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Dielectric permittivity of porous Si (PSi) layers formed on a low-resistivity p-type Si (0.001-0.005 Ω.cm) is thoroughly investigated using analytical expressions within the frame of broadband transmission line characterization method in the frequency range 1-40 GHz. It is demonstrated that the value of Si resistivity is critical for the resulting PSi layer permittivity even within the above limited resistivity range. The real part of PSi dielectric permittivity changes monotonically between 1.8 and 4 by changing the Si resistivity between 0.001 and 0.005 Ω.cm. The above study was made for porosities between 70% and 84%. The quality factor and attenuation loss of the investigated coplanar waveguide transmission lines were found to be Q=26 and a=0.19 dB/mm, respectively, at 40 GHz. These values are competitive to those obtained on quartz, which is one of the off-chip RF substrates with the lowest losses. This confirms the superiority of the PSi material, mentioned above, for use as a local substrate for the on-chip RF device integration.