A CAD software tool based on ray tube tracing technique has been successfully developed for anechoic chamber design which can deal with a chamber of an arbitrary shape and arbitrary layout of radio absorbing material (RAM). The normalised site attenuation (NSA) characteristics of a semianechoic chamber were estimated using the ray tube tracing method. The NSA values obtained for 3 m and 10 m semianechoic chambers with different height of transmitting antennas were compared with the values given in the CISPR (Comité International Spécial des Perturbations Radioélectriques) standard and a good agreement was obtained. It is shown that this software is robust and accurate for chamber simulation.