NSA Calculation of Anechoic Chamber Using Method of Moment

Abstract

NSA characteristics of an anechoic chamber were calculated by using the MoM (Method of Moment), and they were compared with those calculated by the FDTD method as well as the measured results in fully anechoic chamber. Next, we calculated of an anechoic chamber with complicated shapes. We found that first imitating a wave absorber using wire meshes with a limited electric conductivity is effective by controlling the wire interval. The NSA characteristics calculated by the MoM are as well as those calculated by the FDTD method. The NSA characteristics calculated by the MoM also agree with the measured results. And NSA characteristics of an anechoic chamber with complicated shapes were able to be calculated by using the MoM. Based on the results, it is confirmed that the MoM can be used to calculate NSA characteristics for an anechoic chamber. An anechoic chamber is a test room specially designed for completely shielding out external disturbances and for suppressing the echo caused by internal electromagnetic waves. The anechoic chamber for the radiated emission measurement specified by CISPR is usually evaluated by NSA (normalized site attenuation), which is a measure of the transmission characteristics between the standard antennas set inside the anechoic chamber. NSA is usually analyzed by using the ray tracing method (1), but the analytical accuracy of the NSA deteriorates in the low frequency band below several hundreds MHz because the ray tracing method approximates an electromagnetic wave as an optical ray. However, the FDTD (finite difference time domain) method is an analytical method that treats electromagnetic waves purely as waves. Therefore, the FDTD method can analyze NSA characteristics in the low frequency band below several hundreds MHz. The NSA of the anechoic chamber using the FDTD method was obtained by Holloway (2) and Takiguchi (3).