One-Port Tapered Rectangular Waveguide Microwave Permittivity Sensor

This work presents a novel one-port microwave permittivity sensor. The sensor is formed by a non-uniform longitudinal section rectangular waveguide (RW) and makes use of the group delay of the reflected signal at its input as sensing magnitude. With the proposed sensor it is possible to measure the relative permittivity of low-loss dielectrics filling the waveguide. The theoretical background explaining the behavior of the sensor is given, along with expressions for the sensor sensitivity as a function of its geometrical dimensions. Guidelines are provided for designing the proposed sensor structure depending on the targeted range of material permittivities and the desired sensing frequency bandwidth. In order to verify the correct operation of the suggested sensor scheme, a proof-of-concept tapered RW (TRW) one-port sensor has been fabricated to measure relative permittivities between 3.5 and 6.5, for a frequency span from 4.8 GHz to 5.8 GHz. The implementation includes a microstrip to TRW transition for the determination of the dielectric permittivity of different low-loss materials under test.