A novel technique for permittivity and moisture measurements using a planar hybrid junction

A novel technique for permittivity and moisture measurements overcoming the main limitations of conventional methods is introduced. The technique employs a simple measuring device consisting of a planar hybrid junction with the direct and coupled ports terminated by two identical open-ended transmission lines, one being loaded with the Material Under Test (MUT). The complex permittivity of the MUT is evaluated by simply measuring the amplitudes of the reflected and the transmitted signals of the measuring device. The proposed technique can be adopted to estimate the relative moisture content of solid samples, a simple linear fitting being sufficient to model the relation between tandelta and the moisture percentage. A prototype has been fabricated in microstrip technology employing a multi branch line coupler as hybrid junction. Measured permittivities at 3.5 GHz have been compared with data from the literature, resulting in a good agreement for both epsiv_r and tandelta.