Analysis of Planar and Circular Cylindrical Multilayer Structures with Application to Soft and Hard Surfaces

Sammanfattning: This thesis deals with the analysis of planar and circular cylindrical multilayer structures, such as strip-loaded dielectric layers, corrugated surfaces and microstrip antennas. In particular, we study corrugations and strip-loaded grounded dielectric slabs when they are used to realize soft and hard surfaces. Soft and hard surfaces have polarization independent boundary conditions and find applications in antenna design to improve symmetry of the radiation pattern, to reduce the sidelobe level, and to reduce scattering from the supporting structure of antennas.

The analysis of multilayer structures is done by using an integral equation approach and the moment method. We simplify the analysis of strips and corrugations by using the asymptotic boundary conditions, and we derive closed-form spectral domain Green's functions of multilayer structures which contains a corrugated surface or a strip layer. The boundary conditions are asymptotic in the sense that the exact solution approaches the asymptotic one if the periodicity of the corrugations or strips goes to zero. The fulfillments of the soft and hard boundary conditions are investigated by considering the electric field in both the near- and far-field regions. The results are strongly influenced by the presence of surface waves supported by the corrugations, dielectric slab and the strips. The surface wave properties are determined by studying the poles of the spectral domain Green's functions. The results represent a contribution to the understanding of soft and hard surfaces, in particular the hard ones.

We introduce an algorithm for calculating the spectral domain Green's functions of planar, circular cylindrical and spherical multilayer structures. The algorithm is based on subdividing the original problem into equivalent subproblems, one for each layer, with the tangential field components at the layer boundaries as unknowns. The algorithm is implemented into three versions of a Fortran routine, one for each geometry. The only difference between them is in two subroutines which calculate the electromagnetic field in homogeneous media due to a current sheet, current tube and current shell for the planar, cylindrical and spherical geometry, respectively. The asymptotic boundary conditions for strips and corrugations are also implemented in the numerical routine. The routine is used in different moment method programs for calculating characteristics of microstrip antennas and periodic structures. We give the results of scattering from a periodic strip grid on planar and cylindrical substrates, and of the input impedance and radiation pattern of a cylindrical patch antenna. Furthermore, we use the routine to calculate the absorbed power in a lossy half-plane, cylinder and sphere, when there is a dipole or a Huygen's source in the vicinity of them. We also consider symmetry properties of the Green's functions and of the elements of the moment method matrix.