Cylindrical resonator principle can be used in GPR asphalt quality measurement calibration. This method relies on ordinary drill core samples that are regularly taken from measured road sections, but now only analyzed for dimensions, density and sometimes chemically. If such a drill sample is covered with proper conductive surfaces, a cylindrical cavity resonator is formed. The baseline of the GPR permittivity recordings can so be found by measuring the resonance behaviour of this covered sample, which can later still be used for those traditional analyses. A clear benefit is the resonator's 1-2GHz frequency range which equals that of common commercial GPR systems. Example results and reference readings from known dielectric material are shown. The obtained uncertainty in this case study is 0.02 units of permittivity, when measuring the same sample repeatedly.