Yasuo Kuga

Retroreflectance from a Dense Distribution of Spherical Particles

The backscattered intensity from a dense distribution of latex microspheres is measured near the retroreflection direction. It is shown that a sharp peak appears in the retroreflection direction when the volume density is above 1%. The angular width of this peak is much smaller than (wavelength)/(particle size) and cannot be explained by Mie theory, double-passage effects, or radiative-transfer theory. When the particle size D is less than the wavelength λw, a small peak appears at the retroreflection direction. When D is 2–4 times greater than λw, the peak becomes large as the density increases. When D is many times greater than λw, the sharp peak at the retroreflection direction is superimposed upon the Mie-scattering pattern. The angular width of the peak is of the order of (a wavelength)/(a mean free path).

Attenuation constant of a coherent field in a dense distribution of particles

In a dense distribution of particles, the propagation characteristics of the coherent field are strongly affected by the pair-correlated distributions of scatterers. This paper presents an optical experimental study to show that, when the particle density is greater than about 0.1%, the attenuation constant departs markedly from the formula based on an uncorrected scatter assumption. It decreases sharply when ka < 1, whereas it shows a slight increase when ka ≫ 1. Experimental data are shown for the volume densities ranging from 10−3 to 40% and ka ranging from 0.529 to 82.793. Comparisons are given with some theoretical calculations.

Backscattering enhancement of electromagnetic waves from two-dimensional perfectly conducting random rough surfaces: a comparison of Monte Carlo simulations with experimental data

Predictions of an exact numerical model for scattering from a surface randomly rough in two directions are compared with experimental data. The numerical model is based on Monte Carlo simulation using an iterative version of the method of moments known as the sparse-matrix flat-surface iterative approach (SMFSIA). Experimental data is obtained from millimeter wave laboratory experiments in which the bistatic scattering patterns of fabricated surfaces with known statistical parameters were measured. The surfaces studied have both a Gaussian height distribution and correlation function, so that their statistics are characterized by an rms height and correlation length. An rms height of 1 wavelength and correlation lengths ranging from 1.41-3 wavelengths are investigated in this paper, and the phenomenon of backscattering enhancement is observed both in the numerical predictions and experimental data. A comparison of the absolute value of the bistatic scattering coefficient as normalized by the incident power shows the theory and experiment to be in good agreement.

An imaging technique using confocal circular synthetic aperture radar

This paper presents a theory and its experimental demonstration of an imaging technique based on three-dimensional (3D) space-time confocal imaging and circular synthetic aperture radar (SAR). The theory is an extension of the conventional straight-path SAR-to-SAR on an arbitrary curved path. Next, a general formulation for the curved SAR is applied to circular SAR geometry, which has two important features. First, it allows the maximum attainable resolution to be an the order of a wavelength. Second, it makes 3D confocal imaging possible, X-band (7-13 GHz) imaging experiments are conducted to demonstrate this technique.

GENERALIZED SURFACE PLASMON RESONANCE SENSORS USING METAMATERIALS AND NEGATIVE INDEX MATERIALS

Optical surface plasmon resonance sensors have been known for a long time. In this paper, we discuss the use of metamaterials to construct a surface plasmon sensor which can be used at microwave frequencies. We review the conditions for the existence of surface plasmon and the use of the forward and backward surface waves. A sharp dip in the reflection coefficient occurs when the propagation constant of the incident wave along the surface is nearly equal to the propagation constant of the plasmon surface wave and may be used to probe bulk material characteristics or to determine metamaterial characteristics. Numerical examples are given to illustrate the basic characteristics.

Generalized constitutive relations for metamaterials based on the quasi-static Lorentz theory

This paper presents a method of calculating the elements of the generalized matrix representation of the macroscopic constitutive relations for a three-dimensional (3-D) array of non-magnetic inclusions with arbitrary shape. The derivation is based on the quasi-static Lorentz theory and the inclusions are represented by electric and magnetic dipole moments. The 6/spl times/6 constitutive relation matrix is expressed in terms of the interaction matrix and the polarizability matrix, which can be numerically calculated using the sum and the difference of opposing plane wave excitations. Numerical examples are given for split ring resonators and a chiral medium consisting of an array of helices to illustrate the usefulness of the formula and to verify the consistency constraint and reciprocity relations for a bianisotropic medium.

