An Insight into the Practical Problems of Near-field to Far-field Transformation in the Time Domain for EMI Test

Far-field prediction was achieved using time domain near-field on a rectangular Huygens surface. Both electric and magnetic fields are measured in the time domain and processed to predict far-field radiation. An example is used to verify the method and practical issues are addressed to guide the reader on how to perform a time domain near-field measurement. The predicted far-field results are validated by simulation and anechoic chamber measurements. It is shown at low frequencies, here 30 MHz, at which the DUT hardly radiates ($195 \times 150 \times 200 \mathrm{~cm}$ large), the transformation fails as the reactive near field is much stronger than the radiating components of the near field.