CISPR22 measurement distance is 10m. Section 11.2.1 of the standard does allow for a test distance of less than 10m under certain conditions (e.g. high ambient noise levels). Any measurements made at a distance other than 10m should be corrected by 20log(10/d) (where d is the measurement distance used) to obtain the theoretical field strength at a 10m distance and then compared with the limit.
It should be noted that the theoretical drop (10dB) in field strength does not always happen in practice and may be as little as 5dB. For this reason it is wise to test at 10m whenever possible. There is no real need to test at 3m since the FCC will accept radiated emissions data against the CISPR22 limits provided that the test methods and requirements of ANSI C63.4 are met and that conducted emissions are also taken against the CISPR22 limit. Mark Briggs
