Analysis of the measurement uncertainty associated with 1/R extrapolation of radiated emission measurements on an open area test site (OATS)

Radiated emission measurements made on an open area test site (OATS) are often made at distances other than that associated with the specification limit. In such cases, the measured value is extrapolated to the limit distance using an inverse linear-distance extrapolation factor, 1/R. However, this extrapolation is not accurate on an OATS, where the ground plane introduces reflections that result in distance losses that do not follow a 1/R relationship. In such cases, the linear extrapolation introduces a measurement error that affects the overall accuracy of the measurement. This paper analyzes the sources of these errors, and shows that this error is dependent on the frequency, polarization, and height of the source above the ground plane. Based on the analysis, an estimate of the measurement uncertainty associated with distance extrapolation is derived.