Calculation of scattering parameters in multiple-interface transmission-line transducers

Transmission-line scattering parameters becomes unpractical to calculate as one includes more interfaces. Thus, to get an analytical expression, approximations are required. In this paper, an approximation methodology for the calculation of scattering parameters of transmission-line transducers is presented. The transducer cell is partitioned at each interface and the partial scattering equations are calculated, considering two interfaces at a time. Next, standard techniques are applied to solve the signal-flow diagrams to obtain the full scattering equations. Transmission-line transducers are used for the measurement of power absorption and reflection of different materials, such as: liquids, granular medium, and ground, in the RF/microwave range. Such measurements are used to extract materials properties. The proposed methodology has been applied to a coaxial transducer cell filled with different low-loss liquids. The results have been validated with computer simulations and experimental measurements. Measurements and simulations were carried out in the 300 kHz to 3 GHz frequency range.