Kwok Wa Leung
Prof. Kwok Wa Leung
Department of Electronic Engineering
City University of Hong Kong
83 Tat Chee Avenue, Kowloon Tong, Hong Kong
Kwok Wa Leung was born in Hong Kong. He received the B.Sc. degree in Electronics and Ph.D. degree in electronic engineering from the Chinese University of Hong Kong, in 1990 and 1993, respectively.
From 1990 to 1993, he was a Graduate Assistant with the Department of Electronic Engineering, the Chinese University of Hong Kong. In 1994, he joined the Department of Electronic Engineering at City University of Hong Kong (CityU) and is currently a Professor and an Assistant Head of the Department. He is also the founding Director of the Innovation Centre of the Department. From Jan. to June, 2006, he was a Visiting Professor in the Department of Electrical Engineering, The Pennsylvania State University, USA.
Professor Leung was the Chairman of the IEEE AP/MTT Hong Kong Joint Chapter for the years of 2006 and 2007. He was the Chairman of the Technical Program Committee, 2008 Asia-Pacific Microwave Conference, Hong Kong, the Co-Chair of the Technical Program Committee, 2006 IEEE TENCON, Hong Kong, and the Finance Chair of PIERS 1997, Hong Kong. His research interests include RFID tag antennas, dielectric resonator antennas, microstrip antennas, wire antennas, guided wave theory, computational electromagnetics, and mobile communications. He was an Editor for HKIE Transactions and a Guest Editor of IET Microwaves, Antennas and Propagation. Currently, he serves as an Associate Editor for IEEE Transactions on Antennas and Propagation and received Transactions Commendation Certificates twice in 2009 and 2010 for his exceptional performance. He is also an Associate Editor for IEEE Antennas and Wireless Propagation Letters. He has been appointed as a Distinguished Lecturer by the IEEE Antennas and Propagation Society for 2012-2014.
Professor Leung received the International Union of Radio Science (USRI) Young Scientists Awards in 1993 and 1995, awarded in Kyoto, Japan and St. Petersburg, Russia, respectively. He received Departmental Outstanding Teacher Awardsin 2005, 2010, and 2011. He is a Fellow of IEEE and HKIE.
Development of the Dielectric Resonator Antenna
The fundamentals and development of dielectric resonator antenna will be discussed in this talk. For many years, dielectric resonators (DRs) have only been used as high-Q elements in microwave circuits until S. A. Long and his collaborators showed that they can also be used as efficient radiators. The studies were motivated by an observation that carrier frequencies of modern wireless systems had gradually progressed upward to the millimeter-wave region, where efficiencies of metallic antennas can be reduced significantly due to the skin effect. In contrast, DR antennas (DRAs) are purely made of dielectric materials with no conductor loss. This feature makes DRAs very suitable for millimeter-wave systems.
As compared to the microstrip antenna, the DRA has a much wider impedance bandwidth (~ 10 % for dielectric constant ~ 10). This is because the microstrip antenna radiates only through two narrow radiation slots, whereas the DRA radiates through the whole DRA surface except the grounded part. Avoidance of surface waves is another attractive advantage of the DRA over the microstrip antenna. Nevertheless, the DRA and microstrip antenna have many common characteristics because both of them are resonators. For example, both of them can be made smaller in size by increasing the dielectric constant because the dielectric wavelength is smaller than the free-space wavelength. Furthermore, basically all excitation methods applicable to the microstrip antenna can be used for the DRA.
Although the DRA received attention originally for millimeter-wave applications, it is also widely investigated at microwave or even RF frequencies. It is because the DRA is a volume device that offers designers more degrees of freedom than 2D-type antennas (e.g., microstrip antennas) or 1D-type antennas (e.g., monopole antennas). Other advantages of the DRA include its light weight, low cost, low loss, and ease of excitation.
The following DRA topics will be covered in this talk:
- Basic theory
- Frequency-tuning techniques
- Circularly polarized DRAs
- Dualband and wideband DRAs
- Dualfunction DRAs
- Omnidirectional DRAs
- Higher-order-mode DRAs
Transparent Antennas: From 2D to 3D
Transparent antennas are very attractive. They can be integrated with clear substrates such as window glass, or with solar cells to save surface areas of satellites. Transparent antennas are normally realized using (2D) planar structures based on the theory of patch antenna. The optical transparency can be obtained by fabricating meshed conductors or transparent conductors on an acrylic or glass substrate. Transparent designs using the meshed-conductor approach are straightforward because optical signals can pass through the opening of the meshes, while microwave signals can be transmitted or received by the conductors. The transparency and antenna property can be optimized by refining the width of the mesh. In this talk, results of a transparent antenna with meshed conductors will be presented.
