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Unformatted text preview: 56 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 43, NO. 1, FEBRUARY 2001 Statistical EM Field Models in an Externally Illuminated, Overmoded Cavity Richard Holland , Member, IEEE, and Richard H. St. John Abstract— A description is given of the statistics of EM fields and power fluxes inside a highly overmoded, leaky, externally driven cavity. We explore various modeling combinations of chi square (with two and six degrees of freedom), normal, and log normal dis- tributions. Convolution (in an unusual, logarithmic sense) of the two chi square models (with two and six degrees of freedom) proves to provide the best match with observation. Statistically generated field models are then used as drivers of cable codes describing the wiring in an aircraft shell. Modeled cable-current cumulative dis- tribution functions match experimental results within a factor of two, which is commensurate with the configuration and measure- ment uncertainty. Index Terms— Cable currents,-square distributions, Kolmog- orov–Smirnovtest, distribution, Lehmandistribution, log normal distributions, statistical electromagnetics (STEM), statistical inde- pendence. I. INTRODUCTION D ETERMINISTIC formulation of Maxwell’s equations does not behave efficiently for predicting the current response of cables in a complicated, highly overmoded, exter- nally illuminated chamber. Our alternative approach is to work out a description of the probability density function (pdf) of electromagnetic fields or power-flux densities in the chamber –. This pdf, in general, may have position or frequency as the independent parameter. Mode-stirred chambers can also be treated by these techniques, with paddle angle the independent parameter in mechanically stirred chambers –, and frequency or phase the independent parameter in electronically stirred chambers , . (Refer to  and  for an es- pecially lucid description of mode-stirred EM field statistics.) Depending on the nature of the enclosure (frequency-dependent , degree of homogeneity, etc.) the electromagnetic fields have a zero-bias normal pdf, a bipolar log-normal pdf, some mix of the two, or some highly arcane pdf with the property that the associated power-flux-density pdf looks like a modified Bessel function of the second kind (see our subsequent section on the lambda or Lehman statistical model , , , , ) mean (1) where-subscripted quantities are related to fields squared or currents squared (i.e., power). Manuscript received October 17, 1996; revised October 26, 2000. This work was supported by the Air Force Phillips Laboratory (now the Air Force Research Laboratory) under Contract F29601-95-C-0045....
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