Development of Complex Relative Permittivity Measurement System Based on Free-Space in 220-330-GHz Range

For measuring complex relative permittivity of more than 300 GHz in the THz region, time-domain spectroscopy (TDS) is usually used. On the other hand, a free-space method using a vector network analyzer (VNA) is used below 300 GHz. However, these methods have not been compared and continuity of complex relative permittivity measurement around 300 GHz has not been evaluated. We developed a system for measuring complex relative permittivity that can be operated at 220-330 GHz. This system is based on the free-space method. We compared the complex relative permittivity using a VNA and by TDS to evaluate our system and the continuity of complex relative permittivity measurement in the frequency domain. We first compared relative permittivity and dielectric loss between both methods. We then evaluated the measurement uncertainty in consideration of the thickness of materials under test (MUT), time span of the time-domain gating, linearity, stability, aperture alignment, and measurement repeatability. The dispersion in MUT thickness measurement was found to be the dominant source of uncertainty in measuring complex relative permittivity measurement. The maximum difference in relative permittivity and dielectric loss between both methods was less than 0.22 and 0.17 when MUT E was measured. For the measurement result with expanded uncertainty, the relative permittivity of VNA was 3.92±0.44 and that of TDS was 3.70±0.16, the dielectric loss of VNA was 0.30±0.52 and that of TDS was 0.13±0.02, respectively. The measured complex relative permittivity by using the VNA and by TDS were observed within the expanded uncertainty. We verified the availability of the measured complex relative permittivity by using our measurement system and the continuity of complex relative permittivity measurement at 300-GHz band.