Limitations of Low-Frequency Magnetic Field Immunity Testing with Small Radiating Loop

Use of a small radiating loop (6 cm radius) is recommended in EMC standards (Mil-Std-461G:2015(RS101) and IEC 61000-4-39:2017) for immunity testing with low-frequency magnetic fields. We investigated the limitations of this method using finite-element simulations. We studied the effects of fields from radiating loops with different radii and their induced voltages in different radius receiver loops that represented wiring of equipment under test (EUT). We also studied the windowing-method recommended in those standards. It involves positioning the loop successively over all locations on each face of the EUT. Our results show that this radiating loop can only simulate exposure to larger real-world EM fields when the EUT's wiring area is smaller than the radiating loop. Another limitation is that the magnetic field from the radiating loop drops significantly with distance perpendicular to the loop surface. Therefore, the windowing-method with a small radiating loop is only suitable for simulating exposures to real-world sources with fields that do not extend a large distance from the loop. In addition, the field distribution (width and depth) of the real-world EM source must be accounted for before deciding to use a small radiating loop for immunity testing of an EUT.