Numerical simulations for site VSWR with consideration of diffracted wave of pyramidal electromagnetic wave absorber

It is known that a reflected wave from an electromagnetic wave absorber on a wall of an anechoic chamber includes higher-mode diffracted waves. Our research group has studied higher mode diffracted waves from pyramidal wave absorber by measurements and simulations. Meanwhile, an evaluation method for anechoic chambers is defined by the CISPR16-1-4 Ed.3 standard. In this method, measured site voltage standing wave ratios (SVSWR) are affected by several factors such as frequency step, antenna directivity, and boundary between wave absorber and floor. There are several reports about measurement results of anechoic chambers where SVSWR values showed higher values at the specific frequencies. In these reports, the authors suspected that these high SVSWR values were caused by antenna directivity or characteristics of wave absorbers. However the actual reason has not been found out. We supposed higher mode diffracted waves affect SVSWR measurements. In this report, we calculated higher mode diffraction coefficients for pyramidal wave absorber using FDTD. And we simulated a SVSWR measurement by using these higher mode coefficients. In the results, we observed that SVSWRs went up sharply higher at specific frequencies as seen in actual measurements.