In recent years, research and development of small and flat antennas are widely performed, especially, antennas on dielectric substrate as patch antennas or printed antennas. Various methods have been presented for antennas on dielectric numerical simulation. The finite difference time domain (FDTD) method[l] has become a powerful tool for analyzing the electromagnetic problems with complex geometries. The strength of the method is it is simple and efficient to model the electromagnetic field within a practical level of the accuracy. However, much smaller cells are required to obtain an extremely accurate result for impedance characteristics of the antennas on dielectric substrates. One reason can considered as follows. The electromagnetic field indicates a singularity at the edge of the conductor. Furthermore, the antenna conductor is attached just on the dielectric surface. Therefore, the electromagnetic field changes very rapidly near the edge of antenna conductor on the interface. In order to overcome this difficulties, two techniques has been proposed. One of which is a subgridding method [2] in which the fine cell is locally used for the region that the field distribution is expected to change seriously. Other technique is a so-called local cell technique in which the field behavior which has to be theoretically expected in advance [3], is incorporated to the FDTD local cell by using integral form of the Faraday's or Ampere's law. The purpose of this paper is to analyze the impedance characteristics of the printed antennas accurately by using the FDTD method. It has been reported that the resonance frequency of the printed antenna does not converge to the carefully measured resonance frequency even if the extremely fine cell is used. In this calculation the cell size was shrunk to some hundredper wavelength. Therefore, other useful method should be investigated. In this paper the so-called local cell method is tested. In this method, how physically correct field distribution introduced to the FDTD cell is essential. A quasi-static field distribution is utilized because this field is dominant in the region near the conductor edge. This paper consists of two parts. First half describes the Quasi-static approximation. In the second half, the FDTD formulation incorporating with the Quasi-static approximation is introduced. The validity and effectiveness of the method are confirmed numerically and experimentally.
展开▼
