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

A novel technique for the measurement of the complex permittivity of materials is presented that overcomes many limitations of the conventional measurement methods. The RF signal transmitted through a transmission line loaded with the material under test is combined with a reference signal using a quadrature hybrid in such a way that the complex permittivity can be measured by simply detecting the two output amplitudes. This technique requires a simple calibration procedure, provides good accuracy, and avoids expensive vector measurements, thus combining the advantages of transmission methods, in terms of good accuracy, with those of resonant methods, in terms of scalar measurements. Two microstrip implementations have been realized for measurements at 2.35 and 10 GHz, showing very good accuracy to be achieved in both frequency ranges. The measured permittivities have been compared with those obtained with a resonant and transmission method and with data from literature, resulting in a very good agreement for both epsilon(r) and tan delta.