A Microwave Stripline Ring Resonator Sensor Exploiting the Thermal Coefficient of Dielectric Constant for High-Temperature Sensing

A stripline transmission-line (TL)-based temperature sensor for use in harsh environments is designed to exploit the thermal coefficient of the dielectric constant (TCD) of the microwave (MW) substrate material. Rogers 3210 substrate is selected owing to its high TCD of -459 ppm/degrees C and a dielectric constant of 10.8. A ring resonator is designed for 2.4 GHz, with the ring selected for the simple geometric dependence of its resonant frequency on radius. TLs are gap-coupled to the ring, with widths designed for 50-Omega impedance matching and lengths optimized for a compact form factor. Upper and lower ground planes shield the dielectric, resonator, and TLs from environmental disturbances, such as debris and moisture, which could otherwise disrupt temperature measurement. Furthermore, the electric field is completely contained within the homogeneous dielectric, providing improved sensitivity compared to similar designs using microstrip technology. The fabricated sensor requires uniform compression of layers to mitigate the effects of air pockets and thermal expansion of materials. It is found that the sensor requires temperature-conditioning, approximately 70 h cycling between 30 degrees C and 80 degrees C, before its resonant frequency reaches a steady state suitable for instantaneous temperature measurement. Subsequent 10 degrees C- and 2 degrees C-step experiments are performed in the 30 degrees C-80 degrees C and 30 degrees C-40 degrees C ranges, respectively. As a result, a linear sensitivity of approximate to 500 kHz/degrees C is identified. The duration of these experiments and time-based data are representative of applications where long-term temperature monitoring is required.