Abstract :
Present EMC military and industrial standards fail to cope with the problems involved in performing radiated field measurements in shielded enclosures. Repeatability errors of 25 db are not uncommon, and relating the data to open-field values is practically impossible. New measurement methods based on the resonance characteristics of the enclosure were proposed in a paper presented by the author at WESCON 1968, giving the theoretical background and the results of preliminary experiments. This paper, a continuation of the WESCON paper, begins by presenting a general solution to the problem of radiating sources in shielded enclosures using Green´s function method. A particular case, that of a vertical antenna in a rectangular enclosure, is then solved in detail, showing full agreement with measured values. Previous attempts to match theory and experiment failed due to the fact that most of the published literature asserts the completeness of the solenoidal eigenvectors E and H for modal field expansions. Actually, in the presence of current and charge distributions, irrotational eigenvectors constitute not only a necessary part of the solution, but they become the dominant terms at frequencies below cutoff. The theory also corroborates the experimental finding that the field strengths very near the source are essentially the same, whether inside an enclosure or in an open field. With this background, a detailed field-strength measurement procedure for frequencies below cutoff is worked out (the first resonant frequency for typical shielded rooms falls in the range of 20 to 50 MHz).
