We describe a variable path length waveguide setup developed to accurately measure the complex dielectric permittivity of liquids. This is achieved by measuring the complex scattering parameter of the liquid in a waveguide section with a vector network analyzer in combination with an E-band frequency converter. The automated measurement procedure allows fast acquisition at closely spaced intervals over the entire measurement bandwidth: 60-90 GHz. The presented technique is an absolute method and as such is not prone to calibration errors. The technique is suited to investigate low-loss as well as high-loss liquids in contrast to similar setups described previously. We present measurements for a high-loss liquid (water), an intermediate-loss sample (ethanol), and for nearly loss-less n-octane. Due to the available phase information, the present data have an improved accuracy in comparison with literature data.

Rev. Sci. Instrum.
Ultrafast Spectroscopy

Hunger, J, Cerjak, J, Schoenmaker, H, Bonn, M, & Bakker, H.J. (2011). Precision waveguide system for measurement of complex permittivity of liquids at frequencies from 60 to 90 GHz. Rev. Sci. Instrum., 82(10). doi:10.1063/1.3647982