The relative static permittivity, ε, can be measured for static electric fields as follows: first the capacitance of a test capacitor, C, is measured with vacuum between its plates. Then, using the same capacitor and distance between its plates, the capacitance C with a dielectric between the plates is measured. The relative permittivity can be then calculated as
For time-variant electromagnetic fields, this quantity becomes frequency-dependent. An indirect technique to calculate ε is conversion of radio frequency S-parameter measurement results. A description of frequently used S-parameter conversions for determination of the frequency-dependent ε of dielectrics can be found in this bibliographic source. Alternatively, resonance based effects may be employed at fixed frequencies.