Correlation and Spectral Density Functions in Mode-Stirred Reverberation – I. Theory

Auto- and cross-spectral density functions (SDFs) for dynamic random fields and power are derived. These are based on first- and second-order Padé approximants of correlation functions expanded in terms of spectral moments. The second-order approximant permits a characterization of stir noise observable at high stir frequencies in the autospectral density. A relationship between stir imperfection and spectral kurtosis is established. For the latter, lower bounds are established. A novel alternative measure of correlation time for mean-square differentiable fields is introduced as the lag at the first point of inflection in the autocorrelation function. A hierarchy of Padé deviation coefficients is constructed that quantify imperfections of correlations and spectra with increasing accuracy and range of lags. Analytical models of the spectral densities are derived and their asymptotic behavior is analyzed. The theoretical spectral density for the electric field (or
$\mathrm{S}_{21}$
) as an input quantity is compared with that for power (or
$|\mathrm{S}_{21}|^{2}$
) as the measurand. For the latter, its inverted-S shape conforms to experimentally obtained stir-spectral power densities. The effect of additive noise on the stir autocorrelation function and SDF is quantified.