The use of fully anechoic rooms for full compliance EMC testing

This paper gives the background to work is in support of a proposed CENELEC Basic Standard to enable the use of fully anechoic rooms (FARs) for full compliance emission testing without reference to the existing 10 m OATS limits. The standard site for full compliance radiated emission testing in the frequency range 30 MHz to 1 GHz is the 10 m open area test site (OATS). With congestion of the airwaves, testing in the presence of ambient RF interference is increasingly time consuming. This has led to the use of 10 m semi-anechoic rooms (SAR). For operation at frequencies as low as 30 MHz these facilities are expensive. The challenge was to develop limits for a less costly 3 m FAR so that a product has the same probability of complying with the limit as it does for the current limits on a 10 m OATS. New limits need to be set for a FAR because of the absence of a ground plane. A two year study initiated by the EC SMT programme was carried out by 8 partners in 6 European countries in order to establish the appropriate limit for FARs. The conclusion of this study is that the existing 10 m OATS limits, when raised by 5 dB for use in a 3 m FAR, gives good agreement on the two sites. The study showed that the different treatment of cables had a major impact on the comparison and highlighted the need for a review of cable treatment on OATS as well as in FARs