Polymer Materials Characterization Using a Novel C-Shaped Dual-Band Transmission Based Differential Sensing

Various environmental effects, including temperature-induced deterioration in polymers, impact their performance in various applications. This paper presents a novel planar microwave sensor that is sensitive to complex permittivity changes in polymers, which can be correlated with temperature-induced material degradation. The design is based on three principles: sensitive sensor units, multiband operation, and differential sensing.A highly sensitive sensor unit design is developed based on a C-shaped resonator with N floating fingers, allowing for the tracking of minute permittivity changes. The number, size, and location of these fingers increase the design's degrees of freedom and enhance sensor sensitivity. By combining multiple units with different resonance frequencies, enhanced accuracy and sensitivity are achieved. Differential operation is conducted by designing mirror parts of the sensor to compare the sample under test (SUT) and the reference sample (RS). Simulation analysis of this sensor design, using dual-band operation (at 3.84 and 4.48 GHz), is tested on multiple polymer samples representing various temperature loading history profiles. The overall size of this sensor is 100 × 50 mm
²
. An appropriate level of sensitivity for each resonance frequency is achieved, with frequency shifts typically exceeding 300 MHz per unit variation in εr (relative permittivity) in terms of absolute sensitivity.The equations required for determining the permittivity of the material using the resonance frequency are obtained through various runs using time-domain electromagnetic models. These models are developed and used to characterize the effect of material property variation on the frequency shift of the two resonance frequencies. The results are used to build a sensor model that predicts temperature effects on material properties based on the frequency shift of the resonance frequency. The obtained results reveal the advantages of the proposed sensor in terms of simplicity of use and fabrication, as well as enhanced sensitivity and accuracy. This makes the sensor appropriate for large polymer-based structures exposed to outdoor temperature deterioration, such as long pipelines.