Millimeter-Wave Material Characterization Using Laminated Waveguides

This paper presents a material characterization method in the millimeter-wave (mm-wave) range. Laminated waveguides (LWGs), a “quasi” planar form of the metallic rectangular waveguide, were chosen as transmission lines for the test structures. Using transmission lines with a metallically surrounded cross section, the difficulty of calculating the relative permittivity ε r from the measured effective relative permittivity εr,eff (which arises in microstrip-line (MSL)-based methods) can be avoided. Employing transmission lines with different cross-sectional geometries for the characterization, separation of the dielectric loss from the conductor loss is possible, the parameters such as surface impedance and roughness factor of the conductor need not to be determined in advance. Compared to choosing strip-lines, using LWGs is more accurate and less sensitive to fabrication tolerances. LWGs of different lengths and thicknesses were designed for the characterization. For S-parameter measurements based on air-coplanar probes, three MSL-to-LWG transitions were designed to guide the signal into the LWGs. The test structures were realized in a low-temperature co-fired ceramics (LTCC) technology. After co-firing, the LTCC materials were characterized at a working frequency of 79 GHz. The results show that the conductor and the dielectric losses have been separated and the accuracy of the derived loss tangent has been well improved. This method can also be applied to organic materials and offers opportunities for further research on the roughness factor of conductors in the mm-wave range.