10 Electromagnetic simulation
10.1Concept of electromagnetic simulation
Generally, there are some analytical methods to obtain antenna characteristics. However, it can be difficult to obtain the characteristics of small antennas, because they are very limited in dimension and influenced quite sensitively by environmental conditions around them. In many cases, it is hard to verify the performance of small antennas by experiments. For example, when the size of the antenna is extremely small, the radiation resistance becomes too small to be determined accurately. Another case is where the antenna structure is so complicated that precise measurement becomes very difficult to perform.
Recently, antenna engineers have become able to rely on highly specialized electromagnetic (EM) simulators to develop and optimize antenna design [1, 2]. Computeraided analysis and optimization have replaced the design process of iterative experimental modification of the initial design. Therefore, the EM simulators become of paramount importance in the efficient research and development of new small antennas. An accurate computer-aided design (CAD) could support a clear physical interpretation for characteristics of small antennas.
The operation of an EM simulator is based on the numerical solution of Maxwell’s equations in differential or integral form.
Antennas for small mobile terminals are typical small antennas in practice and the EM simulators are often used to design them. Here, the concept of electromagnetic simulation is described along with antennas for small mobile terminals.
The EM simulations including antenna and propagation problems are widely used nowadays as the capabilities of personal computers and workstations increase rapidly. On the other hand, mobile communication systems, such as mobile phones recently deployed under various wireless systems having multiple functions including broadband and near-field communications, and also Radio Frequency Identification applications (RFID), have become common. Small and hand-held equipment used in those systems demand reduction in size and weight. Antennas used for such handsets must also follow downsizing of the handset unit, yet must keep the antenna performance unchanged or even improved. Especially, built-in antennas are required for handsets for the sake of convenience and durability. Because the environmental factors such as the handset material itself and the nearby human body affect the sensitivity and performance of the antenna mounted on the handset, it is not easy to realize an optimum antenna system.
390Electromagnetic simulation
To obtain the characteristics of the installed antenna, considered as one unified system composed of an antenna element and a conducting handset case, analysis by using an EM simulator is very effective [2, 3]. However, as the simulators are not perfect for analyzing antenna problems, designers need to have adequate experience using the simulator and be acquainted with the strengths and weaknesses of the various simulation methods. Representative applications of many analyses of small antennas used in mobile handsets by using EM simulators on the basis of method of moments [4, 5] and FDTD (FiniteDifference Time-Domain) method [3, 6] have been reported so far. In the method of moment (MoM), both the antenna element and the conducting handset case are simulated by a wire-grid model and they are treated as one unified system with the entire system made up of wire elements only [4, 5]. By using FDTD method, the analysis of the handset antenna including the effects of the surrounding objects such as the handset unit and a human body can be easily performed instead of applying approximate methods [3, 6]. However, the EM simulators generally are proprietary in nature and the details of their internal workings are not widely available to general users.
10.2Typical electromagnetic simulators for small antennas
The electromagnetic simulator is based on the numerical solution of Maxwell’s equations in differential or integral form. Integral-equation methods make use of Maxwell’s equations in integral-equation form to formulate the electromagnetic problem. The kernel of the integrals is formed by a Green’s function tensor.
Differential-equation methods are derived directly from Maxwell’s curl equations or the Helmholtz wave equations. The most popular differential-equation-based methods are the Finite Element Method (FEM), utilized in Ansoft’s HFSS software package, and the Finite-Difference Time-Domain Method (FDTD), which is employed by CST’s time domain transient solver.
FEM, like FDTD, is a method based on solving a partial differential equation, rather than an integral equation, as in the case for MoM. Both the differential-equation methods are particularly suitable for modeling full three-dimensional volumes that have complex geometrical details.
Here, four types of EM simulators are explained. The IE3D, FIDELITY, HFSS, and Microwave Studio EM simulators, which are commercial software products, are based on MoM, FDTD (Finite-Difference Time-Domain), FEM (Finite Element Method), and FIT(Finite Integration Technique), respectively.
The IE3D simulator is the electromagnetic simulator based on the method of moments, and the primary formulation of IE3D is an integral equation obtained through the use of Green’s function. Primary features are: (a) modeling of three-dimensional metallic and dielectric structures, (b) automatic generation of non-uniform meshing with rectangular and triangular cells, (c) automatic creation of small cells on the edges, and so forth. There is no limitation on the shape and orientation of the metallic structures. In the calculation, the IE3D simulator employs a three-dimensional non-uniform triangular and rectangular mixed meshing scheme automatically, and the shape of the antenna and