Investigation On Novel Three-Dimensional Frequency Selective Structures

Frequency selective surface (FSS) is basically composed of a two-dimensional (2-D) periodic array of identical unit-cells, which are usually patch printed on a dielectric layer or aperture etched out of a conductive surface. The selectivity and propagation characteristics of the incident electromagnetic wave through the FSS for different frequencies, polarizations and incident angles determine its wide applications in electromagnetic technology. FSSs are widely used in antenna radomes, sub-reflectors, and polarizors and so on. For practical applications, it is desirable to realize a thin FSS with high selectivity of filtering performance, and stable response under different incident angles. However, all these features are sometimes difficult to obtain through conventional FSS designs. Recently, innovative concepts of three-dimensional (3-D) frequency selective structures were proposed, which are promising for realization of compact frequency selective structures with high performance, such as quasi-elliptical filtering performance. These3-D frequency selective structures basically consist of a2-D array of cavities whose modes coupling or cavity coupling with air can be controlled to obtain a desirable frequency response. This thesis presents design and analysis of a series of3-D frequency selective structures with high performance, which are based on waveguide filter analysis method and substrate integrate waveguide (SIW) technique. The main contributions of this dissertation are listed below: (1) The efficient full-wave mode matching method (MMM) is used to analyze the3-D dual-band and dual-polarized frequency selective structure, which is based on a two-dimensional periodic array of substrate-filled L-type waveguide elements. Based on Floquet theorem, the frequency selective structure can be analyzed by considering a unit cell of the geometry. The structure is divided into several regions for a simple analysis when there are discontinuities. The mode functions of TE wave and TM wave are obtained by the wave equations, which are matched according to the orthogonality with the equalities of tangential field continuity, and then the eigen values can be calculated. Hence the dispersive diagram can be obtained and the propagation characteristics of the FSS array can be analyzed. The curves obtained from MMM are given and compared with that from finite element method (FEM) for a contrast to prove the high efficiency and accuracy of the method.(2) Dual-polarized frequency selective structures, which are combined with waveguide and conventional aperture FSS, are proposed. The proposed structures are insensitive to different polarizations and incident angles. Using equivalent circuit method and field analysis, the operating principle is explained. SIW technology is adopted for fabrication, and high selectivity and stable performance are realized. The proposed structures have been demonstrated to exhibit a number of attractive features, such as low profile, easy fabrication, steep rejection skirt, stable angular response, and dual polarizations.(3) By combing waveguide filter technology and conventional patch FSSs, high performance frequency selective structures have been proposed, which realized high-order band-pass responses and sharp transition band. FSSs with different patch types have been analyzed. By using symmetric patch, dual-polarized and dual-band frequency selective structures with quasi-elliptic frequency response have been respectively realized. High selectivity frequency selective structure with flat top, two sharp transition bands, and wide pass band and elliptical filtering response is realized through waveguide array loading with vertical U-shaped striplines.(4) Designs of a polarization rotator and a polarization convector are proposed based on a two-dimensional periodic array of cascaded aperture SIW-FSS. The FSSs are based on a dual-mode configuration in order to achieve a second-order resonance and therefore provide a high selective performance. The apertures on the top and bottom surfaces are for selection of a linear polarization of an incident wave and convection of the outgoing wave. Realizations of a45°polarization rotator and a linear to circular polarization convector are proposed. The structures are investigated in detail including the operating principle, the dual-mode configuration selectivity, and the design procedure. A prototype is fabricated and measured to verify the performance.