Influence of resonances on the Huygens´ box method

The Huygens´ box (HB) method of replacing an arbitrary module inside an electronic apparatus with a set of current sources on a closed surface is discussed. A numerical study is performed, with a typical printed circuit board (PCB) representing the module placed inside a tight metallic enclosure. It is observed that the accuracy of the HB method is generally good except at a few resonant frequencies. Even previously proposed correction to the method that consists of including the main features of the module inside HB is not found to be very effective. The traces on the PCB and especially the parallel plate resonator formed by the ground plane of the PCB and the enclosure are identified as causing the resonances. We estimate that it is most likely the narrowband character of the resonances that considerably increases the sensitivity of the HB method toward small differences in the structure.