Innovative applications of metasurfaces to design linear and circularly polarised antennas for modern communication systems

With the recent advancements towards 5G communications and the latest wire-less technologies, metasurfaces have attracted progressively intensive attention and created tremendous interest due to their adept characteristics such as planar configurations, light-weight, and easy fabrication. Metasurfaces also offer a great range of flexibility in manipulating the electromagnetic waves. This property can be effectively utilised to alter the electromagnetic properties and functionalities for designing new antennas, with significantly improved performances. Thus, this thesis demonstrates the innovative applications of metasurfaces manifesting linear (LP) and circular polarization (CP), addressing various limitations prevailing in modern antenna systems, and offering simple yet effective solutions. This thesis introduces strategic design combinations, including substrate-based and superstrate-based metasurfaces that are unique, light-weight, low-cost, and offer significant performance features.

While considering metasurfaces to achieve CP, it is desired to have a simple structure, single feed, and planar design. For this purpose, two different configurations of unit-cell substrate-based metasurfaces are proposed for dielectric resonators (DRs). A hybrid technique involving perturbed probe feed and well-matched metasurfaces is demonstrated to achieve wide-band CP radiation performance and dual-band CP characteristics, respectively. Exploiting Surface waves resonance property, the first design with a rectangular DR over a 7 ✕ 7 plus "+" shaped unit-cells metasurface depicts a wide impedance bandwidth of >32% (3.6 GHz to 7.0 GHz), an overlapping 3-dB axial ratio (AR) bandwidth of 20.4% (4.4 GHz to 5.4 GHz), and a gain of 6-7 dBic. On the other hand, the second design with two stacked DRs over a 7 ✕ 7 square slotted shape metasurface resulted in two overlapping 3-dB AR bandwidths of 25% (4.2 GHz to 5.4 GHz) and 5.1% (7.6 GHz to 8 GHz), with a peak gain of 8.4 dBic, discretely. These antenna designs broadly can be realized for MIMO features, significantly when extending for mmW applications.

Although substrate-based metasurface antennas showed good performances, the protruding nature of DRs is found to be a limitation for wideband satellite communications and radar systems. Therefore, a sequentially rotated array (SRA) is considered here as an alternative. A 2 ✕ 2 SRA is demonstrated using z-shaped patch elements with a parasitic feeding network for wideband AR band-width of 39% (8.1 GHz to 12 GHz). This array also addresses the limitations of conventional SRAs by offering less complexity and possible realization over a sparse area to avoid couplings effects. A superstrate based metasurface validates the proposed SRA's performance and demonstrates their applicability. An 8 ✕ 8 asymmetric array of square patches etched as a metasurface superstrate is designed and placed on top of the SRA, spaced at a distance of half-wavelength (λ/2). The resulting structure is shown to achieve a wideband impedance band-width of 50.3%, a peak directivity of 15 dBi, and a wide 3-dB directivity band-width of 42.5%, respectively. Thus, the newly proposed SRA with superstrate based metasurface significantly improves the performance and offer more freedom and design flexibility.

This thesis also presents superstrate based all-dielectric metasurfaces to realize highly-directive at-panel antenna arrays, as a potential alternative to bulky reflectors and expensive phased arrays. Dielectric metasurfaces with radially varying permittivity are utilised to construct a dense array (having closely spaced 91 elements), which is shown to achieve a peak directivity of 37 dBi with 3-dB directivity bandwidth of >20%. Proposed arrays can outperform standard microstrip patch antenna arrays. This dramatically reduces the feed network complexity by significantly reducing the total number of array elements required. The problems associated with bulky and expensive waveguide-feeds is also addressed here by designing a planar aperture coupled slotted feed. A fabricated prototype demonstrates a peak directivity of 16.2 dBi with 16.2% 3-dB directivity bandwidth. The planar feeding solution is low profile, cost-effective compared to the expensive waveguide feeds, and can be extended for the switched-polarisation arrays or sparse array configurations.

Wide range of metasurface based antennas presented in this thesis demonstrates a great potential to offer simple solutions for various existing challenges in different antenna technologies. Thus, proposed metasurfaces can be integrated for use in a myriad of future wireless communication applications. They tend to achieve notable performances with reduced complexity, antenna profile, and cost.