Development of permittivity measurement system at microwave and millimeter wave frequencies for low-loss substrate characterization

We have developed a permittivity measurement system up to 110 GHz for low-loss substrates by using a balanced-type circular-disk resonator method. By an electromagnetic excitation in the center from 1 mm-coaxial lines, only the TM0m0 modes are selectively excited, so broadband measurements up to 110 GHz can be performed with a single resonator by utilizing higher order modes. Our system have achieved a good measurement repeatability by using a sample holding mechanism that ensures necessary alignments in the resonator and by using a torque-controllable clamping device. We also developed an algorithm for the uncertainty evaluation of measurement results by considering the uncertainty propagations of measured S-parameters and dimensions. We demonstrated that our permittivity measurement system can contribute to antenna design in the millimeter wave bands by performing the gain estimation of the LTCC oversized rectangular waveguide slot array antenna at 60 GHz.