Broadband Measurement of Permittivity and Permeability for Composite Materials Using the Line-Multinetwork Self-Calibration Technique

This article proposes a free-space measurement system designed to improve the precision of the complex permittivity and permeability measurements over a broad frequency band for materials under test (MUTs) using the line-multinetwork (LmN) self-calibration technique. The system employs a pair of double-ridged horn antennas and lenses. Based on the Gaussian beam theory, the MUT is accurately placed at the output waist of the beam, thereby ensuring that the measured area is subjected to plane-wave incidence. The scattering parameters of the MUT are evaluated directly using the LmN self-calibration technique, which relies on measurements of the MUT at multiple distinct positions. The LmN calibration technique effectively mitigates positional errors by deriving an optimal parameter estimate through a weighted average, thereby enhancing measurement accuracy and reliability. The complex permittivity and permeability of the MUT are then derived from the scattering parameters using the Nicolson-Ross-Weir (NRW) algorithm. The measurement results show that the average complex permittivity of the polytetrafluoroethylene (PTFE) is 2.0528 (real part) and 0.0075 (imaginary part). When the same PTFE sample is measured five times, the standard deviations of the real part and imaginary parts of the complex permittivity for five measurements of the PTFE are 0.0075 and 0.0045, respectively. These experimental results, the accuracy and repeatability of the proposed method are verified.