1. A method of identifying complex permittivity of transmission line dielectric by executing computer-executable instructions stored on a nontransitory computer-readable medium, the method comprises the steps of: measuring scattering parameters (S-parameters) for at least two transmission line segments of different length and substantially identical cross-section filled with the investigated dielectric; determining non-reflective, generalized modal scattering parameters of the said transmission line segment difference based on the measured S-parameters of two transmission line segments; and computing generalized modal scattering parameters of the line difference segment by solving Maxwell's equations for geometry of the line cross-section with a given frequency-dependent complex permittivity dielectric model; wherein for the said generalized s-parameter model uses a given frequency-dependent complex permittivity dielectric model and guess values of the model parameters.