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A new time domain method for effective calculation of transient electromagnetic fields in a shielded quasi TEM transmission line is proposed. The method is based on presenting the fields in time domain as expansions over some coupled frequency in-dependent modes. It was shown that just several coupled modes are enough to account for in order to calculate a band limited signal propagation in the structure with high accuracy. The method is demonstrated on example of impulse wave excitation and propagation in a shielded microstrip line. The mode functions are calculated using finite element method for the elliptic type boundary eigenvalue problem, while hyperbolic evolutionary equation for the mode amplitudes is solved using method of finite differences in time domain. The conditions of distortionless propagation in a quasi-TEM line are found and explained.