> I don't quite follow. I think you just mean reactance at higher frequencies (since resistance is DC), and total impedance is then DCR + frequency-dependent re

> I don't quite follow. I think you just mean reactance at higher frequencies (since resistance is DC), and total impedance is then DCR + frequency-dependent reactance. No, he's correct. Ferrites are a specifically-designed lossy inductor over a targeted frequency range. There is more than just DC resistance + a linearly-increasing reactance with frequency. Just like dielectrics exhibit loss and can be characterized by complex permittivity and a loss tangent, magnetic materials are also lossy and have a complex permeability. Here is a ferrite I used on the data lines for a GPS module: https://www.murata.com/en-eu/products/productdetail?partno=BLM15HD102SN1%23 https://www.murata.com/en-eu/products/productdetail?partno=B... Look at the impedance chart at the end and you'll see the resistive component is basically zero at DC and starts to increase in the HF range, peaking close to 1 GHz. The reactance of it is also highly nonlinear.