A Four Port Structure for Comparative Measurement of Complex Permittivity and Moisture

A novel four port structure for complex permittivity measurement is presented, consisting of the cascade of two quadrature hybrids connected through two transmission paths. The material under test (MUT) is coupled to one transmission path, while the other is coupled to a known reference material sample (REF). The complex dielectric permittivity of the MUT is evaluated by simply measuring the amplitudes of the two outputs when air is chosen as REF, thus obtaining a simple and cheap measurement procedure. The structure can be adopted for comparative permittivity measurement, thus allowing one to obtain a material quality parameter as relative moisture estimation. A prototype has been fabricated and experimentally tested exhibiting very good accuracy and sensitivity