Design of Antennas for Multi-mode Orbital Angular Momentum Generation at Microwave and Millimeter-wave Band

Abstract

Electromagnetic (EM) waves have brought about major changes to our daily lives. They have also spurred a wide range of research studies exploring their new characteristics including, but not limited to, wavelengths, polarization and amplitude. Among these properties, orbital angular momentum (OAM), featuring a helical phase front plane, is a new concept compared to spin angular momentum (SAM). In this thesis, three different antennas operating at microwave/millimeter wave bands that can radiate EM waves carrying OAM modes are studied in depth.

A general introduction to the OAM mode and related theories, followed by a brief review of existing applications and means of OAM mode generation is given in Chapter 1. As antennas related to this thesis, a brief introduction to dielectric resonator antennas (DRAs) and horn antennas is also presented. Next, the design of a hemispherical DRA for generating OAM modes with mode indexes of l = ±1 is described in Chapter 2. In contrast to fundamental mode radiation used conventional in DRAs, the high-order resonant mode (TE₂₂₁ mode) of the hemispherical DRA is used to generate OAM modes by appropriate excitation. Full-wave simulation software is used to simulate and optimize the antenna design. A prototype of the presented antenna is fabricated for measurement purposes. When excited by different input ports, OAM modes of different mode indexes, i.e., l = +1 and l = –1, can be obtained using the proposed design. The near-field phase patterns, field density and far-field radiation patterns of the antenna are measured and presented. Other antenna characteristics including antenna input impedance features, radiation efficiencies, antenna gain, etc. are also discussed in detail.

As a basic feature of EM waves carrying OAM mode, the power null in propagation direction limits its application. To address this issue, a horn antenna with a high gain for the generation of EM waves with OAM mode is proposed in Chapter 3. The antenna utilizes the higher-order mode of the circular waveguide, which has long been overlooked. With the excitation of appropriate higher-order modes, OAM waves with arbitrary charge can be generated. We provide computational formulae of the antenna’s radiation field and theoretically prove that the field carries OAM. A prototype of the designed antenna operating within the millimeter wave band is fabricated and measured. The measured results coincide with the simulated results, verifying the design philosophy.

The use of one radiator that can multiplex EM waves in free space is of great significance, as the polarization states of EM waves are limited. This limitation may be addressed by applying the OAM mode. As is shown in Chapter 4, a first single radiator that transmits three independent EM waves simultaneously is proposed. The antenna utilizes a cylindrical DRA operating in the HEM_21δ and TM_01δ modes. The HEM_21δ is used to generate the OAM mode with a mode index of l = ±1 while the non-OAM mode, i.e., l = 0, is generated using the TM_01δ mode. To verify this theory, an antenna that can radiate three OAM modes operating at 2.6 GHz is fabricated and measured. The phase pattern and intensity distribution of the DRA are presented, and the field characteristics are discussed. Finally, an experimental near-field communication link is established, and the channel orthogonality is measured. Measured results show that three independent wireless channels can be obtained.

In this thesis, different antennas that can generate a multi-OAM mode are developed with good performance. All of these antennas have potential applications to wireless communications. Finally, a summary and avenues for future research are given in Chapter 5.

Date of Award	24 Aug 2018
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwok Wa LEUNG (Supervisor)