High-Temperature Measurement of the Complex Permittivity of Flat Wave-Transparent Materials

Accurate high-temperature characterization of the complex permittivity of planar wave-transparent materials is essential for electromagnetic design and performance evaluation. To address measurement-system variations and unstable complex-permittivity retrieval for low-loss plates, a non-contact platform comprising a vector network analyzer, point-focusing lens antennas, and a microwave-transparent insulated chamber was developed. TRL calibration and air-reference normalization reduced measurement-chain and reference-path effects. Complex permittivity was retrieved using PCHIP node parameterization and IPSO-DE hybrid optimization, with the effective band determined from measured S₂₁ data quality. Quartz-fiber-reinforced ceramic and 99% alumina plates were tested from 25 to 1000°C over 8.6–10.9GHz. The former varied little with temperature, whereas alumina real permittivity and loss tangent increased. Within the effective band, alumina real permittivity differed from third-party results by less than 1.1%, while its loss tangent showed the same order of magnitude and temperature trend. Reconstructed S₂₁ agreed well with measurements. Sensitivity analysis showed lower error sensitivity for real permittivity, while loss tangent was sensitive to incidence angle and residual measurement-chain errors. The proposed method is feasible for high-temperature characterization of non-magnetic planar wave-transparent materials.