Millimetre wave permittivity of water near 25 °C

Water is an important substance in every aspect of life. However, many physical properties and processes of water are not well understood; as such it has been and will continue to be a focus of extensive investigation. The broad range of applications to science prescribes the need for experimental verification of theoretical behaviour, specifically the permittivity. Unfortunately, due to the strong absorption across the entire microwave and millimetre wave spectrum there are gaps in available permittivity data. Here an attempt is made to fill a portion of the gap in the millimetre wave range using a free-space method that exploits Fabry–Perot resonances to measure the permittivity of water. This eliminates transmission losses, as well as increases the sensitivity of the measurement. Full-band spectra are collected using a millimetre wave vector network analyzer. The data are fitted using a double-Debye relaxation model, where the fit parameters agree well with published values for water at nearby frequencies.