New Method for Determining Permittivity of Thin Polymer Sheets

We present a new method for measuring thin, polymer sheets using a slotted rectangular coaxial transmission line (R-Coax). This method allows a sheet of material to be inserted into the R-Coax slot, greatly simplifying the measurement procedure over traditional waveguide methods. The permittivity inversion is performed with the aid of computational simulations of the R-Coax conducted across a range of expected dielectric properties. In particular, the slotted R-Coax device was optimized to enhance signal strength but has no simple analytical solutions for inversion. This new measurement technique is demonstrated on several thicknesses of commercial polyethylene terephthalate (PET) films, with a maximum thickness of 10 mils (0.254 mm). Due to the coaxial geometry, this technique does not have an intrinsic lower frequency cutoff and has an upper frequency cutoff near 3 GHz from over-modeing within the transmission line, though this frequency range could be extended by shrinking the fixture. However, the signal strength and calibration stability limit the useful range of permittivity measurement to 0.5-3 GHz for 10 mil thick specimens (and a range of ~1 GHz-3 GHz for 0.5 mil thick specimens). Repeatability for the real part of the permittivity ranged between 2-5% and loss tangents of ~0.006 were measured. Thus, this paper demonstrates the R-Coax measurement technique as a potential QA tool for microwave frequency electrical properties of thin polymer films.