Statistical behavior of electromagnetic fields within aperture-coupled nested reverberant cavities

Abstract

Reverberation chamber (RC) theory shows that the square-magnitude of a single spatial component of a received electric field within an ideally operating chamber follows an exponential distribution. Historical experimentation has shown this ideal distribution is closely approached in well-designed chambers. It has been hypothesized that by nesting another, smaller, reverberant cavity within a larger RC, a multiplicative effect will occur creating double-Rayleigh distributed fields withing the small cavity. A study to measure the nested cavity statistics of the fields when a small chamber is weakly coupled to a large chamber has been performed. Both the external and internal cavities were mechanically stirred during measurements and frequency stirring was applied in post-processing. Three distributions were fit to the measured fields, the double-Rayleigh, exponential, and 2-parameter Weibull distributions. The goodness-of-fit of each distribution was evaluated using rigorous statistical tests. The shielding effectiveness of the smaller chamber was also found.

The double-Rayleigh distribution gave a poor fit to the measured field statistics under all conditions considered. The exponential distribution gave a good fit at higher frequencies and with large coupling, the two chambers are acting as one resonant cavity. Overall, the Weibull distribution gave the best overall fits under all conditions tested. However, the statistical tests still rejected the fits at a higher rate than expected from chance alone. These results suggest that the Weibull distribution can be used to describe the nested-cavity field statistics under general conditions for low precision work but a more accurate statistical description is needed when high-precision results are required.