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Z (Impedance)

The impedance of a bead is the total resistance to the flow of current, including the AC and DC component. The DC component of the impedance is simply the DC resistance of the winding.
The AC component of the impedance includes the inductor reactance and AC loss.
The following formula calculates the inductive reactance of an ideal inductor (i.e., one with no losses) to a sinusoidal AC signal. 
                                                                                 Z = XL = 2πfL
L is in henries and f is in hertz.
This equation indicates that higher impedance levels are achieved by higher inductance values or at higher frequencies.
AC loss of the bead includes the magnetic hysteresis loss, the eddy current loss and dielectric loss.