Simplifying EMI immunity (susceptibility) tests in TEM cells

The author addresses two problems encountered in EMI (electromagnetic interference) immunity tests of printed circuit boards and other small electronic assemblies in TEM (transverse electromagnetic) cells. One time-consuming activity in these tests is the repetitive repositioning and testing of the device under test (DUT) to assure that all likely EMI coupling orientations have been tried. The other problem is spurious results arising from undesired induction of the test signal on the wires which service the DUT. It is shown that the time required to repeatedly test the DUT in different orientations may be reduced by mounting it on a dielectric rotary shaft on its ortho-axis. The ortho-axis makes an angle of 54.7° to the edges and centerlines of each face of the DUT; this angle is the ortho-angle. A single 360° rotation of the DUT around the ortho-axis exposes all its orientations to the TEM-cell fields. The DUT service wires may be brought out of the TEM cell along the ortho-axis and shielded by a metal tube around the axis. Care must be taken in the design of the DUT service wiring to avoid coupling to the H-field. In particular, current loops in the wiring must be avoided or made as small as possible