Near-field measurement based prediction of antenna test results below 30 MHz in CISPR 25 setups

In automotive radiated emission testing, as specified in CISPR 25, the absorber lined shielded enclosure (ALSE) method is commonly used. This requires an expensive chamber. Based on the observation that the dominating coupling is capacitive at frequencies below 30 MHz, the emission tests often suffer from low reproducibility. In order to predict emission in an alternative test method, a Huygens's principle based field prediction method is presented in the paper. This method only requires a set of near-field measurements along a line above the metal table of the CISPR 25-setup. The uncertainties of the predicted fields at the measurement antenna location is reduced, because the near-field measurements dramatically minimize the influences from nearby objects. The higher field strength close to the cable also improve the signal to noise ratio with respect to external noises such that the method can be applied without the use of a shielded chamber. The proposed method defines an open Huygens's surface located in front of the cable harness and the equipment under test (EUT). First, the near-field measurements are conducted using an electrically-short monopole field probe, and second, the field distribution along the whole Huygens's surface can be found using the measured data and applying an appropriate extrapolation function over the Huygens's surface. The field distribution on this surface is finally used to calculate the electric field at the antenna location.