On Measuring Shielding Effectiveness of Sparsely Moded Enclosures in a Reverberation Chamber

Recently, there has been great interest in evaluating the shielding effectiveness of physically small enclosures (all linear dimensions between 0.1 and 2 m) using a reverberation chamber. In cases where the enclosure is also electrically small (linear dimensions on the order of a free-space wavelength or less), the enclosure supports only discrete resonant modes whose lineshapes have little or no overlap in frequency. This sparsely moded or “undermoded” cavity poses a number of complex challenges to defining and measuring shielding effectiveness. This study contributes to the development of a measurement process for evaluating shielding effectiveness in electrically small enclosures. Specifically, we demonstrate the performance advantages of a traveling-wave antenna (long-wire probe) as a means of fully sampling the field throughout the volume of the enclosure without the need for multiple, wall-mounted probes. Furthermore, the good impedance match of the long-wire antenna permits a large dynamic range in the shielding effectiveness measurements. A simple and fast test method is presented that is accurate and repeatable, and embodies the desired “dovetailing” of shielding effectiveness values obtained as frequency increases and the enclosure transitions from undermoded to overmoded operation. Finally, a rudimentary statistical analysis is provided to assess typical uncertainties inherent in the shielding effectiveness evaluation.