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The existing CAD formulae are complicated and inaccurate at high frequencies. In particular, no closed-form expression has been obtained for the effective dielectric constant of microstrip lines on multi-layed dielectric substrate by considering the line-substrate interphase effect, although attempts have been made to study the finite thickness effect of the conductor and dielectric substrate. The present work represents the first attempt to obtain a closed-form CAD formula for the effective dielectric constant of microstrip lines on dielectric substrates by considering the effect of conductor-substrate interphase, in addition to the skin effect losses, dielectric loss, dispersion and radiation losses based on the quasi-TEM assumption and the superposition of partial capacitance. The model is verified by experimental observations on the effective dielectric constant and its dependence on microstrip dimension and frequency.