Reproducibility of Radiated Emissions Measurements in Compact, Fully–Anechoic, Rooms – The Contribution of the Site-to-Site Variations

In this paper, a new procedure is presented for accurately measuring the difference in performance between sites for radiation tests (emission/susceptibility). Differences as low as 0.5 dB can be detected, and this high sensitivity is a consequence of the basic idea of having the entire transmission/reception (T/R) system itinerate through all the sites under investigation. This rules out the otherwise determinant contribution of the system-associated uncertainties (mainly from non-reproducibility of receiving antenna and receiver). The results of the survey, thus, reliably quantify the amount of disagreement that can be accounted for as being due to the site non-ideality alone. Attention was confined to: 1) short-range, fully anechoic rooms (3-m T/R distance) and 2) the lower frequency range (30 to 300 MHz). These assumptions identify a type of site that is in frequent use today and a frequency range where the measurement conditions are usually very critical. Application of this method to different sites or configurations other than those considered here is straightforward. A total of 14 different sites were investigated, and their level of disagreement is collectively described here, in terms of standard deviation of the sample, and individually, in terms of one-to-one deviation, namely, each site against each other (91 pairs). The one-to-one results exclude that the observed collective deviation was due to the presence of a minority of defective sites, and thus, demonstrating that the collective deviation that we derived effectively describes the amount of statistical disagreement in the whole sample. The measured sample standard deviation can be inserted in an overall uncertainty budget together with the independently derived instrumentation uncertainties. All aspects of the physical design of the experiment are analyzed to demonstrate the steps needed to obtain the high sensitivity that is required here.