PROPAGATIONAL CHARACTERISTICS OF CENTIMETER AND MILLIMETER WAVES BY NUMERICAL AND EXPERIMENTAL TECHNIQUES

Abstract

The classical solution of electromagnetic wave inter action with penetrable bodies are limited to simple objects of separable boundaries such as spheres, cylinders etc. Recent developments in scattering of arbitrary shape includes the method of moment, the geometric theory of diffraction, the perturbation method, the point matching method and the Fredhlom-Integral equations approach. The latest and promising methods are the extended boundary condition method and the finite element technique. In the present work,the finite element method has been applied for the two dimensional and axisymmetric cases for the computation of electromagnetic scattering properties. Galerkin«s principle for two dimensions and variational approach for the axisymmetric cases have been used in forming elemental equations. For the application of boundary conditions, cylindrical harmonics for the two dimensional and spherical harmonics for axisymmetric cases have been used. Global matricies are formed by summation technique. Solution routine incorporates Gauss-elimination with banded symmetric matrices and back substitution. Error minimisation is done through least square surface fitting. Complex amplitude expansion coefficients are first computed. From these expansion coefficients, scattering properties are evaluated. The computer programmes developed on the Finite Element technique are general and applicable to any type of elements •avchosen to discretize the finite element region. Test results for the spherical surface of conducting and lossy body have been compared with those obtained by separation of variables technique. In the second numerical computation, extended boundary condition method has been applied for the axisymmetric multilayexed lossy body to compute complex expansion coefficients for scattering and transmitted fields. Matrix elements are evaluated numerically by Gauss-Legendre quadrature method. Spherical vector harmonics are used in computing the matrix elements. The scattering expansion coefficients are used to evaluate scattering properties such as complex far field amplitude and various cross sections. Test results by T-Matrix method for the spherical surfaces are compared with those obtained by separation of variables method for the multi-layered spherical surface. Attenuation of microwave signals has been exhaustively studied considering the effect of rain, snow, fog,iee etc. However, the study of the effect of sand particles in sand/ dust storms on microwave propagation has been made to a limited extent. In the present work this aspect has been further investigated. Assuming sand/dust storm particles to be spherical in shape, asymptotic expansion for the spherical Bessel and Hankel functions of the Mie coefficients of higher order are used for the evaluation of the attenuation of ~\f«- microwave signal through sand/dust storms and compared with the available analytical expressions. Further, T-Matrix method has been applied to evaluate theoretically the scatter ing properties of sand/dust particles covered with water layer of different thickness at various frequencies. On experimental side, confocal Fabry-Perot resonator has been designed, fabricated and used to evaluate the attenuation of microwave signal under simulated sand/dust storm conditions. The measured quality factor is then used for the prediction of attenuation for different sand concentrations.