Grounded Coplanar Waveguides for Material Characterization

When designing microwave frequency sensors mounted on human tissue, electrically accurate models of tissue, i.e. phantoms, are necessary. For applications in measuring internal body temperature, material parameters at 1-1.5 GHz are of particular interest (P. Momenroodaki et al., “Noninvasive Internal Body Temperature Tracking with Near-Field Microwave Radiometry,” IEEE Transactions on Microwave Theory and Techniques, vol. 66, no. 5, pp. 2535–2545, Dec. 2017). Previous research suggests the use of transmission lines to extract the effective permittivity from S-parameters (W. B. Weir, “Automatic measurement of complex dielectric constant and permeability at microwave frequencies,” Proceedings of the IEEE, vol. 62, no. 1, pp. 33–36, Jan. 1974). A grounded coplanar waveguide (GCPW) was chosen for the transmission line due to ease of construction and small form factor. From the measured effective permittivity, the filling factors are necessary to obtain the relative permittivity of the material under test. These filling factors are found using a conformal mapping (J. Hu et al., “A grounded coplanar waveguide technique for microwave measurement of complex permittivity and permeability,” IEEE Transactions on Magnetics, vol. 42, no. 7, pp. 1929–1931, Jul. 2006).