A Feasibility Examination Using Microwave Stripline Resonators for Low-Temperature Sensing
Temperature monitoring is essential in numerous applications, from material storage and process control to safety systems. Conventional sensors, like optical temperature sensors, often struggle in sub-zero conditions due to frost and other environmental interference. Microwave sensors offer a robust alternative, exploiting the temperature-dependent properties of dielectric materials.
The study demonstrates a microwave-based approach to low-temperature sensing feasible for temperatures as low as -70 °C (-94 °F) using inexpensive components and simple fabrication techniques. Stripline is a planar transmission line (TL) technology that symmetrically encases a resonant circuit within a homogeneous dielectric backed with ground planes. The circuit, made of patterned copper traces, is designed to resonate within the 902-928 MHz ISM band by carefully choosing resonator geometries and dielectric substrate. The dielectric substrate is strategically selected as one with a temperature-sensitive dielectric constant. Since the electric field is confined within the stripline and the dielectric, temperature changes affect the dielectric constant, subsequently altering the sensor's resonant frequency.
A chassis is designed and fabricated to protect the stripline from anticipated environmental disturbances such as frost and moisture that will otherwise disrupt temperature measurement. The bottom dielectric layer is placed in the chassis base, followed by the stripline connector, ensuring pin contact with the TL. The upper dielectric layer and chassis top are then added and secured with M2 screws. A connector is connected to the stripline to interface with the readout electronics, monitoring the sensor's resonant frequency changes due to temperature variations.
The sensor system was calibrated using a CM ACM2520 autocalibration module over the 900–955 MHz range. The sensor's frequency response is recorded every 30 seconds using a LabVIEW program, enabling a direct comparison with concurrent thermocouple temperature readings.
Starting with thermal cycling, sensor resonance is shown to change between cycles, reaching consistency in resonant frequency after 11 cycles. This stage is considered an important fabrication step to precondition the sensor and reduce measurement error.
The final experiment consisted of 32 descending steps varying from 10.0 °C to −70.0 °C. A detailed sensor characterization is performed, with long-duration stability measurements, where the temperature is set and the resonant frequency is observed. Averaging the resonant frequency and temperature measurements produces a linear dataset with a sensitivity of S = 199 kHz/°C, closely aligning with the combined dataset's sensitivity (S = 198 ± 1 kHz/°C) derived from previous independent repeatability experiments. Step sizes as small as 1.0 °C are clearly distinguishable, demonstrating the potential for even higher sensing resolution.
The results show that stripline-based microwave sensing using commercial dielectrics is a viable option for low-temperature applications. This method provides a reliable alternative to sensors susceptible to environmental interference and is also cost-effective.
The techniques used in this work may be extended to other temperature ranges by studying different materials, such as the dielectric substrate and metallic resonant layer, to be used in the stripline circuit. Chassis optimization in dimensions and material may offer improvements in sensing response time.