Statistical comparison of site-to-site measurement reproducibility

There have been many attempts to assess the reproducibility of EMC measurements between test sites, with varying degrees of success dependent upon implementation details. Hewlett-Packard has a successful program of site-to-site reproducibility evaluations using a custom-designed RF source as a reference, and applying statistical techniques to include the day-to-day variability of each site in the collected data. Comparing the sites to the average (for lack of a reference site), 95% of the measurements agree within 3.8 dB between 30 and 100 MHz and between 550 and 1000 MHz, and within 2.8 dB between 100 and 550 MHz. The program benefits the participating sites by answering the question of how much variability can be expected from day to day and from that site to another, as well as detecting and helping to identify sources of measurement error. The paper also includes evidence that a 1/r correction going from 3 m to 10 m test distance involves errors of about 8 dB.