Microstrip Coupled-Line Directional Coupler for High-Sensitivity Dielectric Constant Measurement

We propose a simple approach utilizing microstrip coupled-line directional couplers for the dielectric constant measurement of materials. In the proposed method, material under tests (MUTs) are placed on the coupled lines of a coupler as a sensor, and the coupler's coupling (S-31) and isolation factors (S-41) are considered as the sensor's response. The concept is based on changing the effective dielectric constant of the structure that leads to a change in the coupling coefficient (S-31) by putting different MUT. In addition, since the isolation of a microstrip coupled-line coupler depends on the difference between the phase velocity in the substrate and the medium above the strips, by putting different MUTs on the line, the isolation factor (S-41) changes noticeably. This change is significantly greater than the change in the S-21 of a microstrip line when it is loaded with different MUTs. To prove the concept, a high isolation directional coupler working in low gigahertz was designed and fabricated. By putting different MUTs on the fabricated coupler, the coupling and isolation factors were measured. Good agreement between the full-wave solver and measurement results was observed.