Magnetic Field Shielding-kHz
Many times I see people saying and writing that a ferrite absorbs the magnetic field. While this is true in the MHz range where the energy of the electromagnetic field is converted to heat (e.g. absorbed), in the kHz range there is no significant absorption happening!
In this example a 7cm current loop is driven with a 1 Watt signal. At the center of the loop is a ferrite cube of 2 cm edge length. The ferrite is a typical soft ferrite modeled with dispersive properties: static µ_r = 1000, and a peak in µ’’ at 10 MHz. The losses have been scaled to the power accepted by the loop. The losses in the magnetic material start to become visible only above 1 MHz. Below 1 MHz all losses are ohmic!
So how can a ferrite shield magnetic fields in the kHz range? A ferrite has the effect to change the distribution of the magnetic field: At the sides of the cube the magnetic field is reduced: This is what we call shielding. However, above the cube the field is increased: The ferrite did not absorb, it changed the distribution.
So if you have a ferrite wall it will shield your kHz magnetic field, but it will not absorb it.
