Microwave Sensor Based on Transmission Zeros for Material Characterization

In this work, a two-port microwave sensor is presented to characterize different dielectric materials. Most of the resonance-based microwave sensors in the state-of-the-art characterize the materials under test (MUTs) by measuring the relative resonance frequency shift from the unloaded resonance frequency. The resonance frequency of a microwave sensor depends on the substrate’s permittivity. The temperature and humidity of the environment can influence the substrate properties. Hence, in an environment where temperature and humidity frequently change, each time, with the measurement of the loaded resonance frequency, the unloaded resonance frequency is also required to be measured for calculating the exact relative resonance frequency shift, leading toward significant sensor power and time-consuming operations for real-time measurements. We propose a sensor that does not need relative resonance frequency measurement from the unloaded resonance frequency but utilizes the distance between two transmission zeros (TRZs) in the presence of an MUT for material characterization. The design method of the sensor showing increasing frequency distance between the TRZs in the presence of different MUTs with an ascending order of permittivity is also discussed. The measured results show that the technique can be an efficient alternative to the traditional characterization of materials under varying atmospheric conditions.