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This study demonstrates a novel low-loss dielectric measurement technique that utilizes reflections at the backside of a probe in the millimeterwave frequency range up to 300 GHz. The accuracy of measurement was investigated by evaluating the measurement uncertainty contributors individually. Further, the dielectric dispersion of alumina was calculated from a phonon model. The evaluated dielectric constant and dielectric loss tangent corresponded with values in the accuracy range generated by the phonon model, which was constructed from Fourier transform infrared spectroscopy (FT-IR). These results validated the accuracy of the proposed method in the millimeter-wave frequencies. The measurement technique is expected to be useful for the evaluation of dielectric properties of circuit substrates used in "Beyond 5G" communication technologies.

This paper proposes a breakthrough in-situ dielectric permittivity measurement technique for a circuit substrate with various planar circuits on the substrate. The proposed technique uses a quasi-resonator at the backside of an RF probe, which is contacted at the middle of the feed line of the planar circuits. Dielectric permittivity is calculated using the difference between the resonance frequencies of two measurements with different probe-contact positions. The proposed technique can be used for the feed lines of a matched-transmission line and an attenuator. Therefore, it can be used for the evaluation of circuit substrates with various devices. The proposed technique can realize the in-situ dielectric measurement of circuit substrates, and it is expected to be a powerful tool for the investigation of the defects in practical RF devices because it does not require any resonator for a device under test.