Study On The Electromagnetic Characteristic Of Multilayer Planar Periodic Structures

The multilayer planar periodic structure, as a special electromagnetic material, has found wide applications in radar and satellite communication systems. It can be used as frequency selective surfaces (FSSs) to effect the spatial filtering, or as polarizers to effect the polarization conversion. In this dissertation, the analysis of multilayer planar periodic structures and the designs of FSSs and polarizers are investigated. The main contributions are summarized as follows:1. The integral equations (including electric field integral equation (EFIE) and magnetic field integral equation (MFTE)) for single layer planar periodic structures with multilayered isotropic media are deduced by the mode matching method and the spectral domain approach (SDA), respectively. The integral equations are then solved by the the method of moment, and the scattering parameters are obtained. The cascading technique of the generalized scattering matrix is applied to analyze multilayer planar periodic structures. The spectral dyadic Green's function of multilayered bi-anisotropic media is deduced. Then, the SDA is extended to planar periodic structures with multilayered bi-anisotropic media. The resonant characteristics of FSSs on ferrite substrates are also studied. An approximate formula for calculating the resonant frequencies of FSSs on biased ferrite substrates, whose magnetic bias field is parallel to the incident electric field and perpendicular to the incident plane (this implies a TE incidence), is deduced.2. The fractal theory is introduced to the design of miniaturized and multiband FSSs. Fractals are geometrical shapes that are self-similar. The fractal theory has been widely used in the microwave area, especially in the miniaturized, multiband and wideband applications. The transmission characteristics of the FSSs composed of fractal cross dipole loop apertures of various orders are investigated. The results show that miniaturized and multiband FSSs can be accomplished by using high order fractal elements.3. A four-band angle and polarization insensitive FSS is designed. The FSS is composed of novel combined elements, which consist of an outer convoluted loop and an inner cross dipole loop. Each loop is composed of several sections, whose dimensions can be tuned separately, which offers extra degrees of design freedom compared with the square loop. The FSS reflects X and K band waves, and passes L and Ku band waves. In the range of angles of incidence from 0°to 75°, the two resonances in the X and K band and the transmission peak in the Ku band are angle and polarization insensitive. And the transmission coefficients of the TM polarization in the L band are angle insensitive. In addition, the FSS is etched on a single layer substrate. So it has the advantages of low profile, low cost, and ease to conform with other structures.4. The Fabry-Perot interferometer (FPI) theory is applied to design double layer FSSs. First, a design methodology of narrow bandpass FSSs is presented. As an example, narrow bandpass FSSs that operate at 96GHz are designed. Second, a four-band symmetrical double layer FSS is designed. The transmission performance of the FSS in the S band is improved by using the FPI theory. Third, a dual-band band-stop FSS with broad bandwidths and rapid rolloffs is designed. An equivalent circuit model for symmetrical double layer FSSs is presented to give a physical insight into the frequency response of the FSS. The Floquet theory and the cascading technique of the generalized scattering matrix are applied to examine the influence of the metallic offset on the transmission performance of double layer FSSs. The results show that the larger periodicities and smaller distance between the metallic screens, the stronger the couplings are, and the more significant the influence of the metallic offset is.5. The SDA is applied to analyze polarizers. A low-profile circular polarizer and a multi-functional polarizer are designed. Polarizers are often comprised of planar periodic structures, so they can be analyzed by the SDA. High performance polarizers often consist of multilayer metallic screens. In the conventional equivalent circuit method, the couplings between metallic screens are ignored. So it is not appropriate to analyze low-profile polarizers, where the cou-plings are strong. In the SDA, the couplings can be considered through the cascading technique of the generalized scattering matrix. A low-profile circular polarizer is designed by the SDA. The polarizer is composed of two layered meander lines. The total thickness of the polarizer is about 0.14 wavelength. A multifunctional polarizer is also designed. This polarizer consists of two identical circular polarizers, which can operate separately. When only one circular polarizer is working, the multifunctional polarizer is just a circular polarizer, effecting conversions be-tween linear and circular polarizations. When the two circular polarizers are cascaded together, the multifunctional polarizer can effect arbitrary rotation of linear polarizations and conversions between left-hand and right-hand circular polarizations.6.The optimum design of FSSs is investigated. The Powell algorithm combined with the mode matching method is presented for the synthesis of FSSs with circular ring patch elements. The optimized parameters are the inner and outer radii of the ring, the periodicity and skewed angle of the array. The initial design is greatly improved after optimized. Then, the micro-genetic algorithm combined with the SDA is used to synthesize a tri-band FSS with elements of arbitrary geometry.