Higher Symmetries In Eletromagnetic Periodic Structures-Analysis And Applications

The impact that higher symmetries have on periodic structures is a recently emerged topic that has attracted increasing interest in the metamaterial/metasurface community.These symmetries can be geometrical or involve both geometrical and temporal trans-formation.A spatial higher-symmetric structure,studied in this thesis,remains invari-ant under a translation combined with an extra geometrical transformation.This extra operation can be a reflection or a rotation,corresponding to two typical higher symme-tries,i.e.glide symmetry and twist symmetry,respectively.The extraordinary effects of higher-symmetric structures have stimulated extensive studies on their analysis and applications.In this thesis,two efficient methods for Bloch analysis are investigated,and applied to the analysis and synthesis of various one-or two-dimensional,closed or open higher-symmetric periodic problems.Moreover,by introducing higher symmetries in metasurface lenses and leaky-wave antennas(LWAs),we propose several mmWave metasurface antennas that are implemented and integrated in gap waveguide technology.The main contributions and novelties of this thesis are summarized as follows:·The dispersion characteristics of two coaxial waveguides possessing higher symme-tries are investigated.A longitudinal equivalent-circuit-based analytical method is proposed for the accurate and efficient dispersion analysis of the polar glide-symmetric coaxial waveguide.Furthermore,a convenient measurement technique is proposed to accurately extract the dispersion diagrams of the twist-symmetric coaxial waveguide.Based on the proposed theoretical and experimental methods,various higher-symmetric effects have been revealed,such as closed stopbands,low dispersion,increased electrical density,etc.·The dispersion properties of a glide-symmetric holey periodic surface are investi-gated,with special emphasis on the accurate characterization of its stopbands.For this purpose,an efficient simulation-assisted approach based on the multi-mode transfer matrix is studied,presenting a generic and convenient computational tool for higher-symmetric or general periodic problems.This approach also provides a very fruitful physical insight that reveals some previously undetected dispersion behaviors in the structure.Based on this approach,an extensive parametric study is carried out,rigorously establishing a set of critical criteria for the use of such periodic surface as an electromagnetic bandgap structure in groove-gap waveguide(GGW)technology.·Two highly-efficient LWAs with stable radiation patterns are proposed.The side lobe levels(SLLs)of the LWA based on a GGW are optimized with the glide-symmetric holes,thus demonstrating for the first time their capability of leakage control.In addition,a dispersive prism based on the glide-symmetric holey meta-surface is proposed to compensate the inherent dispersion of the leaky-wave mode,and a squint-free bandwidth of 11%is achieved.These fully-metallic and highly in-tegrated designs exhibit a total efficiency of almost 90%at 60-GHz band,presenting a robust,cost-effective,and low-profile solution for point-to-point communications for 5G and beyond.·A novel substrate-integrated-holey metasurface with enhanced dispersion and ro-bustness is proposed.By applying such metasurface to the aforementioned prism-compensated concept,a leaky-lens antenna with a broadened squint-free bandwidth of 20%is proposed.This improved design features low costs and an efficiency of over 80%at 60-GHz band.Also,the multi-mode transfer matrix approach is utilized in the analysis of the leaky-wave feed,which significantly expedites the aperture synthesis.More importantly,this method has illustrated for the first time that glide-symmetric holes provide better control over the leakage than their non-glide counterparts.·A frequency-agile fully-metallic LWA at Ka-band with high efficiency is proposed.Owing to the elevated electrical density in higher-symmetric structures,a backward leaky-wave mode is generated by glide-symmetrically loading metallic pins in a GGW feed.This loading increases the dispersion of the leaky-wave mode,and thus improves the scanning performance.Conceptually contrast to the "prism-compensated" architecture,a generic "prism-augmented" solution independent on the feeding is also proposed to further increase the frequency scanning rate by almost twice.