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This paper describes an investigation into the key factors, which contribute towards an effective mode stirrer. The work concentrates around the lower frequency range, since all stirrers have poorer performance at low frequencies. The stirrer's shape and size have been investigated together with an optimisation of the finer details in the stirrer's shape. The modelling of the mode stirred chamber has been performed using the Transmission Line Matrix method. Software has been developed which, for each position of the stirrer as it rotates, builds the shape of the stirrer using thin perfectly conducting boundaries. Results indicate that the design of the stirrer's basic shape has a small but significant impact on its performance. A genetic algorithm has been used to optimise certain parameters in the shape of the stirrer and a fitness factor based on a free space model of the stirrer has been used. The free space model runs 1500 times faster than the model in the chamber. The optimisation is shown to improve the stirrer's performance in three different sized chambers. Computer modelling has been verified by measurements performed in the chamber at the University of York.

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This paper describes the rationale behind a new proposed measurement of the screening effect of an equipment enclosure that takes into account the contents of the enclosure. The method uses a set of representative contents for enclosures. The representative contents are equipped with surface field probes to measure the power entering the contents. The ratio of this power to the incident power density is used to derive a quantity with the dimensions of area, termed here the Shielding Aperture. The measurement technique is described and examples of measurements are given along with computed comparisons with the conventional Shielding Effectiveness of the enclosures used.

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