Use of a directional coupler in RF/microwave measurements: which quantity (or quantities) can it determine on a transmission line?

Introduction:
A directional coupler samples forward and reverse traveling waves on a transmission line. From the sampled amplitudes, mismatch metrics such as reflection coefficient and VSWR can be derived.

Given Data / Assumptions:

• Coupler provides isolated forward and reverse ports.
• Line is driven at a known frequency with steady conditions.
• Calibration accounts for coupling factor and directivity.

Concept / Approach:

Let Vf be forward and Vr be reverse wave voltages. The magnitude of reflection coefficient is |Γ| = |Vr / Vf| at the reference plane. VSWR is then VSWR = (1 + |Γ|) / (1 − |Γ|). With proper calibration, the coupler thus yields both |Γ| and VSWR; phase requires a vector measurement or adjustable line.

Step-by-Step Solution:

Verification / Alternative check:

Slotted lines and reflectometers implement the same principle; vector network analyzers generalize it with phase.

Why Other Options Are Wrong:

• L/C and wave velocity require additional fixtures/time-of-flight methods; a coupler alone does not directly yield them.
• Source resistance alone is not directly measured without further setup and modeling.

Common Pitfalls:

Ignoring coupler directivity; poor directivity corrupts reverse-power readings and thus |Γ|/VSWR.

Final Answer:

Reflection coefficient and VSWR