Metasurface Apertures for Wireless Power Transfer and Computational Imaging

Abstract

Metasurface apertures provide an alternative approach to the very commonly used phased arrays or electronic scanned antennas (ESA) for wireless power transfer (WPT) and imaging applications. Array antennas use radiating elements which are often spaced at half-wavelength and uses active phase-shifter at each module to control the phase. However, in a metasurface antenna, the required phase is obtained from the sampled reference wave which propagates over the aperture providing an advance phase to each radiating elements. Metasurface apertures have very low manufacturing costs, planar form factor making them a suitable candidate for applications involving beamforming and wavefront shaping. The thesis is divided mainly into two parts consisting of designing metasurface apertures for WPT applications and computational imaging purposes. In the first part, the proposed WPT system operating by focusing fields in the Fresnel region is presented with two proof of concept demonstrations. The first demonstration includes patch array antennas as the transmit and receive aperture and a half-wave rectifier to convert the RF to DC. The frequency of operation is 5.8 GHz (C-band) and the design of the patch array antenna is tedious and not suitable for a dynamic aperture which is possible by making use of metasurface apertures. The second demonstration consists of a metasurface aperture which uses a holographic technique to achieve focusing of microwaves at a particular focus distance for the transmit aperture. The receiving aperture is a metamaterial absorber which is connected to a rectifying circuit to harvest the power, thereby completing the WPT system. A LED connected as the load is illuminated which indicates the basic functionality of the WPT system. The RF-DC power transfer efficiency is in good agreement between simulations and experiments. The proposed system consisting of metasurfaces for both the transmit (focused aperture) and receive aperture (absorber) operates at 20 GHz (in K-band) has not been demonstrated in the literature and is a suitable candidate for higher frequencies (W-Band).The second part consists of designing metasurface apertures demonstrating monostatic and bi-static microwave imaging systems where the metasurface apertures are frequency-diverse and operating at K-band frequencies (18-26.5 GHz). The metasurface apertures consist of radiating irises distributed over the sub-apertures in a periodic pattern. This frequency-diverse aperture produces distinct radiation patterns as a function of frequency that encode scene information onto a set of measurements; images are subsequently reconstructed using computational imaging approaches. In the monostatic case, the metasurface aperture is used as the transit aperture and 4 open-ended waveguides are used as receive aperture. In the case of bistatic case, both the transmit and receive apertures are metasurface apertures which result in increased mode diversity resulting in improved image reconstructions.