Noniterative complex permittivity retrieval using calibration-independent waveguide measurements

Calibration-independent nonresonant measurements can be employed to eliminate the need for calibration before microwave measurements. These methods generally assume that the location of the sample in its measurement cell is known after the sample is positioned or shifted within the cell and that the length of the measurement cell is known. In addition, in general these methods necessitate some sort of numerical analysis to retrieve the complex permittivity of the sample. In this research paper, we propose a calibration-independent nonresonant waveguide method to first eliminate measurement errors arising from inaccurate knowledge of the location of the sample within its cell (after shifting) and arising from improper knowledge of the length of the measurement cell and second to determine the complex permittivity explicitly. In addition, our proposed method needs scattering parameter measurements from three different measurement configurations – another improvement with respect to a similar calibration-independent method requiring measurements from four different configurations. Furthermore, it does not necessitate that the sample be flushed to the measurement cell. We analyzed repeatability of cell connection and investigated uncertainty in the measurement of complex permittivity for a change in length of the sample and the cell and when air gap present between sample surfaces and guide walls. We validated the proposed method by measuring the complex permittivity of polyethylene sample and distilled water at X-band (8.2–12.4 GHz).