The dielectric properties of coplanar propagation waveguides (CPW) designed and fabricated on high resistivity silicon substrates are investigated. These CPWs exhibited transmission losses around 0.4 dB/mm at 50 GHz. In particular, temperature-dependent measurements show very low deviations. The complex relative permittivity and loss angle tangents are extracted through adequate TEM analytical modeling. It is shown that the results are affected by parasitic peaks that are intrinsically linked with the extraction of the characteristic impedance. To overcome this issue, a numerical resolution is proposed, based on a modelling of the transmission parameter S21 associated with the implementation of a bidimensionnal Newton-Raphson algorithm for the resolution of the inverse problem. Both methods give nearly identical results in terms of dielectric properties, confirming the validity and complementarity of the analytical and numerical models.