A study of electric field behind isolated metal plate in a radiated immunity test set up

In a radiated immunity test, the equipment under test (EUT) failed at 300 MHz when the vertically oriented antenna was facing a vertical metal plate mounted above a baseplate on the EUT. The two edges of the plates were located a few mm apart from each other, on one side of the equipment and the plates and the frame of the EUT were isolated from each other. The EUT passed above the specified test level when the antenna was facing exposed wiring, connectors and enclosures on the other three sides. The test personnel expected the opposite to be true. When the gap between the two plates was electrically closed then the EUT passed the radiated immunity above the specified test level. This result prompted measurements of a solid 0.6 m x 0.3m plate versus two 0.3 m x 0.3 m plates with a gap between them of 2mm and 4 mm. The results of these measurements may be a surprise to some and are described in this paper.