A Modified Open-Ended Rectangular Waveguide Based Reflection Approach for Dielectric Constant Characterization of Low-Loss Slab Materials

A reflection approach utilizing an open-ended rectangular waveguide (ORW) for dielectric constant characterization of low-loss slab materials is presented. A calibration technique is proposed to reduce the effects of the higher order mode fields in dominant-mode analysis. In addition, the effects of undesired reflections caused by the finite-sized flange are also reduced in the extraction of the dielectric constant. Moreover, a small perturbation is introduced in the calculation of the reflection coefficient to eliminate the singularities as well. The dielectric constant of material under test (MUT) is extracted utilizing the measured reflection coefficient by assuming that the loss tangent of MUT is zero. Simulation results are presented to validate the calibration process. The selection of suitable airgap thickness and the effects of lossy MUT are also discussed. The measured dielectric constants of MUT made of polymethyl methacrylate (PMMA) and polytetrafluoroethylene (PTFE) are presented as well, which agrees excellently with the results achieved from the free-space method.