Modulated metasurface antennas : analysis and direct numerical synthesis

Metasurfaces (MTS) offer an innovative way for the generation and manipulation of electromagnetic (EM) waves. The EM properties of usual surfaces/materials are dictated by their atomic/molecular constituent. Therefore the achievable performance of the resulting EM components (lens, absorbers, waveguides, antennas, etc.) are fundamentally limited by the properties of available materials. MTSs inherit EM characteristics from their internal micro-structure, rather than from the chemical composition of the surface. As a result, the achievable properties can surpass the limitations of those of usual surfaces, thus pushing forward the performance of the feasible EM components. This thesis is devoted to the analysis and design of aperture antennas making use of artificially engineered boundary conditions provided by modulated MTSs. MTS aperture antennas are getting a considerable interest from the antenna community. Such antennas present the advantage of being extremely flat and thin by nature since they rely on the excitation of SWs with a feed located within the substrate layer (MTS plane). They also exhibit very low ohmic losses and can be easily manufactured with standard printed circuit board (PCB) technology. The radiation characteristics of modulated MTS antennas result from the progressive interaction between the excited SW and the spatially modulated surface impedance. A class of orthogonal, entire-domain, and divergence-conforming basis functions (named Fourier-Bessel) is first proposed for the efficient analysis of the surface current distribution, in the Method of Moments (MoM) solution of the electric field integral equation (EFIE). The surface impedance is further expanded into Fourier-Bessel basis functions (FBBF), then allows us to address the inverse problem, which consists of computing the MTS EM properties on the basis of a desired aperture current distribution. The antenna synthesis problem is thereafter formulated, first by computing the objective current distribution based on the desired radiation pattern. Secondly, the relevant integral equation is numerically solved while constraining the tensorial surface impedance to be anti-Hermitian so as to avoid losses on the sheet. This results in a systematic and direct algorithm (optimization is not required) able to provide an arbitrary radiation pattern in amplitude, phase and polarization. The derived impedance profile (described in Fourier-Bessel basis) is no longer necessarily locally sinusoidal, which significantly widens the class of synthesizable radiation patterns as compared to the techniques proposed in the literature. Challenging shaped beams and multiple beams in linear and circular polarizations have been designed and numerically validated using independent electromagnetic (EM) simulation software tools. The developed EFIE based formalism is thereafter extended to deal with multiple beams with multiple feeds synthesis, dual-polarization capability, and multiple frequency-band operation. The thesis also introduced a new MTS beam scanning paradigm in which the antenna is fed at multiple points. Beam scanning is obtained after generating proper embedded element patterns with opposite phase slopes, followed by a phasing of those radiation patterns with respect to each other. Near-field applications are tackled as well through the generation of Orbital Angular Momentum (OAM) beams for near-field multiplexing, and Bessel beams for near-field focusing. Free-space transmittance (power transfer) between Fourier-Bessel fields (a particular class of OAM beams) is analyzed and a closed-form expression of the transmittance (valid for the intermediate-to-far field) is derived. Finally, an antenna prototype for radar applications has been developed, experimentally validated, and the power balance in MTS antennas has been rigorously analyzed.(FSA - Sciences de l'ingénieur) -- UCL, 202