Investigation of relations between measurement distance and electric field strength for radiated emission tests using artificial radiation sources

Abstract

The electric field strength that is allowed to be radiated from electronic equipment is specified at distances of 10 or 3 m. However, it has been pointed out that test results obtained at 3 and 10 m are not always correlated. In this study, an artificial radiation source for which the radiated electromagnetic field can be analytically calculated is used, and the relation between the electric field strength and the measurement distance is investigated by theoretical analysis and measurement. The following results are obtained. Defining the electric field strength at a distance of 10 m as the reference, the average electric field strength from 30 MHz to 1000 MHz on a relative basis changes in proportion to the −1.17 power of the distance for horizontal polarization, and as the −0.71 power for vertical polarization. The standard deviation of the relative electric field strength increases as the distance ratio relative to 10 m increases, being 5 dB for horizontal polarization and 2 dB for vertical polarization at a distance of 3 m. Analysis shows that the distance dependence can be improved by raising the height of the antenna sweep or by moving the antenna at a fixed distance from the equipment. The dependence can be improved sufficiently that the electric field strength is proportional to the −1 power of the distance down to a distance of 2 m for horizontal polarization. But for vertical polarization, the electric field strength is not proportional to the −1 power of the distance at distances of up to 5 m, even though the effect of the near field is small. © 2005 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 88(7): 41–51, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.20171