Wideband Circularly Polarized Elements and Arrays for Wireless Systems

Circularly polarized (CP) antennas have received increasing interest during recent decades due to their unique features such as the mitigation of multi-path fading, reduction of the "Faraday rotation" effect when signals propagate through the ionosphere and immunity of the polarization mismatching between transmitting and receiving antennas. Due to the requirements of high date rate and large system capacity, CP antennas deployed in various wireless systems are always demanded to have wide bandwidth. Furthermore, other system requirements such as polarization diversity, wide-angle beam scanning and low power consumption impose additional requirements to CP antennas. Therefore, it is becoming a more stringent requirement to design wideband CP antennas with diverse features to fulfil the requirements of various wireless systems.

In this thesis, six different types of wideband CP antenna elements and arrays are designed, fabricated and characterized to meet the different demands of wireless systems. Chapters 3-5 investigate three different types of wideband CP antenna elements while Chapters 6-8 investigate three different kinds of wideband CP array antennas. In Chapter 3, an ultra-wideband CP element with a bandwidth of 100% (3:1) is proposed. It over-comes the problem of limited 3 dB axial ratio (AR) bandwidth for single-feed CP antennas and achieves high front-to-back ratio (FBR) by using a novel ground plane with simple configuration, which makes it a good candidate for high-performance Global Navigation Satellite System (GNSS) receivers. Chapter 4 presents a wideband loop antenna with electronically switchable circular polarizations. It solves the issue of narrow overlapped bandwidth under different polarization states for a polarization reconfigurable CP antenna. Because of the available orthogonal polarizations across a wide bandwidth, this antenna can be deployed in wireless communications which implement polarization diversity. The third antenna element investigated in Chapter 5 tackles the difficulty of designing wide-band wide AR beamwidth CP antennas. It achieves wide AR beamwidth within a 42% bandwidth, which is suitable for wideband wide-angle CP beam-scanning applications.

The second main part of this thesis focuses on the investigation of wideband CP arrays. In Chapter 6, a dual-CP beam-scanning array is investigated, which can scan its beam independently in right-hand circular polarization (RHCP) and left-hand polarization (LHCP) from 27 GHz to 30 GHz. It tackles the problem of low isolation between the two orthogonally polarized ports across a wide bandwidth at Ka-band. A single-layer high-efficiency CP reflectarray is proposed in the following Chapter. The proposed design solves the issues of bandwidth limitation and low aperture efficiency for single-layer CP reflectarrays. It achieves the widest bandwidth compared with other CP reflectarrays reported in terms of 3 dB AR bandwidth, 3 dB gain bandwidth, larger than 50% aperture efficiency and undistorted radiation pattern bandwidth. In Chapter 8, we investigate the first application of tightly coupled array (TCA) concept into ultra-wideband arrays with CP radiation. Instead of trying to reduce the mutual coupling among the elements, it exploits the strong mutual coupling to improve the bandwidth of a CP array. By using the strong coupling in a constructive way, it overcomes the bandwidth limitation of CP arrays which are constituted by narrow-band elements.