Microstrip Line loaded with Series Gap and Dumbbell Defect-Ground-Structure (DB-DGS) Resonator for Highly Sensitive Sensing Based on Resonance/Antiresonance : Application to Humidity Measurements

A new planar structure for the implementation of highly sensitive single-frequency microwave sensors is proposed. The key aspect to obtain high sensitivity is to generate closely spaced resonance and antiresonance frequencies, achieved by means of a microstrip line with a series gap etched on top of a transversely oriented dumbbell defect-ground-structure (DB-DGS) resonator, the sensitive element. By this means, both the magnitude and the phase of the transmission coefficient exhibit a high phase slope between such frequencies. The result is a high sensitivity of either the magnitude or the phase of the transmission coefficient at the operating frequency with the dielectric constant of the environment surrounding the DB-DGS resonator. Advantages of the proposed device are size and simplicity, and isolation between the analyte (or material under test) and the line strip. Two sensors with different DB-DGS geometries are reported to validate the proposed approach. Using the phase as the main output variable, the maximum sensitivity in one of the prototypes is found to be 88.7° per unit of dielectric constant variation. This sensor has been applied to relative humidity (RH) measurements by using a functional film of polyvinyl alcohol deposited on top of the sensing DB-DGS resonator. The average sensitivity in this case is 0.33°/% (note that RH is given in %).