Coupled-Line Directional Coupler Permittivity Sensors

Coupled-line directional couplers have played a significant role in microwave systems since their introduction by Oliver [1]. This chapter presents a new application of this component: dielectric constant sensing. To this end, the material under test is placed on the top of a microstrip coupled-line section, and by analyzing the coupler’s response, the dielectric constant of the sample is detected. The response can be either the coupling or isolation levels of the coupler. This sensing approach benefits from high sensitivity, particularly when the isolation level is used. In the following, we will first review the theory of coupled lines and the basics of directional couplers, then present the idea of using a coupler for dielectric constant sensing.