Kranti Kumar Katare, Gästforskare
The exponential growth in wireless communication system requires the integration of multiple antennas into a single platform for maximizing the connectivity. In the last decade, a number of antenna configurations have been proposed for the wireless communication system. However, the performance of any modern wireless communication system is usually limited due to usage of the particular antenna providing some fixed radiation characteristics in the specified frequency band. In recent years, this restriction is relaxed up to a large extent by introducing the concept of reconfigurable antenna system, whose characteristics can be adjusted as per system requirement and required application. Hence, these types of reconfigurable antenna systems appear to be quite appropriate and versatile for modern wireless communication as they can easily fulfill the requirements of increased functionality using a compact structure without substantially increasing the overall footprint. Broadly, the reconfigurable antenna system can be realized either using an active device leading to electronic tuning or using some kind of mechanical movement leading to the passive antenna system.
The active antenna configurations usually comprise of various electronic components and a complex feeding network with the extra biasing arrangement, which increases the cost and reduces the overall efficiency of the system. Hence, to obtain better efficiency with the high gain performance, the passive mechanical reconfigurable antenna systems appear to be better choice. In order to realize passive reconfigurable antenna configurations, different schemes such as the antenna array, the reflector antenna, the dielectric lens antenna etc. have been reported in the literature. It is observed that the performance of most of the aforementioned reconfigurable configurations deteriorate either due to high losses in the feed structures (in case of feed networks) or due to the radiation blockage (in case of reflectarray antennas).
From above discussion, it emerges that there are still lot of challenges to design passive, simple and low loss reconfigurable antenna system. To this end, our primary focus here is on the design and investigation of various types of metasurface based structures to realize the passive low loss reconfigurable antenna system. A metasurface modifies the wavefront of impinging waves by introducing abrupt changes in their properties (amplitude, phase, polarization), which facilitates a new degree of freedom in the radiation characteristics (radiation pattern and polarization) of the resultant antenna system. These distinguish properties of the metasurface manifests its potential for a number of applications demanding passive reconfigurable antennas. In my PhD thesis, a number of novel metasurface based passive antenna configurations are proposed to realize the simple pattern and polarization reconfigurable antenna systems. To this end, special class of metasurfaces such as radially gradient hybrid metasurface (RGHMS), semicircular radially gradient metasurface (SCRGM), bianisotropic metasurface (BMS), digital metasurface (DMS) and anisotropic metasurface (AMS) are employed to realize a number of novel reconfigurable antenna systems.
In the first configuration, a circularly shaped radially gradient hybrid metasurface (RGHMS) structure comprising of two different phase profiles in a single lens structure is proposed. In-plane movement of this RGHMS in-front of a stationary feed antenna encapsulates the microwave beam steering capability in both elevation and azimuth planes. In the second configuration, the radially gradient metasurface only partially covers the antenna aperture leading to the semicircular radially gradient metasurface (SCRGM) structure. The realization of SCRGM structure basically facilitates a compact configuration with a comparatively large beam scanning range in the elevation plane. The above two antenna configurations (RGHMS and SCRGM) providing beam steering are based on the translation of the metasurface parallel to the antenna aperture which normally increases the overall footprint of the structure. Therefore, in the next part, a novel bianisotropic metasurface (BMS) based beam-switching/steering technique utilizing the Fabry-Perot (FP) cavity antenna configuration is demonstrated. The beam steering in this case is obtained by slowly rotating the BMS over the patch antenna in the elevation plane. After utilizing a number of metasurface based antenna configurations to realize beam steering, the concept of the digital metamaterial (DMS) is introduced here to obtain multiple beam radiation pattern. The multiple beams in this case are basically achieved by splitting the high gain radiation beam (originating from the subsystem of patch antenna and planar lens) with the help of proposed DMS configurations. After considering a number of reconfigurable antenna configurations dealing with the polarization independent pencil beams discussed above, our work is focused towards the generation of dual-polarized fan-beam patterns with the independent beam-scanning capability using the anisotropic metasurface (AMS) based reconfigurable antenna system. This type of fan-beam configuration is required for various industrial applications such as the imaging system, the radar technology, just to name a few. In the last part of this thesis, a simple directive array based wideband mechanical beam steering configuration providing a frequency dependent radiation characteristics, is proposed. In this structure, a monopole antenna in combination with twelve parasitic elements are employed to steer the direction of main beam in the entire azimuth plane.
To summarize, the work presented in this thesis investigates several new metasurface based antenna configurations to steer or redirect the direction of main beam in the elevation/azimuth plane for the X-band frequency range. A simple directive array based passive system is also proposed to steer the direction of main beam in the entire azimuth plane for the S-band frequency range.