We design a four-reflector offset antenna satisfying the cross-polarization elimination condition and the broadband characteristics condition which consists of one primary horn, three subreflectors and one main reflector. The cross-polarization elimination condition for the four-reflector offset antennas is expressed by the equations of hyperbolas with the coordinate axes of the reciprocal of equivalent focal lengths. The configurations of the reflector system are derived simply from the graphical representation because four-reflector offset antennas satisfying these relationships exist on the hyperbolas with the coordinate axes of the reciprocal of equivalent focal lengths. Furthermore, we clarified that the derived condition for having planar phase front applying the broadband characteristics condition is independent of frequency. An actual design example for the four-reflector offset antennas satisfying the cross-polarization elimination condition and the condition for having planar phase front, both of which are independent of frequency is shown. The design method using the graphical representation is simpler than that of the tri-reflector offset antennas.

Scattering responses from a dielectric sphere are analyzed in the time-frequency domain by using two types of wavelet transform in order to reveal the scattering mechanisms. In the resulting time-frequency displays, various scattering processes including reflection, refraction, and diffraction can be clearly resolved and identified. The delay time of each scattering process agrees well with that obtained by the ray theory. Furthermore, the natural frequencies that are not easy to extract by the conventional Fourier analysis can be extracted.

A simple numerical method for calculating paths of creeping rays around an arbitrary convex object is presented. The adventage of this method is that the path of creeping ray is iteratively determined from initial values of incident point and incident direction of the creeping ray without solving differential equation of geodesic path. As the numerical examples, the path of creeping ray on the prolate spheroid and the resonance path of natural modes are shown.

Scattering data from radar targets are analyzed in the time-frequency domain by using wavelet transform, and the scattering mechanisms are investigated. The wavelet transform used here is a powerful tool for the analysis of scattering data, because it can provide better insights into scattering mechanisms that are not immediately apparent in either the time or frequency domain. First, two types of wavelet transforms that are applied to the time domain data and to the frequency domain data are defined, and the multi-resolution characteristics of them are discussed. Next, the scattering data from a conducting cylinder, two parallel conducting cylinders, a parallel-plate waveguide cavity, and a rectangular cavity in the underground are analyzed by using these wavelet transforms to reveal the scattering mechanisms. In the resulting time-frequency displays, the scattering mechanisms including specular reflection, creeping wave, resonance, and dispersion are clearly observed and identified.

The Yasuura method is effective for calculating scattering problems by bodies of revolution. However dealing with 3-D scattering problems, we need to solve bigger size dense matrix equations. One of the methods to solve 3-D scattering is to use multipole expansion which accelerate the convergence rate of solutions on the Yasuura method. We introduce arrays of multipoles and obtain rapidly converging solutions. Therefore we can calculate scattering properties over a relatively wide frequency range and clarify scattering properties such as frequency dependence, shape dependence, and polarization dependence of 3-D scattering from perfectly conducting scatterer. In these numerical results, we keep at least 2 significant figures.

A high-frequency approximate method for calculating the diffraction by a smooth convex surface is presented. The advantage of this method is the validity of it in the caustic region of the creeping rays where the Geometrical Theory of Diffraction (GTD) becomes invalid. The concept used in this method is based on the Method of Equivalent Edge Currents (EEC), and the equivalent line currents for creeping rays which are derived from the diffraction coefficients of the GTD are used. By evaluating the radiation integral of these equivalent line currents, the creeping ray contribution which is valid within the caustic region is obtained. In order to check the accuracy and the validity of the method, the diffraction problem by a perfectly conducting sphere of radius a is solved by applying the method, and the obtained results are compared with the exact and the GTD solutions. It is confirmed from the comparison that the failure of the GTD near the caustic is removed in this method and accurate solution is obtained in this area for high-frequency (ka8). Furthermore, it is also found that this method is valid in the backward region (0θ90, θ is an observation angle mesuered from an incident direction), whereas not in the forward region (90θ180).

Electromagnetic wave scattering from a perfectly conducting indented body of revolution is analyzed both in the frequency and time domains. The three-dimensional (3-D) scattering process and the effect of polarization on scattering characteristics are revealed. A well-defined scattering matrix representation is adopted for investigating the polarimetric property of the 3-D scattering. First, the scattering cross sections are calculated in the frequency domain by using the Yasuura method (Mode-Matching Method) which is a powerful and reliable numerical method for solving electromagnetic boundary value problems. Next, co-polarization and cross-polarization components of the pulse response waveforms are calculated from the scattering matrix by using the Fourier synthesis technique. It is found out from the numerical results that the pulse responses from the indented objects can be interpreted as three distinctive waves: conventional specularly-reflected wave, creeping wave and complex specularly-reflected wave which is reflected from a complex specular reflection point. These scattering processes are consistently explained by employing the extended ray theory. And the radar polarimetries of these three distinctive waves are clearly observed both in co-polarization and cross-polarization components of scattering cross sections and pulse responses.

We present a finite-difference time-domain (FD-TD) method with the perfectly matched layers (PMLs) absorbing boundary condition (ABC) based on the multidimensional wave digital filters (MD-WDFs) for discrete-time modelling of Maxwell's equations and show its effectiveness. First we propose modified forms of the Maxwell's equations in the PMLs and its MD-WDFs' representation by using the current-controlled voltage sources. In order to estimate the lower bound of numerical errors which come from the discretization of the Maxwell's equations, we examine the numerical dispersion relation and show the advantage of the FD-TD method based on the MD-WDFs over the Yee algorithm. Simultaneously, we estimate numerical errors in practical problems as a function of grid cell size and show that the MD-WDFs can obtain highly accurate numerical solutions in comparison with the Yee algorithm. Then we analyze several typical dielectric optical waveguide problems such as the tapered waveguide and the grating filter, and confirm that the FD-TD method based on the MD-WDFs can also treat radiation and reflection phenomena, which commonly done using the Yee algorithm.