  This paper introduces a novel approach to experimentally characterize
effective human skin permittivity at sub-Terahertz (sub-THz) frequencies,
specifically from $140$~to $210$~GHz, utilizing a quasi-optical measurement
system. To ensure accurate measurement of the reflection coefficients of human
skin, a planar, rigid, and thick reference plate with a low-loss dielectric is
utilized to flatten the human skin surface. A permittivity characterization
method is proposed to reduce permittivity estimation deviations resulting from
the pressure effects on the phase displacements of skins under the measurements
but also to ensure repeatability of the measurement. In practical permittivity
characterizations, the complex permittivities of the finger, palm, and arm of
seven volunteers show small standard deviations for the repeated measurements,
respectively, while those show significant variations across different regions
of the skins and for different persons. The proposed measurement system holds
significant potential for future skin permittivity estimation in sub-THz bands,
facilitating further studies on human-electromagnetic-wave interactions based
on the measured permittivity values.
