Non-Invasive Near-Field Measurement Setup Based on Modulated Scatterer Technique Applied to Microwave Tomographhy

Résumé L’objectif principal de cette thèse est d’aborder la conception et le développement d’un montage d’imagerie en champ proche (CP) basé sur la technique de diffusion modulé (TDM). La TDM est une approche bien connue et utilisée pour des applications où des mesures précises et sans perturbations sont nécessaire. Parmi les applications possibles disponibles pour la fabrication d’une sonde TDM, que ce soit électrique, optique, mécanique, le diffuseur optique modulé DOM a été pris en considération afin de fournir des mesures quasi sans-perturbations en raison de l’invisibilité des fibres optiques face aux champs radiofréquence électromagnétiques. La sonde est composée d’une puce photodiode commerciale “off-the-shelf” (dispositif non-linéaire), d’une antenne dipôle courte agissant comme diffuseur et un réseau d’adaptation (cir¬cuit passif). Cet dernieér améliore les propriétés de diffusion et augmente également la sensibilité de la sonde DOM dans la bande de fréquence pour laquelle le réseau correspondant est optimisé. Les caractéristiques de rayonnement de la sonde, y compris sa réponse de polarisation croisée et sa sensibilité omnidirectionnelle, ont été théoriquement et expérimentalement étudiés. Enfin, la performance et la fia¬bilité de la sonde a été étudiée en comparant des mesures de distribution de champs proche avec une distribution de champs simulé. Une vitesse d’imagerie accrue a été obtenue utilisant un réseau de sondes DOM, ce qui réduit les mouvements mécaniques résultant ansi en une amélioration remarquable de la vitesse de mesure. Le couplage mutuel, le temps de commutation et l’effet d’obscurité, des effets qui peuvent affecter les performances du réseau ont été explorés. Ensuite, les résultats obtenus par le réseau ont été validé par une imagerie CP en mesurant la distribution des champs E d’une antenne sous test (AST) et la comparant à des résultats de simulation. Une calibration et un calcul de moyenne ont été appliqués à des données brutes pour com¬penser pour les incertitudes dans la fabrication et l’interaction entre réseau/AST et réseau/antenne de réception. La plage dynamique et la linéarité de la réponse de l’imagerie CP ont été améliorées en ajoutant un circuit suppresseur de porteuse en avant de l’antenne. Le suppresseur élimine la porteuse sur laquelle aucune information n’est transmise et laisse les bandes latérales intactes.----------Abstract The main focus of this thesis is to address the design and development of a near-field (NF) imaging setup based on the modulated scatterer technique (MST). MST is a well-known approach used in applications where accurate and perturbation-free mea¬surement results are necessary. Of the possible implementations available for making an MST probe, including electrical, optical and mechanical, the optically modulated scatterer OMS was considered in order to provide nearly perturbation-free measure¬ment due to the invisibility of optical fiber to the radio-frequency electromagnetic fields. The OMS probe consists of a commercial, off-the-shelf (COTS) photodiode chip (nonlinear device), a short-dipole antenna acting as a scatterer and a match¬ing network (passive circuit). The latter improves the scattering properties and also increases the sensitivity of the OMS probe within the frequency range in which the matching network is optimized. The radiation characteristics of the probe, includ-ing cross-polarization response and omnidirectional sensitivity, were both theoreti¬cally and experimentally investigated. Finally, the performance and reliability of the probe was studied by comparing measured near-field distributions on a known field distribution with simulations. Increased imaging speed was obtained using an array of OMS probes, which re¬duces mechanical movements. Mutual-coupling, switching time and shadowing effect, which all may affect the performance of the array, were investigated. Then, the re¬sults obtained by the array were validated in a NF imager by measuring the E-field distribution of an antenna under test (AUT) and comparing it with a simulation. Cal¬ibration and data averaging were applied to raw data to compensate the probes for uncertainties in fabrication and interaction between array/AUT and array/receiving antenna. Dynamic range and linearity of the developed NF imager was improved by adding a carrier canceller circuit to the front-end of the receiver. The canceller eliminates the carrier on which no information is transmitted and leaves the sidebands intact. This enables us to increase the amplification gain to achieve better signal-to-noise ratio (SNR) and more importantly to expand the imager’s dynamic range

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Theoretical and experimental advances in two-port injection matched antennas

The bandwidth enhancement of two-port single radiator patch antennas for wireless communications is studied through single resonance and frequency tunability using the injection matching technique. The approach uses the variations of the relative amplitude $\gamma$ and phase shift $\varphi$ between the two coherent excitation signals at port two of the antenna to modulate the terminal impedances at the two ports, which simultaneously matches the two ports of the antenna to the 50 $\Omega$ coherent sources. Particular focus is paid to the active modulation of the electrical length of the patch antenna to enhance the bandwidth of an electrically small antenna. Wide bandwidth greater than 64% is attained from the two-port proximity-fed patch antenna with approximately stable omnidirectional H-plane radiation patterns, radiation efficiency higher than 99% and total efficiency higher than 89% across the antenna passband through excitation of two characteristic modes of the antenna with $\gamma=1$ and $\varphi=180^o$ applied to the antenna. Also, the fractional bandwidths of 67% and 19% with radiation efficiency higher than 98% and total efficiency higher than 84% across the antenna passband are attained from a patch antenna through a non-progressive frequency tuning of the antenna modes with $\gamma=1$ and $\varphi=180^o$, and $\gamma=1$ and $\varphi=0^o$, respectively. It is further shown that other $\gamma$ and $\varphi$ combinations apart from $\gamma=1$ and $\varphi=180^o$ or $\gamma=1$ and $\varphi=0^o$ can only affect the matching of the attained differential or common mode resonances. Besides, an electrically small operation with electrical length $ka=0.81$, radiation and total efficiencies of 93%, and a -10 dB fractional bandwidth of 2.7% at a 1.423 GHz centre frequency has been attained on a patch antenna which is incorporated with a slow-wave structure, through the application of the injection matching technique. It is further shown that the use of lumped elements for tuning the two-port injection matched antennas to an electrically small frequency of operation is not feasible because of the narrowband matching behaviour of the lumped elements, which causes significant variations in the applied $\gamma$ and $\varphi$. Also, the reciprocity analysis of the two-port injection matched antennas shows that the same feeding network arrangement used by the antenna in transmit mode can be fed by an injection matched antenna in the receive mode. Besides, the two-port injection matched antennas can work reciprocally with any other antenna with similar radiation characteristics. The high bandwidths attained by the two-port injection matched antennas are attributed to the excitation of the multiple characteristic modes of the antenna through the application of the injection matching technique. However, all these features are attained at the expense of complex driving circuitry, which is not always advantageous to realise

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium