I. Introduction
Water is one of the most abundant molecules on Earth with great significance. Its relative complex permittivity helps us to understand the microwave propagation in water or water-carrying media; thus, needs to be characterized as a function of frequency and temperature. Previous studies, often decades-old, have reported measurements of the static permittivity of water versus temperature [1], complex permittivity measurements across a range of microwave frequencies at a fixed temperature [2], and indirect measurements of the water permittivity versus frequency and temperature with non- uniform frequency and temperature intervals and resolutions [3]. On the other hand, a complete set of water permittivity measurements with the state-of-the-art equipment is missing in the literature. To fill this gap, this paper presents the relative complex permittivity of distilled water at frequencies up to 50 GHz and temperatures up to 50 °C. A revised analytical permittivity model is also given, and the differences between the current and the new model are discussed in the context of important practical applications.