We present a method for extracting the magnetic parameters of a soft magnetic Co-based alloy (Co$_68.25$Fe$_4.5$Si$_12.25$B$_15$) in the high-frequency regime (100 MHz–2.5 GHz). The method uses the magnetic sample (as-cast ribbon) as the conductor of a terminated microstrip transmission line. It uses the complex propagation constant of the line, obtained from open/short circuit input impedance measurements, to extract the per-unit-length series impedance. It then uses a theoretical model, based on the distributed parameters and the skin effect, to extract the bulk transverse relative permeability. The model considers the geometrical configuration of the magnetic conductor (ribbon) when determining the series impedance as well as the complex transverse relative permeability$(mu_ tr= mu'_ tr- j mu''_ tr)$. We report on the effect of applying and varying an external dc magnetic field.