1.1 BACKGROUNDS
Over the past few decades, the field of antenna engineering has undergone significant progress. Many new techniques and design concepts have been developed to overcome a myriad of challenges experienced in antenna engineering. Amongst the advancements, the increasing characteristic mode (CM) theory study, focusing on its extensive implementations in many critical antenna designs, is one of the exciting breakthroughs in antenna engineering. Its promising potentiality has been constantly attracting the attention of antenna engineers. The CM theory and its applications in antenna engineering are the topics of this book.
The booming of wireless communication is an important driving force for the advancement of antenna technology. Antennas are the sensors of wireless communication systems. They find wide range of applications from terminal devices (such as mobile phones) to advanced communication systems on aircrafts, ships, and so on. Antennas transfer microwave energy from transmission system to propagating waves in free-space and vice versa. Strong demands like small physical size, low weight, low cost, wideband/multiband bandwidth, reconfigurable capabilities, or even aesthetic consideration are increasingly specified as a must in modern antenna designs. The inherent challenges in these demands thus have further propelled the advance of antenna technology.
The rapid growth of numerical electromagnetic (EM) modeling techniques plays another vital role in antenna technology advancement. The numerical EM modeling techniques can provide an accurate way to validate antenna performance before carrying out expensive fabrications and measurements. Consequently, numerous in-house or commercial software packages based on the method of moments (MoM) [1] , the finite element method (FEM) [2, 3], and the finite difference time domain (FDTD) method [4–6] are extensively used in antenna designs. Given the antenna geometry and excitation structure, numerical techniques are able to simulate any antenna parameters.
However, from the practical point of view of antenna design problem, these numerical EM modeling techniques provide little information on the physical aspects of an antenna to be designed. The lack of physical insights brings difficulties in the further optimization of the antenna structure and feedings for achieving enhanced radiation performance. Therefore, antenna designs are heavily reliant on the designer’s experience and knowledge. In the worst cases, antenna designs become a trivial task where antenna engineers blindly modify the antenna and feeding structures and simulate the antenna performance via numerical EM modeling tools iteratively.
For the sake of convenience, numerical EM modeling techniques are extensively combined with modern evolution optimization algorithms such as the genetic algorithm (GA) [7] to help mitigate the heavy workload in antenna tunings. The assumption is that the optimization algorithm will eventually arrive at the expected antenna performance after the exhaustive search in their decision space. However, this is not always true in all antenna design problems. More often than not, the automatic optimization algorithm returns to a complicated antenna structure with the satisfactory level of performances. However, the complexity of the resultant antenna structure makes it too hard to understand the underlying radiation mechanism. In this case, the design will be generally regarded as a lack of scientific knowledge. Therefore, over-dependence on such brute force techniques is not a good way in antenna research. At least, it should not become antenna engineers’ primary choice.
It is evident that a successful antenna design is highly dependent on previous experiences and the physical understanding of antennas. To grasp such knowledge may require many years of practical exercise. The experience, however, is hard to be imparted from one to another, as such personal experience is usually formed based on one’s understanding of conventional antenna design concept introduced in textbooks. With such experience, solutions to some critical antenna design problems (e.g., the problems in Chapter 6) are usually not available.
Based on authors’ personal understanding, an ideal antenna design methodology should allow one to achieve optimal antenna performance in a systematic synthesis approach with very clear physical understandings. However, such antenna design methodology does not exist until the antenna community recognizes the great potential of the CM theory in antenna engineering. In the past decade, the extensive applications of the CM theory in antenna designs have witnessed the roadmap of the development of this ideal antenna design methodology.
In the new millennium, studies on the CM theory have revealed its promising potential in a variety of antenna designs. The CM theory makes antenna design much easier than ever as antenna engineers need not depend heavily on personal experiences or brute force optimization algorithms. Meanwhile, the CM theory provides an easy way to understand the physics behind many key performances such as the bandwidth, polarization, and main beam directions. These physical understandings provide a greater degree of freedom in terms of design. As compared to traditional antennas, the antennas designed with the CM theory have more attractive electrical performances and configurations. Based on the recent advances made by the authors from the Temasek Laboratories at National University of Singapore (TL@NUS), this book discusses the CM theory and the CM-based design methodologies for a wide range of antenna designs.
1.2 AN INTRODUCTION TO CHARACTERISTIC MODE THEORY
The CM theory is a relatively new topic in the antenna community. Its great potential in antenna engineering has not been widely recognized till 2000. In the following subsections, several well-known modal methods for the analysis of particular antenna problems are being reviewed first providing preliminary knowledge about what modal analysis is about. It also illustrates how these modal methods help in practical antenna design. Next, we address some fundamental questions: why the characteristic modes were proposed and what are characteristic modes? Furthermore, the primary features of the CMs would be discussed. Finally, we briefly introduce CM variants that are distinct in terms of structures and materials to meet different radiation/scattering requirements. Detailed formulations and applications of these CM variants are discussed through Chapters 2–6.
1.2.1 Traditional Modal Analysis in Antenna Engineering
There is a long history in the development of modal analysis methods for a variety of problems in electromagnetics. The most famous one would be the modal expansion technique for infinite long waveguides [8–11] . This modal expansion technique handles the electromagnetic field inside a closed structure. It calculates the cut-off frequency and modal field pattern of each propagating mode inside the waveguides. These modes are primarily determined by the boundary conditions enforced by the cross-section of the waveguide. However, it is exceptionally difficult to implement modal analysis for open problems (radiation or scattering). Thus if the resonant behavior and radiation performance of an antenna can be interpreted in terms of the mode concept, great convenience will be brought to the analysis, understanding, and design of antennas.
In antenna engineering, there are three well-known modal analysis methods for particular antenna structures, namely, spherical mode, cavity model, and dielectric waveguide model (DWM). They will be briefly reviewed to show their attractive features and primary limitations.
1.2.1.1 Spherical Mode
The first modal analysis method for antenna problems was the spherical mode method proposed in 1948 [12]. It discussed the physical limitations of the omnidirectional antennas in terms of antenna quality (Q) factor. It is now known as the Chu’s Q criterion. Later in 1960, Harrington used the spherical mode analysis to discuss the fundamental limitations with respect to the gain, bandwidth, and efficiency of an antenna [13]. These pioneering works were based on the assumption where the radiating fields can be expanded using the spherical modes within a sphere that completely encloses the antenna. Therefore, the radiated power can be calculated from the propagating mode within the sphere. The limitations of an electrically small antenna in terms of the Q factor can be determined from the size of t...
