Chapter 6: RF Emissions Measurements

Overview

One of the aspects of electromagnetic compatibility that is most difficult to grasp is the raft of techniques that are involved in making measurements. EMC phenomena extend in frequency to well beyond 1GHz and this makes conventional and well-known techniques, established for low frequency and digital work, quite irrelevant. Development and test engineers must appreciate the basics of high frequency measurements in order to perform, or at least understand, the EMC testing that will be demanded of them. This chapter and the next will serve as an introduction to the equipment, the test methods and some of the causes of error and uncertainty that attend high frequency EMC testing.

For ease of measurement and analysis, in the commercial tests radiated emissions are assumed to predominate above 30MHz and conducted emissions are assumed to predominate below 30MHz. There is of course no magic changeover at 30MHz. But typical cable lengths tend to resonate above 30MHz, leading to anomalous conducted measurements, while measurements of radiated fields below 30MHz will of necessity be made in the near field if closer to the source than ?/2 ? (see section 10.1.4.2), which gives results that do not necessarily correlate with real situations. In practice, investigations of interference problems have found that controlling the noise voltages developed at the mains terminals has been successful in alleviating radio interference in the long, medium and short wave bands [85]. At higher frequencies, mains wiring becomes less efficient as a propagation medium, and the dominant...