Time-Domain Measurement and Analysis of Mechanical Step Transitions in Mode-Tuned Reverberation: Characterization of Instantaneous Field

We report on time-domain measurement and statistical analysis of field transients that are caused by mechanical transitions between the stationary states of an overmoded resonant cavity. These transitions and field transients are produced by the stepwise rotation of a mode tuner (reflective paddle wheel) inside a reverberation chamber excited by an uninterrupted time-harmonic source. It is shown that the cross-correlation function between the so-called stir sequences enables the reconstruction of the averaged mechanical motion of the mode tuner. Evolutions of the probability distribution and statistics of the transmission S-parameter show correlation with tuner motion and demonstrate nonstationarity of the cavity field. The influence of the operating frequency and various motor parameters is investigated. In estimating the evolving maximum-to-average ratio, the variations in the underlying (parent) distribution are shown to be considerably more influential than the variations of the number of independent samples. Semi- and fully empirical estimates of this ratio are compared with the directly measured values, and fully empirical estimation allows for more accurate prediction. It is shown that the measured local minima and local maxima of this ratio can be identified with short- and long-tailed instantaneous parent distributions, respectively. The effect is physically interpreted as modal bunching and dilation. The results are relevant to EMC immunity testing in reverberation chambers.