Saburo Adachi

FDTD calculation of radiation patterns, impedance, and gain for a monopole antenna on a conducting box

The ability of the finite-difference-time-domain (FDTD) method to calculate radiation patterns, input impedance, and gain for a monopole antenna on a conducting box is demonstrated. Results are given for the bare box and with the box coated with a dielectric layer. Radiation patterns are compared with measurements and with the method of moments for the bare box. Radiation patterns for the dielectric-covered box and all impedance and gain results are compared with measurements only. Good agreement is obtained in all cases. The FDTD approach includes a dielectric covering quite easily, while this would be quite difficult for a method of moments approach. The FDTD method requires similar computer time as the method of moments for a single-frequency result, but produces wide-bandwidth impedance and gain results with much less computer time. >

FDTD analysis of resistor-loaded bow-tie antennas covered with ferrite-coated conducting cavity for subsurface radar

This paper presents a full-wave analysis of a ground penetrating radar (GPR) using the finite-difference time-domain (FDTD) method. The antenna treated here consists of a resistor-loaded bow-tie antenna, which is covered with a rectangular conducting cavity of which inner walls are coated partially or fully with ferrite absorber. Some techniques are introduced into the FDTD analysis to obtain the accurate results and to save the computer resources. The validity of the FDTD analysis is confirmed experimentally. Furthermore, the effects of the ferrite absorber on the GPR characteristics are theoretically investigated in detail. The FDTD results indicate that the remarkable improvement of the antenna characteristics for the GPR system cannot be attained by the ferrite absorber.

Perfectly matched layer absorbing boundary condition for dispersive medium

Berengers perfectly matched layer (PML) absorbing boundary condition (ABC) has been found very effective for truncating the unbounded spatial domain in the finite-difference time-domain (FDTD) computation. The PML ABC was originally introduced for a free-space spatial domain and later extended to a lossy medium using the stretched coordinates. In this paper, we propose a novel PML ABC for a dispersive medium in an ordinary Cartesian coordinate. It is also shown that the PML for the lossy medium can be easily derived from our formulation.

Inverse scattering method for one-dimensional inhomogeneous layered media

An inverse scattering method to reconstruct simultaneously the permittivity profile and the conductivity profile of one-dimensional inhomogeneons medium which makes use of the transverse electric (TE) wave and/or transverse magnetic (TM) wave, is proposed. The medium is illuminated by the TE and/or TM plane wave at oblique incidence, and the data are taken as the reflection coefficients for a set of discrete frequencies and/or a finite number of incident angles. Furthermore, the reflection coefficient data contain the Gaussian noise. The nonlinear integral equation relating the unknown constitutive parameter of the medium to the reflection coefficient for TE wave and/or TM wave is solved by the Newton iteration method. The inverse operator in the Newton method is determined by the regularization method. It is demonstrated in terms of the numerical examples that this method utilizing both polarizations and the incident angle of the incident plane wave is very effective even if the reflection coefficient contains the practical measurement error, or the phase of the reflection coefficient is unknown. Moreover, the relationships between the errors of reconstructed profile and the measured reflection coefficient are also discussed.

FDTD calculation of transient pulse propagation through a nonlinear magnetic sheet

The application of the finite-difference-time-domain (FDTD) method to propagation through a highly conductive nonlinear magnetic material is described. The FDTD calculations will require changing the permeability of the material at each time step based on the value of the magnetic field at the previous time step. The effects of the high conductivity and magnetic material nonlinearity on the stability of the FDTD calculation is investigated. Stability requires reduction of the time-step size to well below the Courant limit. Accuracy is demonstrated by comparison with previously published results. >

Optimization of aperture illumination for radio wave power transmission

A basic design of an aperture illumination of antennas used for radio wave power transmission is presented. In the power transmission using microwave such as solar power satellite, not only the transmission efficiency, but also the transmitting power capacity allowed by the necessary electromagnetic environment is a very important factor for realization. In this paper the circular antenna aperture illumination is optimized in order to give the maximum transmission efficiency with the constraint on the radiation levels at the edge of the receiving aperture and at the radiation peaks of the subsequent sidelobes. This constrained optimization problem is solved by successive use of the sequential unconstrained minimization techniques (SUMT method). The maximum transmission efficiency and receivable average power density are obtained for various constrained levels of the spilled radiation, and are compared with the values in the unconstrained case.

Impedance of a large circular loop antenna in a magnetoplasma

The input impedance of a large circular loop antenna with arbitrary orientation in a cold magnetoplasma is calculated by using a transmission line theory. New impedance resonances for antennas of finite size in a magnetoplasma in the frequency region below and near the electron cyclotron frequency are indicated theoretically. The resonance peak of the impedance at the lower hybrid resonance frequency is also predicted to exist for arbitrarily oriented antennas of finite size. The experiments on the impedance of a large circular loop antenna are carried out for the cases of normal and parallel orientation of the magnetic field with respect to the plane of the loop immersed in a radio frequency-generated laboratory plasma. The newly predicted impedance resonances for the antenna of finite size are observed. It is also shown that the measured impedances agree fairly well with the calculated ones.

InGaAsP/InP buried‐heterostructure lasers (λ = 1.5 μm) with chemically etched mirrors

Buried‐heterostructure InGaAsP/InP lasers with chemically etched mirrors are fabricated successfully. The lasers emit light at ∼1.5 μm. The threshold current of these lasers is nearly the same as that of conventionally fabricated cleaved‐mirror lasers. This fabrication procedure enables us to obtain low‐threshold‐current devices and allows a much wide variety of device design and fabrication compared with the conventional cleaving technique.

Electromagnetic wave propagation along a conducting wire in a general magnetoplasma

The propagation constant of the electromagnetic wave along a very thin conducting wire immersed in a magnetoplasma is obtained explicitly for a general case where the dc magnetic field is applied with an arbitrary inclination with respect to the wire.

Self-oscillations excited by two stream ion-ion instability

In an ion beam-plasma system, low frequency self-oscillations are observed when an ion beam with a velocity near the ion acoustic velocity is injected into the plasma. This oscillation has a discrete spectral structure which results from the fact that the system is axially bounded. A real wave number for each discrete excited mode is determined by the bounded length. The self-oscillation frequency is approximately proportional to the wave number and to the ion beam velocity. These properties of the oscillation are quantitatively explained by the kinetic dispersion relation for the two stream ion-ion instability in the ion beam-plasma system.