Capacitively-Loaded Slow-Wave Transmission Lines for Sensitivity Improvement in Phase-Variation Permittivity Sensors

This paper proposes a strategy to implement permittivity sensors exhibiting high sensitivity. The considered structures belong to the category of microwave sensors based on phase variation. In such devices, the dielectric constant of the material under test (MUT) is inferred from the phase variation experienced, e.g., by a transmission line, when the MUT is in contact to the sensitive region of the structure (typically a meandered line section). The proposed approach exploits phase variation experienced by a capacitively-loaded microstrip line. Due to the presence of shunt capacitors at periodic positions, such artificial lines exhibit a slow-wave effect, and thereby a small phase velocity as compared to the one of ordinary lines. By virtue of this small phase velocity, the phase (or electrical length) of the proposed structures is very sensitive to variations in the dielectric constant of the surrounding medium. Consequently, these capacitively-loaded slow-wave transmission lines are very useful for the implementation of highly sensitive permittivity sensors. To demonstrate the potential of the approach, a prototype sensor is designed and fabricated. The measured sensitivity is superior to the one of the sensor based on an ordinary (meandered) line occupying the same sensing area.