Steerable metasurface antennas

Steerable metasurface antennas Metasurface antennas are planar multi-layer configurations of sub-wavelength building blocks (i.e. unit cells) typically printed on grounded dielectric slab. Metasurfaces possess, among others, the ability to manipulate surface waves. Metasurface antennas radiation is well described in terms of a continuous slowly varying electric sheet tensorial reactance, interacting with a surface wave, which is gradually transformed into a leaky wave. Indeed, a leaky-wave antenna is a waveguiding structure which sustains a wave that radiates energy while propagating along the antenna itself. As for metasurface antennas, such radiating mechanism is achievable through Sinusoidally-Modulated Reactance Surface (SMRS) placed at the upper interface of a grounded dielectric slab. This work focuses on the steerability of metasurface antennas via a reconfigurable ground plane. Thus, the goal is to reconfigure the ground plane in a manner that for different values of the equivalent plane, the antenna radiates at different controllable angles. Therefore, the idea is to add between the SMRS and a fixed ground a reconfigurable periodic arrangement of sub-wavelength unit cells, which will form a uniform reactance surface. The reconfigurability will be achieved through the insertion of varactor diodes in the unit cell design of such texture. Ergo, the steerable metasurface antenna will be composed by three different layers: on the bottom there is a bare ground plane, separated by an air gap from the reconfigurable metasurface, which is surmounted by a dielectric layer and topped by a SMRS. The working principle is the following: changing the sheet impedance of the reconfigurable plane allows for the modification of the wavevector of the surface waves sustained by the structure and, consequently, of the surface impedance ”seen” by such modes. The modulation of the overall surface impedance gives rise to a leaky wave radiating away from the surface at a certain angle, which depends on the average impedance and, ultimately, on the varactor diodes’ capacitance. State of the art systematic procedures allow to derive the admittance of a patterned metallic sheet, whether this is printed on a grounded dielectric substrate or not, starting from the scattering parameters. These techniques are tested and used to determine the sheet admittance of the upper layer of the proposed antenna. Then, studying the solutions of the transverse resonance equation (TRE) of the structure, the optimal values of the admittance of the reconfigurable plane are identified in order to achieve the maximum possible steerability of the main beam. Finally, a proper geometry is found to implement such admittances, given the range of capacitance of a commercial varactor diode. The complete antenna is simulated and designed using both full-wave electromagnetic solver CST and in-house solvers. Despite some discrepancies between the expected results and the simulations, a clear steerability of the radiating beam is obtained.