Ionospheric effects on synthetic aperture radar at 100 MHz to 2 GHz

Recently, there has been increasing interest in the use of spaceborne synthetic aperture radar (SAR) for measuring forest biomass. However, it is noted that conventional SAR using C-band or higher frequencies cannot penetrate into foliage, and therefore the biomass measurements require longer wavelengths, typically P-band (500 MHz). It is also known that the ionosphere is highly dispersive, causing group delay and broadening of pulses. The variance of the refractive index fluctuations due to turbulence is approximately proportional toƒ−4. In addition, the Faraday rotation due to the geomagnetic field in the ionosphere becomes significant. This paper presents an analysis with numerical examples of the following effects in the frequency range from 100 MHz to 2 GHz in order to show the frequency dependence and the effects of total electron content (TEC) of the ionosphere. First, the ionospheric turbulence can reduce the coherent length below the equivalent aperture size, and the azimuthal resolution becomes greater than D/2 where D is the antenna aperture size. Second, the ionospheric dispersion causes a shift of the imagery due to the group velocity. Third, the dispersion also creates broadening of the pulse. In addition, multiple scattering due to ionospheric turbulence gives rise to pulse broadening. Fourth, we consider the Faraday rotation effect and show that the ellipticity change is negligible, but the orientation angle changes significantly at P-band. Numerical examples are shown using typical ionospheric parameters, turbulence spectrum, and TEC values.

Scattering and diffusion of a beam wave in randomly distributed scatterers

Diffusion theory is applied to the transmission of an optical beam through randomly distributed particles, and the theoretical calculations are compared with experimental data for an optical beam at 0.6 μm propagating through latex scatterers of sizes 0.109 and 2.02 μm. It is shown that, for particles small compared with the wavelength, the diffusion theory gives good agreement with experimental data; whereas for particles large compared with the wavelength, the diffusion theory is applicable when the optical depth is greater than about 20. For shorter optical depth, experimental results are also compared with the first-order scattering theory.

Ionospheric effects on SAR imaging: a numerical study

There has been an increasing interest in the use of spaceborne very high frequency ultra high frequency (VHF-UHF) synthetic aperture radar (SAR) for measuring forest biomass and for detecting underground facilities. The propagation characteristics of the low-frequency electromagnetic wave are severely affected by the ionosphere. Recently, Faraday rotation effects and SAR image degradation have been studied using an analytical model and a homogeneous ionosphere. In this paper, a numerical model is developed to investigate the SAR image degradation caused by an inhomogeneous ionosphere. Both horizontal and vertical structures of the ionosphere are considered in this model. Three different cases are studied. The first is a vertically homogenous ionosphere, where the simulation condition is the same as in the analytical study by Ishimaru and others. The second is a vertical profile, which is introduced using the Chapman formula. The ray-bending effect is added for the ionosphere with a layered structure. Finally, both the vertical profile in electron density and the horizontal gradient in total electron content are considered in the simulation. Simulation results show good agreement with the theoretical analysis under the same conditions of the ionosphere. When both horizontal and vertical structures and the inhomogeneity of the ionosphere are considered in the model, the simulation result shows further image degradation and shift caused by the ray-bending effect. The simulation results also show the strong frequency dependence of the SAR image resolution.

Polarized pulse waves in random discrete scatterers

In recent years there has been increasing interest in the use of polarization for imaging objects in a cluttered environment. Examples are optical imaging through clouds, optical detection of objects in a biological medium, and microwave detection of objects in clutter. We extend previous studies of continuous-wave scattering to pulse-polarization scattering in discrete scatterers. We solve the time-dependent vector radiative transfer equation for a plane-parallel medium by using Mie scattering and the discrete ordinates method. The time-dependent degree of polarization and cross-polarization discrimination are calculated and verify the advantages of circular over linear polarization in maintaining greater copolarized components rather than cross-polarized components.