A novel technique for complex permittivity measurement based on a planar four port device

A novel four port device for complex permittivity measurement is presented consisting of the cascade of two 3 dB branch line directional couplers connected through two U-shaped transmission line lengths. The material under test (MUT) is brought into contact with one U-shaped section. The complex dielectric permittivity of the MUT is evaluated by simply measuring the amplitudes of the two outputs, thus obtaining a simple and cheap measurement procedure. A prototype has been fabricated and experimentally tested exhibiting very good accuracy.