In this thesis it is shown, that complex permeability (CP) in the form of complex initial permeability (CIP) as intrinsic property (InP) of polycrystalline ferrites (PF) is possible to model basing on effects following from polycrystal grain size distribution. In order to prove this there were experimentally studied a large amount of magnetic spectra of different ferrite sample groups. The studied groups of samples were selected both from available industrially made ones and from specially made samples (so to get similar microstructure within the group of samples). In its turn the study of CIP as extrinsic property (ExP) is performed experimentally. In particular it shows, that dimensional resonance depends on cross section L, number of turns N of its winding and nonmagnetic gap width. The obtained CIP as InP is viewed as the reference dependence allowing for better insight in ExP phenomena. Also, there was developed, and implemented in MATLAB®, an accurate and effective method for verification of quality of incomplete, noisy or rather doubtful (low quality) spectra by means of Kramers-Kronig relations.