Use of a directional coupler in RF/microwave measurements: which quantity (or quantities) can it determine on a transmission line?
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AReflection coefficient only (|Γ|)
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BReflection coefficient and VSWR
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CInductance and capacitance directly (L and C)
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DWave velocity directly without calibration
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ESource internal resistance only
Answer
Correct Answer: Reflection coefficient and VSWR
Explanation
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:
1) Measure coupled forward power Pf and reverse power Pr.2) Correct for coupling factor to estimate |Vf| and |Vr|.3) Compute |Γ| = |Vr / Vf|.4) Compute VSWR = (1 + |Γ|) / (1 − |Γ|).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