Numerical analysis and optimization of anechoic chambers for EMC testing

Anechoic chambers for EMC testing in the frequency range from 30 MHz to 1 GHz have to fulfil stringent requirements before they are accepted for standardized emission and immunity measurements. In Europe, these requirements will lead to the exclusion of about 90% of the existing and 50% of the new chambers from accreditation due to the formerly lower standards. This paper presents for the first time a complete, accurate and validated numerical analysis method of EMC anechoic chambers, which focuses especially on the problems below 200 MHz where the efficiency of most absorbers is quite poor. Examples of computed field configurations, normalized site attenuation (NSA) deviations, and field homogeneity zones are presented. They show the complexity of the frequency dependent field propagation mechanisms in anechoic chambers. Computational results coincide excellently with measurements obtained by scanning the chamber with a small field probe. Unlike trial and error or ray optical methods, often used for this purpose, the developed code can accurately predict the performance of new chambers during their design, before the realization. Measures are proposed for the functional improvement of existing as well as new chambers toward an accreditation.