In the transparent-conductor approach, transparent conductive films are used as radiators. Commonly used transparent conductive films include indium tin oxide (ITO), silver coated polyester film (AgHT), and fluorine-doped tin oxide (FTO). A sheet resistance of at least 1-2 ohm/square is required to obtain an optical transmittance of better than 70%. However, antennas made of such transparent conductor films are not efficient because of the high sheet resistance. This is one of the major obstacles to the widespread application of transparent antennas. A method that alleviates this problem will be discussed in this talk.
For a long time, transparent antennas have been of planar (2D) structures. Very recently, 3D transparent antennas have also been developed. This is a new topic. The principle of 3D transparent antenna is based on the theory of dielectric resonator antenna; the resonance is caused by the whole 3D structure rather than a confined cavity as found in the patch-antenna case. For glass, it is usually assumed that its refractive index is ~1.5, giving a dielectric constant of ~ 2.25. This value is too low for a DRA to have good polarization purity. However, it was generally overlooked that this dielectric constant was obtained at optical frequencies instead of microwave frequencies. Recently, a dielectric constant of ~7 was measured for glass at 2 GHz and this value is sufficient for obtaining a good radiator. Since crystals are basically glass, they can also be used for antenna designs. In this talk, the characteristics of glass DRAs will be shown. In addition, the idea of using a 3D glass antenna as a light cover will be presented. It has been experimentally found that the lighting and antenna parts do not affect each other because they are operating in totally different frequency regions. Interesting results will be presented in this talk.
Finally, it will be shown that 3D transparent antennas can be designed as aesthetic glass (or crystal) wares or artworks. This idea is especially useful when invisible antennas are needed due to psychological reasons. The idea has been demonstrated successfully using a glass swan and apple bought from the commercial market. The results will be presented in this talk.
Analyses of Spherical Antennas
The spherical antenna is an interesting and useful topic. For example, a spherical helical antenna can radiate circularly polarized fields over a wide beamwidth. An antenna array with its elements distributed over a spherical surface is able to determine the direction-of-arrival and polarization of an incoming wave. Further, a spherical antenna array can be used to avoid the scanning problem of a planar array at low elevation.
The spherical antenna is also important from the theoretical point of view. Since a spherical structure does not have any edge-shaped boundaries as found in cylindrical and rectangular structures, its closed-form Green’s function is obtainable. As a result, an exact solution of a spherical problem can exist, and the solution can be used as a reference for checking the accuracy of numerical or approximation techniques.
In this talk, the general solution of Helmholtz equation in the spherical coordinates will be briefly reviewed. The solution will be used to solve different spherical antenna problems, including the spherical slot antenna, spherical microstrip antenna, and grounded hemispherical dielectric resonator antenna (which is equivalently a dielectric sphere after imaging). Derivations of their exact modal Green’s functions will be described. Both electric and magnetic current sources will be considered, and their integral equations will be formulated using the Green’s functions. The method of moments (MoM) will be used to solve for the electric or magnetic current sources. From the currents, the input impedances and radiation patterns of the spherical antennas can be obtained easily.
When a field point coincides with a source point, the Green’s functions will become singular and care has to be exercised in evaluating their MoM integrals. Around a singular point, an extensive number of modal terms are needed to calculate the Green’s functions accurately. This may lead to practical problems because amplitudes of high-order Hankel functions can be too large to be handled numerically. A method that tackles the singularity problem will be presented. In this talk, integrals involving spherical Bessel functions or associate Legendre functions will be evaluated rigorously through analytical integration or their recurrence formulas. Since numerical integration is avoided, the evaluations of the integrals are computationally very efficient. Numerical convergence of the modal solutions will also be examined. Excellent agreement between theory and experiment is observed and the results will be presented in the talk. Finally, it will be shown that a spherical solution can be used to solve a planar annular problem.