Investigating the Challenges of Near-Field to Far-Field Transformation at Low Frequencies in Electromagnetic Compatibility Testing

Near-field to far-field transformation is a widely used technique in electromagnetic compatibility testing to predict far-field radiation from near-field measurements. However, at low frequencies, where the device under test is small relative to the wavelength, the transformation often fails, resulting in significant discrepancies between calculated and the actual far-field results. Based on a test case, this paper investigates the underlying causes of this failure by analyzing the effects of cable radiation, limited scan area, probe sensitivity, and other contributing factors. Time-domain near-field measurements of the magnetic and electric fields are conducted using a calibrated scanning system, and the results are compared with full-wave simulations. The findings reveal that improper scan area selection significantly contributes to transformation inaccuracies. Furthermore, the study demonstrates that increasing the scan height can reduce errors caused by the dominance of reactive near-field components which worsens with reduced frequency for a small DUT. A detailed error analysis is provided, offering practical guidelines to improve near-field to far-field transformations in low-frequency electromagnetic compatibility applications.