Evaluation of present limit for radiated emission measurement in FAR Test Site and proposal for new limits according to the polarization plane

Radiated emission test in a FAR (Fully Anechoic Room) site has been adopted as the latest version of CISPR32, IEC61000-6-3 and IEC61000-6-4 standards. Test in a FAR site without metal ground plane is expected to the measurement time reduction, since the height scan is not required to find the maximum emission because there is no reflective wave from the floor. Consequently, the severe limit of 5 dB than a test in OATS (Open Area Test Site) / SAC (Semi Anechoic Chamber) has been introduced to a test below 1 GHz in FAR. However, based on the comparative simulation results, the radiated emission limit in FAR site has been relaxed to plus 7 dB at 30 MHz from the 5 dB severe value at 230 MHz linearly with respect to the logarithm of the frequency, since the disturbance source in the equipment is affected by a metal ground plane of the test in OATS/SAC. The rationale of FAR test limit was indicated as a calculation result, but actual measurement data are not enough. Under this situation, we have performed comparison experiment measurement between SAC site and FAR site by using a same equipment to investigate the validity of the relaxed limit based on the calculation. From the experiment measurement result, the limit according to the polarization plane should be specified because it has a high possibility to mislead the product conformity assessment in a polarization of vertical to the ground plane.