Perturbations of Electric and Magnetic Fields Due to the Presence of Materials in TEM Cells

This paper investigates in detail the impact of electric (E) and magnetic (H) field distributions inside transverse electromagnetic (TEM) cells, below and above the corresponding dominant modes of the cells, in conditions differing from free space, i.e., in presence of representative ferromagnetic and dielectric materials. Simulations and measurements are performed from hundreds of MHz to GHz using IEC 61967-2 (closed) and open TEM cells. Whatever the frequency and equipment under test location, unlike dielectric material, which only locally changes the norms (||.||) of E (and H depending on permittivity) at its location, the presence of ferromagnetic material inside the cell changes both ||E|| and ||H|| distributions: locally below the dominant mode frequency and globally at the entire bottom of the cell above that frequency. This demonstrates that a local distortion of ||H||-field, due to ferromagnetic material, has a stronger influence than a local distortion of ||E||-field, disregarding frequency, location, and magnetic losses. Moreover, the demand to use a TEM cell below the dominant mode frequency, as mentioned in the IEC 61967-2 and 62132-2 standards, may not be relevant, provided that both electromagnetic field inhomogeneities are considered, and the presence of an integrated circuit package in the cell is accounted for in the equivalent ||E||-field level around the pins in immunity testing.