Beam shaping metasurfaces
Creators: name: AHMED, Asif
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400601
subject: Antennas and propagation
classificationCode: 400899
subject: Electrical engineering not elsewhere classified

description: Metasurfaces (MTS) are two-dimensional metamaterials (MTM) that are a periodic composition of thin, subwavelength and periodic inclusions widely known as unit-cells or meta-atoms. Amongst the vast range of research outcomes accomplished so far, there still is keen interest in the further investigation of metasurfaces to be utilized in various unique methods and applications. The purpose of the thesis is to explore the applicability of metasurfaces towards the avenue of electromagnetic (EM) beam shaping. This will be embodied in the form of gain enhancement, leaky radiation, dual-beam operation, beam reconfiguration and lensing for integrated antenna and guiding structures. The novelty and challenge of the research stands in the effective structuring of the planar surface to support microwave energy propagation with very low-loss and high directivity, along with its compact planar formation.  The practical outcomes of this thesis are the realization of metasurface-based beam-formers which are capable of being mounting and integrated on several different types of feeding structures, e.g. monopole radiators, rectangular waveguides, etc.



Initially, this thesis introduces an azimuthally reconfigurable and dual-beam lensing
structure through the utilization of an anisotropic impedance surface based on electromagnetic bandgap (EBG) theory. The lensing structure controls the propagation of a surface-wave (SW) across the interface between impedance surface and free-space. Reconfiguration is achieved along two orthogonal spaces, and is activated through switching between the frequencies 4.0 GHz and 4.5 GHz. The achieved directivity at 4.0 GHz and 4.5 GHz is found to be 11.2 dBi and 10.7 dBi, respectively.  The orthogonal spaces are located in textured and periodic arrangement along y-axis and x-axis, and have mutually exclusive EBG responses with inhomogeneous and anisotropic impedance responses.  This enables them to produce a single pair of dual-beam radiation lobes per operating frequency corresponding to the respectively operational orthogonal spaces. Such  lenses can be utilized as reconfigurable antennas that have applications in  spatial multiplexing of radiated beams and beam steering in wireless communication systems. 


Two transmitarray (TRA) lens antennas that utilize a dielectric-backed multilayered MTS and operate between 11.75 GHz and 12.5 GHz are also conceived in this research. The TRA lenses are integrated with a flanged rectangular-waveguide, and operate through the manipulation of the boresight wavefront radiated from the waveguide aperture. The peak measured gain is found to be 15.1 dBi with aperture efficiency  58% at 12.5 GHz. These types of antennas may be suitable for use as a horn-replacement antenna with comparably higher radiation gain and aperture efficiency.


A rectangular-waveguide mounted MTS is proposed that produces dual-beam and end-fire radiation that is anti-parallel in orientation and perpendicular to the boresight direction. The MTS is loaded on a dielectric slab that, in tandem with the MTS structure, acts as a substrate integrated waveguide (SIW)-like architecture. The structure is integrated with a flanged waveguide and designed to inhibit the boresight emission whilst enabling symmetric wave splitting at the open-ended waveguide aperture. The dual-beam radiation operates between 10.5 GHz and 12.4 GHz. The device exhibits a peak measured gain of 6.15 dBi having a peak-to-boresight intensity ratio of 5.73 dB at 12.4 GHz.


Finally, the thesis presents a MTS design capable of producing dual-beam leaky radiation with frequency triggered beam switching. The beam shaping is activated by the leaky radiation of the textured periodic structure, whilst the beam is spatially switched in quadrature with a frequency trigger utilizing the EBG of the structure. The MTS consists of a finite two-dimensional periodic arrangement of subwavelength, homogeneous and anisotropic split ring resonators (SRR) as unit-cells. The subwavelength SRRs satisfy leaky-wave theory at 1.8 GHz and generate two anti-parallel beams directed at 0 degree and 180 degree azimuth angles.  However, at 2.6 GHz the SRRs create a complete operating band-gap to steer the dual-beam leaky-waves towards the 90 degree and 270 degree azimuth angles. The peak measured gain is found to be 9 dBi and 10.2 dBi at 1.8 GHz and 2.6 GHz respectively.