Spherical dipole antenna has been used to evaluate EMC performance because it can consider small dipole source. In this paper, site attenuation of spherical dipole antenna is calculated using wire grid model. It is also measured in semi-anechoic chamber for 3m distance, and ihe result indicates the deviation between calculation and measurement result is within 4dB from 30 MHz to I OOOMHz. The site attenuation distribution is measured in a small semi-anechoic chamber and compared with the measured one. The result indicates that the large reflection loss absorber is efective to improve the performance of the site and the site attenuation is improved when transmitting and receiving antenna place at line unsymmetricalposition.

SUMMARYSite attenuation has been evaluated by means of a half-wave dipole antenna. Since this antenna type requires a size on the order of a half wavelength, it is not suitable for evaluation of small anechoic chambers. In this paper, a method of evaluation of small anechoic chambers using a spherical dipole antenna with a diameter of 15 cm is discussed. First, the spherical dipole antenna is modeled by wire grids and the site attenuation and antenna factor are derived. It is found that in the range of 30 MHz through 1000 MHz the discrepancy between the analytical values and the measured values of the site attenuation is less than ±4 dB, while that of the antenna factor is less than ±2 dB. Next, the site attenuation taking account of the reflection characteristics of the anechoic chamber is analyzed by the ray tracing method. As parameters, the antenna factor and the radiation pattern derived from the wire grid model are used for the antenna characteristics, and the reflection coefficient derived from the multilayer plane approximation method is used for the reflection characteristics of the absorber. It is found that the discrepancy between the analytical values and the measured values for the site attenuation distribution at a frequency of 600 MHz is less than ±3 dB, and the ray tracing method is found useful for the analysis of the site attenuation distribution using a spherical dipole antenna. Further, when the new approach is applied to a small anechoic chamber in which absorbers with different characteristics are included, the present method is found effective for evaluation of the areas with good properties and of the effect of the absorbers.