Millimeter wave device for water content determination based on dielectric waveguide

Summary form only given. The device consists of a dielectric waveguide and a cell with sample under test which adjoins the wall of the waveguide. We present such a device elaborated for water content determination in water solution of ethyl alcohol. In the dielectric waveguide made of high-resistivity silicon (ε=3.42, tanδ=0.001) with cross-section dimensions of 1.4×2.8 mm, the E₁₁^y mode propagates. The frequency range is 26-36 GHz. The broad wall of the waveguide ( parallel to the y-axis) adjoins the cell with a liquid under test. The alcohol concentration W was varied from 0 to 96%. In this interval of W, the real and imaginary parts of the permittivity of the water-alcohol solution ranges from 19.6 to 4.1 and from 29.0 to 0.8, respectively. The losses in the waveguide due to the cell have a maximum in the vicinity of W=50-60% with a sharp decrease at higher values of W and a slow decrease at smaller W. For the length of a polyethylene cell of 25 mm and the wall thickness of 1 mm, the sensitivity of the device for determination of water content in alcohol at W=85-96% was about 0.5-0.7 dB for 1% of water. The losses in a dielectric waveguide associated with the fact that a lossy solution contacts its wall is in a good agreement with the theoretical results for a two-dimensional dielectric waveguide situated on a lossy substrate (with tanδ ranging from 0 to 10¹⁵)