Designs of Antenna Array and Metasurface for Wireless Communications

應用於無綫通信中的天綫陣列和超表面設計

Student thesis: Doctoral Thesis

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Award date28 Jul 2022

Abstract

This thesis presents some novel designs of antenna and array for the fifth-generation (5G) wireless communications applications at millimeter-wave (MM wave) frequencies. These designs are excited by two techniques: slot-fed excitation and microstrip line for antenna and array. Three antenna arrays utilize parasitic patches to achieve wide bandwidth, low-profile, low-cross polarization level, and stable radiation pattern across their operating frequency bands for single or dual-polarizations. These antenna arrays are low-cost and easy to fabricate.

First, a slot-fed antenna element is proposed to generate wideband, low-cross polarization level linearly-polarized (LP) radiation. The antenna consists of a cavity, two vias, a printed dipole, and four parasitic patches. The dumbbell-shaped aperture etched on the top wall of a substrate integrated waveguide (SIW) is used to provide differential feed. A prototype of a single element and 8 × 8 array based on the low-side-lobe design method were fabricated using standard printed-circuit-board (PCB) technology. Measurements of the array show a bandwidth of 24.8% from 23 to 29.5 GHz with an average sidelobe below −17dB. In addition, the array achieves a maximum gain of 26.2 dBi and radiation efficiency of 80%.

Second, extended from the LP design above, a dual-polarized array based on SIW technology is developed to satisfy increasing channel capacity requirements. The operation of each polarization is like that of the last one. The dual-polarized element consists of four L-shaped strips and four parasitic patches. The element is fed using SIW with a coupling cross-slot etched on the SIW. This proposed element owns all the merits of the last LP design while achieving dual-polarization operation. In addition, in the dual-polarized feeding network design, an anomalous response at a frequency point is alleviated using our proposed method, achieving wide bandwidth in such SIW dual-polarized design. A prototype was fabricated, and the test results show that it can achieve an overlapped bandwidth of 28.4% with good low cross-polarization levels and maximum gains of 18.9 and 19 dBi, respectively. In addition, the radiation pattern is stable across the whole operating frequency band.

Third, a novel differential fed dual-polarized shared-aperture antenna array fed by a microstrip-line network is proposed. Each polarization of the single element is differentially driven. Then, elements are arranged tightly to form a 4 × 4 array. The mutual coupling between adjacent elements could help to improve the antenna gain. The proposed tightly coupled array is compared with an array with the same unit design having conventional element spacing, and an obvious aperture efficiency improvement could be observed. Two microstrip-line networks are arranged in different layers to avoid interference between the two polarizations. A prototype was fabricated and experimentally tested. A hybrid provides the differential feed with a ±14.5° phase difference and a 1.2 dB magnitude imbalance. The measurement results show an overlapped bandwidth of 25.3% and antenna size of 3.32λ0 × 3.28λ0 × 0.087λ0 in terms of wavelength at the measured center frequency.

Fourth, a dual-polarized reconfigurable metasurface (RM) is proposed to be compatible with the upcoming sixth-generation (6G) communications. Particularly, to address the issues from the non-line-of sight propagation. The metasurface array comprises two layers of PCBs and two slices of bonding film. For the unit cell of each metasurface, four octagon patches are used and connected with four varactor diodes. The placement of four varactor diodes and the bias voltage feeding method enables this metasurface to manipulate incident plane waves of any polarization. A prototype of a 15 × 15 array composed of 900 varactor diodes was fabricated and tested with the help of a field-programmable gate array (FPGA)-based control panel. Three functions are experimentally demonstrated. The dual-polarized RM can achieve beam steering with x- or y-polarized incident wave or plane wave in any polarization. In addition, it can perform as a polarizer, i.e., linearly-polarized to orthogonally-polarized, linearly-polarized to right-hand circularly-polarized, and left-hand circularly-polarized operations in the working frequency band.