Simple and High-Sensitivity Dielectric Constant Measurement Using a High-Directivity Microstrip Coupled-Line Directional Coupler

Simple methods using a microstrip coupled-line directional coupler (CLDC) are presented for dielectric constant measurements. The material under test (MUT) is placed on the coupled-line section of the coupler, and either the coupler’s coupling (<inline-formula> <tex-math notation="LaTeX">$\lvert S_{31} \lvert $ </tex-math></inline-formula>) or its isolation level (<inline-formula> <tex-math notation="LaTeX">$\lvert S_{41} \lvert $ </tex-math></inline-formula>) is considered as the sensor’s response. Putting different MUTs on the microstrip line leads to a change in the effective dielectric constant of the structure and consequently causing a change in the coupling coefficient. In addition, since the isolation level of a microstrip coupled-line coupler depends on the phase velocity difference between the substrate and the medium above the signal strips, putting different MUTs on the line significantly changes the isolation level. This change is significantly greater than the change in <inline-formula> <tex-math notation="LaTeX">$\lvert S_{21} \lvert $ </tex-math></inline-formula> level of a microstrip line when loaded with different MUTs. Validation of the method is presented through measurements for both solid and liquid MUTs.