An analytical calculation of the permeability frequency spectra using Landau–Lifshitz theory is presented and compared to experimental results obtained on soft amorphous ferromagnetic thin films with an in-plane uniaxial magnetic anisotropy. The disagreement between theory and experiment is attributed to a small magnetic anisotropy dispersion in the film (magnetization ripple). We have measured the real part of the transverse biased permeability μt′ at different frequencies of the exciting field in order to estimate the ripple effects in our films and to improve our model. We have plotted the value of the bias field Hm, at which μt′ versus exciting field frequency reaches a maximum. Hm is found to be frequency independent at low frequencies (up to 280 MHz). Above a critical frequency it increases rapidly, suggesting that ripple effects are strongly frequency dependent.