Design of antennas based on metasurfaces at millimeters and terahertz waves
Authors:   , Pérez Quintana Dayan 
Published: November 2022

This thesis presents the evolution of an investigation line guided by the development of metallic structures to improve the radiation properties of devices operating at millimeter wave (mmWaves). It covers different aspects of high frequency solutions such as Gap Waveguide (GW) technology to guide waves with low loss and high efficiency overcoming the limitations of classical waveguides. Using GW as a common link, three different antennas have been developed to generate circular polarization (CP) in a simple and compact way using a feeding technique based on the length difference of orthogonal arms exciting a slot on the top plate. Each of these antennas follows a natural evolution from the initial design that presents the basic operation fundamentals towards more complicated topologies aiming to increase the directivity. Metasurfaces (MTS) are another hot topic in electromagnetics research due to the possibilities they offer for the control of the electromagnetic wave radiation. In particular, MTS at mmWaves are key elements in the context of modern communication systems (5G, 6G) as they open new ways to steer the beams in arbitrary directions as well as improve in general the radiation characteristics of wireless devices. In the thesis MTS have been combined with GW to extend the functionality of compact devices in a fully metallic design. A flat Luneburg lens (LL) antenna based on a bed of nails fed by a GW horn antenna has been designed and experimentally demonstrated. The refractive index needed to modulate the Luneburg equation has been implemented using small pins with different heights. This design presents a significant bandwidth and good directivity values in a compact implementation. Also related with lenses but using a different concept, a compact pillbox reflector based on a geodesic lens has been proposed at 60 GHz. A geodesic lens is based on a parallel plate waveguide which achieves the focusing properties by introducing a curved shape and hence varying effective optical path. The system has been implemented in a dual-layer parallel plate waveguide. In the proposed design the wave stemming from the geodesic lens in a first layer is reflected by a parabolic mirror connecting the rims of the two layers that directs the wave towards the second layer where it is radiated. To make the system more compact, the lens is designed with a virtual focus so that the actual source can be placed closer to the lens. An open technological challenge is to achieve high gain antennas at the higher end of the mmWaves band, close to the terahertz range (300 GHz), where a wide bandwidth is available thanks to the increased operation frequency. An initial step to cover this gap has been done in the thesis by proposing an antenna lens system based on a metamaterial flat hyperbolic lens illuminated by an H-plane horn antenna designed at 300 GHz using groove GW technology. Both devices, the metalens, and the GW antenna achieve excellent radiation results when combined. Furthermore, the manufacturing process is analysed in detail and a realistic proposal based on metallized micromachined silicon is presented. Finally, reconfigurable and active metasurfaces are a disruptive topic with multiple applications in smart wireless systems. Here, a dynamically reconfigurable reflectarray (RA) using liquid crystal (LC) operating above 100 GHz has been numerically and experimentally demonstrated. This is a relevant and challenging result due to the stark difference between the operation wavelength (of the order of millimeters) and the typical thickness of a LC substrate (some micrometers). The good performance of the prototype paves the way towards more ambitious designs aligned with the requirements of modern communication systems at mmWaves.