How to handle a Huygens' box inside an enclosure

It has been suggested that it is possible to replace printed circuit boards with a Huygens' box (HB) representation obtained from a near-field scan in simulation of far-fields from an apparatus. However, the surface equivalence theorem requires that the environment outside HB is the same in the near-field scan and in the apparatus. This is seldom the case in common type of apparatus. This paper discusses how to handle HB inside typical enclosures. It is demonstrated that if the most important features of the printed circuit board are included inside HB, the introduced error in radiated fields caused by violating the surface equivalence theorem can be lower than 2 dB. It is also demonstrated that if the printed circuit board is galvanically connected to the enclosure, the near-field scan must be performed under same conditions.