Coaxial probe dielectric measurements in the presence of airgaps

Interpolation formulas for retrieval of the dielectric constant of the substrate from the measured probe impedance within a specified frequency range are introduced. The proposed formulas were tested for retrieval of dielectric constant from impedance measurements for the substrates with known properties. It was found that the accuracy of the proposed interpolation formulas is better than 5% within the frequency range of 0.5-20 GHz.