CHARACTERIZATION of electromagnetic interference (EMI) generated withinpower electronic systems is an important activity in electromagneticcompatibility (EMC). However, EMC behaviors are generally treated after thedevelopment of a prototype; this leads to additional costs and significantdelays in manufacturing in case of malfunctioning due to EMI. In order tostudy the problems of EMC directly at the design phase, EMC predictivemodeling methods must be developed.One of the important characteristics to predict is the near magnetic fieldcoupling. A new method has been developed in our laboratory based on themultipolar expansion of the radiation near field around the element. Thisallows to calculate the near-field coupling with other radiating elements.The method gives an equivalent source, which is valid only outside a regionwhose shape is defined by the function basis used in the harmonic expansion.In the case of spherical coordinates, the smallest sphere that covers the sourceis called the “sphere of validity”. However, if two devices are too close, theintersection between the spheres of validity is not empty and therefore themodeling by our method becomes invalid.A first approach to solve this problem is to change the geometry of themultipolar expansion. For example, the oblate spheroidal coordinate may beadapted when dealing with a flat source such as an electronic board, contraryto the prolate spheroidal coordinate, which is adapted to an elongated object.There are eleven coordinate systems allowing simple separation of the Laplaceand Helmholtz equations.A second approach is to translate the center of the multipolar expansion. Forexample, to know the near magnetic field in the sphere of validity of a flatsource, the solution is to increase the radius of the sphere and translate itscenter so that the flattened source is close to the surface of the sphere.The aim of our work is to find the more convenient function basis and centerof the multipolar expansion to model the sources in typical cases in powerelectronics. The objective of our method is to use one set of magneticmeasurements all around each source to calculate the multipolar expansion.Thus, the near magnetic field coupling of any set of sources in any positionbecomes possible using only the multipolar expansion. Finally, we comparethe accuracy and the computational speed of the different expansion methods.