Beam-steering accuracy and polarisation agility in near-field antenna systems

With a substantial part of the world’s population still not connected to the internet, satellite-on-the-move (SOTM) communication is the choice of connectivity in regions where terrestrial communication systems are not available. However, the cost of such connectivity is far beyond the affordability limit of billions of people worldwide. One limiting but major component in connectivity through satellites is the antenna terminal. Predominantly, reflectors or dish antennas are used. However, their bulky size, heavy weight, and costly beam steering technology are less desirable for mass adoption. This thesis explores alternative antenna systems that are low profile, low weight, portable, and relatively cheap to fabricate. The thesis focuses on two aspects of the antenna system: (i) the base antenna for transmission and reception and (ii) low profile beam steering techniques. To increase the communication capacity and enhance the usage of the antenna, it is designed with a shared-aperture approach with polarisation agility and independent control. The antenna can operate with linear vertical and horizontal polarisation. Moreover, it can operate in circular polarisation with the correct input provided. The antenna array also incorporates optimised tapering to achieve low sidelobe levels and relatively stable operation over a ≈ 17% bandwidth.

A substantial part of the thesis is dedicated to discussing, analysing, and comparing near-field metasurface-based beam steering systems. Enhanced approaches to achieve higher accuracy and lower profile of the overall system are also presented. In particular, the traditional cell design is considered to construct a near-field metasurface as a reference. Then, a new cell design called Flanched-Cross is investigated for the construction of improved and more accurate near-field beam steering metasurfaces. Moreover, an out-of-the-box approach is presented in this thesis, whereby multiple cell designs are combined to obtain excellent steering results. This is the first time in the literature of the beam steering metasurfaces and frequency selective surfaces (FSS) for such an approach to be reported. The traditional single-geometry cell design has a limited degree of freedom and restrictive phase range. In the hybrid approach, on the other hand, several geometries are considered for the cell design to maximise the phase range with high transmission magnitude (greater than -1 dB). This thesis provides a detailed comparison among the traditional square patch, Flanched-Cross, and hybrid geometry approaches. Results are verified through simulation and measurement of fabricated prototypes.

The dissertation also provides a comprehensive overview of currently available analytical models for near-field two-dimensional beam steering metasurfaces. Results are verified through simulation of a pair of metasurfaces and a base antenna array. The dissertation also highlights the bottlenecks and assumptions made during the analytical modelling of two-dimensional steering.