitomi

Efficient calculation of lattice sums for free-space periodic Green's function

An efficient method to calculate the lattice sums is presented for a one-dimensional (1-D) periodic array of line sources. The method is based on the recurrence relations for Hankel functions and the Fourier integral representation of the zeroth-order Hankel function. The lattice sums of arbitrary high order are then expressed by an integral of elementary functions, which is easily computed using a simple scheme of numerical integration. The calculated lattice sums are used to evaluate the free-space periodic Greens function. The numerical results show that the proposed method provides a highly accurate evaluation of the Greens function with far less computation time, even when the observation point is located near the plane of the array.

Numerical, analytical, and experimental studies of scattering from very rough surfaces and backscattering enhancement

Abstract This paper Presents numerical simulations, theoretical analysis, and millimeter wave experiments for scattering from one-dimensional very rough surfaces. First, numerical simulations are used to investigate the effects of roughness spectrum, height variation, interface medium, polarization, and incident angle on the backscattering enhancement. The enhanced backscattering due to rough surface scattering is divided into two cases; the RMS height close to a wavelength and RMS slope close to unity, and RMS height much smaller than a wavelength with surface wave contributions. Results also show that the enhancement is sensitive to the roughness spectrum. Next, a theory based on the first- and second-order Kirchhoff approximation modified with angular and propagation shadowing is developed. The theoretical solutions provide a physical explanation of backscattering enhancement and agree well with the numerical results. In addition to the scattering of a monochromatic wave, the analytical results of the ...

Analysis of TM scattering from finite rectangular grooves in a conducting plane

TM-wave scattering from rectangular grooves in a perfectly conducting plane is investigated. A Fourier-transform technique is employed to express the scattered field in the spectral domain as a superposition of the parallel-plate waveguide modes. The boundary conditions are enforced on the conducting surface and the groove apertures to obtain simultaneous equations for the transmitted field inside the grooves. The simultaneous equations are solved to obtain the transmitted field in a series representation that reduces to a closed form in a high-frequency scattering regime. With the use of the stationary phase approximation, the far-zone scattered field is obtained, and its scattering behavior is studied in terms of the scattering angle. The diffraction efficiency for the finite grating as the number of grooves increases is presented.

An analysis of transverse electric scattering from a rectangular channel in a conducting plane

The problem of transverse electric plane wave scattering from a rectangular channel which is engraved in a perfect conducting plane is investigated. A Fourier transform technique is employed to express the scattered field in the spectral domain in terms of parallel-plate waveguide modes. The boundary conditions are enforced on the conducting surface and the channel aperture to obtain simultaneous equations for the transmitted field inside the channel. The simultaneous equations are solved to represent the transmitted field in a series form. By using the stationary phase approximation, the exact expression for the far-zone scattered field is obtained and the echo width behavior is numerically studied in terms of the scattering angle, frequency, and channel size. It is found that the behavior of the echo width versus the channel depth exibits resonance irrespective of the size of the channel width. An approximate series expression (Kirchhoff solution) for the echo width is valid for high-frequency scattering regimes (a > 2 λ). We also present an approximate closed form solution for nadir backscattering which is valid when a > 0.2λ.

An analytic solution for transverse‐magnetic scattering from a rectangular channel in a conducting plane

The problem of transverse‐magnetic plane‐wave scattering from a rectangular channel in a perfect conducting plane is investigated. A Fourier‐transform technique is employed to express the scattered field in the spectral domain in terms of parallel‐plate waveguide modes. An approximate series solution for scattering is presented in a closed form that is valid for a high‐frequency scattering regime. The presented formulation is computationally much more efficient than other existing numerical solutions.

Radiation from a rectangular waveguide with a lossy flange

The radiation from a rectangular waveguide with a lossy flange coating is studied both analytically and experimentally. An analytic method using Fourier analysis and an impedance boundary condition is proposed. The experiment was performed using a flange covered with a rubber ferrite sheet at an X-band frequency. The numerical and measured results agree well and show that the lossy flange improves the E-plane radiation pattern and the crosspolar radiation. A boresight gain drop of about 2 db is predicted theoretically and measured experimentally. >

RIGOROUS ANALYSIS OF RECTANGULAR WAVEGUIDE JUNCTIONS BY FOURIER TRANSFORM TECHNIQUE

A rigorous and efficient method for characterizing rectangular waveguide junctions is presented. Using the image theory, in this method, the junction problem is transformed into an aperture problem in a uniform rectangular waveguide. The fields in the cavity region expressed by Fourier integrals are matched to those on apertures expanded by the normal modes of arm-waveguides. Then a system of linear equations with very fast convergence is obtained to determine scattering parameters of the junction. The scattering parameters are calculated for right-angle corner bends, symmetric E- and H-plane T-junctions, asymmetric E- and H-plane T-junctions, and asymmetric series E-plane T-junctions, which agree very well with literatures.

RIGOROUS ANALYSIS OF E-/H-PLANE JUNCTIONS IN RECTANGULAR WAVEGUIDES USING FOURIER TRANSFORM TECHNIQUE

A rigorous and efficient method for characterizing E-/H-plane multiport junctions in rectangular waveguides is presented by using the mode-matching method combined with Fourier transform technique. The method is based on a field-theoretical full-wave approach. The electric and magnetic fields of a main waveguide in the junction are expressed in terms of Fourier integrals and matched in the Fourier transformed domain to the aperture fields expanded by the normal modes in each of arm waveguides. This yields a set of linear equations in analytically closed form to calculate the scattering parameters of the multiport systems. The numerical examples for a Magic-T junction and an asymmetric E-/H-plane three-port junction verify the accuracy and usefulness of the present method.

Radiation from a slot in an impedance surface

The effects of a lossy-ground plane on the radiation from a slot are analyzed by using a Greens function of an impedance surface. Numerical results of the radiation field, radiated power, and loss power are presented for various lossy ground planes. In particular, the radiation field and the radiated power from a short slot are given by simple closed expressions. The analytical model of a slot in an impedance surface is validated by radiation field measurements.

Surface roughness determination using spectral correlations of scattered intensities and an artificial neural network technique

An artificial neural network (ANN) technique is applied to the determination of the RMS height and the correlation distance of one-dimensional rough surfaces. The surface is illuminated by a beam wave, and the intensity correlations of the scattered wave at two wavelengths in the specular and backward directions are used to determine the roughness parameters. Scattered intensity correlations calculated by Monte Carlo simulations are used to train the ANN, and two methods, the explicit inversion method and the iterative constrained inversion method, are used to perform the inversion. The inversion values are compared with the target values, and the iterative constrained method is shown to give smaller errors, but it requires longer computer CPU time. >