SUMMARYDielectric resonator antennas (DRAs) are a relatively new class of antenna that utilizes the radiation phenomena of dielectric resonators in open space. Since mainly analytical approaches have been applied to investigate DRA designs, the current DRA forms are limited to simple geometric shapes and highperformance DRAs with complex shapes have not yet been developed. Topology optimization is capable of yielding high-performance structures, and has been extensively applied to a variety of structural optimization problems. Applying it to the task of DRA design may be extremely useful for the design of high-performance antennas. On the other hand, the finite difference time domain (FDTD) method has been used to numerically evaluate general antenna performance, since it is numerically robust during time domain analyses and can handle complex models. Thus, the integration of topology optimization with the FDTD method has the potential to enable innovative designs of advanced antennas that offer exceptional performance. In this research, we propose a new topology optimization method for the design of DRAs that aim to operate with enhanced bandwidths, using the FDTD method. First, the concept of topology optimization is briefly discussed, and a way to integrate topology optimization with the FDTD method is proposed. Next, design requirements are clarified and the corresponding objective functions and the optimization problem are formulated. An optimization algorithm is constructed based on these formulations. Finally, several DRA design examples are presented to confirm the usefulness of the proposed method.

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The reactions for the deoxidation and the desulfurization of liquid iron by calcium are respectively expressed as follows:CaO(s)ϭCa(mass%, in Fe)ϩO(mass%, in Fe) .....(1) ISIJ International, Vol. 43 (2003) It is important to reduce the oxygen potential in steel in order to lower the sulfur content, and the addition of the strong deoxidizing agent, such as calcium, is considered to be most effective. The deoxidation and the desulfurization equilibria of liquid iron by calcium have been investigated at 1 873 K. The following thermodynamic values on the calcium desulfurization have been derived from the experimental results in the present study and in the previous studies by others.CaS (s)ϩCa (mass%, in Fe)ϩS (mass%, in Fe) log KϭϪ6.23 (Ϯ0.56) e S Ca ϭϪ22.4 (Ϯ6.4) , e S Ca ϭϪ28.0 (Ϯ8.0) (at 1 873 K) Using the thermodynamic data derived in the present study, the deoxidation and the desulfurization equilibria of liquid iron by calcium are proposed in the Fe-Ca-O-S system. The deoxidation and the desulfurization limits of liquid iron by calcium are calculated to be 1.6 mass ppm O (at 15 mass ppm Ca) and 3.5 mass ppm S (at 40 mass ppm Ca) under the condition of a CaO ϭa CaS ϭ1 at 1 873 K.