Emission and immunity testing: test object electrical size and its implication

As the electrical size of a test object increases, so does the complexity of its radiation/receiving pattern. The complexity of the coupling pattern affects the number of orientations necessary to determine minimum immunity and maximum emission. This issue has implications for electromagnetic compatibility (EMC) standards. If the true minimum immunity or maximum emission is to be determined, then a sufficient number of measurement samples (orientations of the test object) over the test object´s coupling pattern need to be taken. If fewer samples are used in an immunity or emission test (e.g., the four orientations presently recommended by IEC standard 61000-4-3), then the difference between the estimate (based on the low sample number) and the true value needs to be recognized and, if possible, quantified. Examples suggest that, for test objects of modest size (an example of a test object 50 cm in major dimension is considered), maximum coupling may be underestimated by up to 4 dB for present immunity test methods (e.g., IEC 61000-4-3 with an upper frequency limit of 1 GHz). The underestimation increases to 6 dB if the upper frequency is extended to 5 GHz. This result suggests that present immunity and emission standards should consider including the effect of the size of the test object on EMC emission and immunity pass/fail limits.