Normalized site attenuation calculations using standard spreadsheet analysis

Normalized site attenuation (NSA), has become the standard method for determining the adequacy of EMC test sites to perform EMI emission measurements. The measurement of the NSA parameter for a test site and the subsequent required data processing and analysis can sometimes be complicated, tedious, and time-consuming. This is particularly so for alternate test sites (e.g., semi-anechoic chambers) where 'volumetric' NSA analysis is required and also when swept frequency measurements are performed that contain large amounts of data. In cases such as this the management of the large amounts of data can be just as critical as the actual measurements. This paper proposes a method by which the large amount of NSA data and calculations can be managed in a reasonably simplistic manner through the use of standard spreadsheet analysis. This method has the distinct advantage (in contrast to special custom software) of providing an open framework model whereby all calculation details are readily available and visible for review and inspection, including all formulas used in the calculations. The foundation for this spreadsheet analysis discussion is based on the author's recent work involving NSA qualification testing of a new 10-meter semi-anechoic chamber for a major manufacturer.