High-gain antennas play a crucial role in modern wireless communication systems, as they can compensate for path losses in the air. Typically, antenna gain enhancement is achieved by increasing the physical aperture size, such as in reflector antennas, antenna arrays, and transmitarrays/reflectarrays. However, these antenna arrays often suffer from large bulky sizes or complex feeding networks. Lenses have been widely used in antenna designs to improve aperture efficiency and enhance antenna gain. However, traditional lenses are separate components that can add extra size to the overall antenna system. This thesis focuses on the integration of metal lenses with antennas to address size limitations and achieve compact high-gain antennas.

Chapter 1 provides a comprehensive review of existing methods for antenna gain enhancement. Chapter 2 investigates a novel H-plane metal lens and its application in unidirectional antennas. This metal lens has distinctive characteristics, incorporating wrinkled sub-channels with varying lengths. These features play a crucial role in compensating for phase errors and facilitating the radiation of a nearly planar wavefront across an extensive frequency band. As a result, the designed lens antennas exhibit remarkable advantages in terms of broadband capabilities and high gain. A detailed explanation of the operational principles, alongside comprehensive design guidelines, is methodically presented within this chapter. Moreover, the chapter proposes a compact wideband open-horn antenna, integrated seamlessly with the metallic lens, optimized for efficient wireless communication applications. This antenna consists of a spatial power divider and an H-plane metal lens. Both components, crafted from metallic materials, are produced utilizing advanced 3-D printing technology. Following the design guidelines, two exemplary antenna models exhibiting disparate E-field magnitude distributions have been rigorously studied and consequently manufactured for potential Ku-band applications. The primary antenna design exhibits a uniform E-field magnitude distribution, resulting in significant gain enhancement and high aperture efficiency. The secondary antenna design incorporates a tapered E-field magnitude distribution. As expected, it has improved side lobe levels and acceptable gain enhancement. Both designs demonstrate measured 3-dB gain bandwidths exceeding 46.7%. Furthermore, the proposed designs exhibit axial lengths significantly shorter than their optimal horn antenna counterparts, exceeding a reduction of 60%.

In Chapter 3, a novel annular metal lens and its applications in omnidirectional antennas are investigated. A biconical antenna, seamlessly integrated with a metal lens, is proposed to be employed in millimeter-wave wireless communications. The antenna is laden with an annular metallic lens, an amalgamation of rotationally symmetric annular metallic plates. This design allows for a nearly uniform E-field distribution at the radiating aperture across a broad frequency range, thereby facilitating a wideband high-gain omnidirectional antenna. This chapter provides the operating principle and design flowchart of the lens. To validate the idea, a vertically polarized prototype operating in the Ka-band was fabricated. Both its measured -10-dB impedance and 3-dB gain bandwidths are 43.9% (25.6-40.0 GHz), with the maximum measured gain given by 9.2 dBi. Compared with the conventional biconical antenna, our antenna can obtain a higher gain by ~4 dB across the entire impedance passband.

Chapter 4 focuses on a novel geodesic metal lens antenna and its application in wideband antennas. A wideband metal-lens integrated antenna is proposed, consisting of a conventional horn antenna and a geodesic metal lens. The geodesic lens, characterized by a spatially curved flared-out structure, enables the antenna to obtain a quasi-uniform E-field distribution at the radiation aperture, thereby enhancing the horn antenna gain. The metal lens offers advantages such as geometric integration and air filling, reducing design costs and eliminating the need for additional length from antireflection coatings. To achieve a broadband geodesic lens, an exponential gradient structure is introduced. The original feed structure and radiation aperture are modified by adding a back cavity and curved opening section to improve the impedance match. A prototype of the antenna was fabricated and tested to demonstrate the feasibility of the proposed design.

In this thesis, all designed lenses are integrated into antennas, resulting in improved performance and compact axial lengths. These findings make them promising candidates for compact wireless communication systems. Chapter 5 presents a summary of the research and some potential topics for future exploration.