Mode-stir measurement techniques for EMC theory and operation

A mode-stir chamber is simply an RF shielded room that has the properties of a high Q cavity. It must be electrically large, the smallest dimension of a normally rectangular chamber being the limiting factor of operation. The unique feature of a mode-stir chamber is a rotating paddle wheel or tuner. This has to be electrically large and physically large with respect to the dimensions of the chamber. The purpose of the tuner is to perturb the boundary conditions of the chamber changing the standing wave patterns set up. This has the effect, if we can stir the energy in the chamber sufficiently, to yield an ensemble illumination of the equipment under test (EUT) over 4π steradians and all polarisations during one rotation of the paddle wheel. Mode-stir or reverberation chambers are now becoming established as a powerful tool for EMC susceptibility and emission measurements also for measuring shielding effectiveness. The mode-stir technique has many advantages over plane wave measurements including: robust testing of EUT susceptibility, high field capability, repeatability, broad-band frequency measurement and low cost. This paper reviews the theory of operation of mode-stir chambers; to includes chamber design, measurement technique, the evaluation of measurement errors and statistical methods for evaluating the ensemble field uniformity