The paper presents an improved technique for determining complex permittivity using the transmission/reflection (TR) method. The authors address the issue of instability in the commonly used Nicolson-Ross-Weir equations at frequencies corresponding to integer multiples of one-half wavelength in the sample, which is a common problem for low-loss materials. They propose new robust algorithms that eliminate this instability and derive equations that are independent of reference plane position and sample length. The paper includes an error analysis to identify sources of uncertainty in scattering parameters, length measurement, and reference plane position. The authors also discuss the transformation of S-parameter measurements from calibration reference planes to the ends of the sample and present equations that are invariant to reference plane position and sample length. The goal is to provide a stable and accurate method for determining complex permittivity over a broad frequency range, suitable for various applications.The paper presents an improved technique for determining complex permittivity using the transmission/reflection (TR) method. The authors address the issue of instability in the commonly used Nicolson-Ross-Weir equations at frequencies corresponding to integer multiples of one-half wavelength in the sample, which is a common problem for low-loss materials. They propose new robust algorithms that eliminate this instability and derive equations that are independent of reference plane position and sample length. The paper includes an error analysis to identify sources of uncertainty in scattering parameters, length measurement, and reference plane position. The authors also discuss the transformation of S-parameter measurements from calibration reference planes to the ends of the sample and present equations that are invariant to reference plane position and sample length. The goal is to provide a stable and accurate method for determining complex permittivity over a broad frequency range, suitable for various applications.