Directional coupler — primary purpose in microwave measurements In RF and microwave engineering, a directional coupler is used for which of the following tasks?

Introduction / Context:
A directional coupler is a four-port passive device that samples a defined fraction of power travelling in a transmission line, with strong directivity so that forward and reverse waves can be observed separately. Engineers rely on it to characterize systems (VSWR, return loss, gain, isolation) without interrupting the main signal path.

Given Data / Assumptions:

• Ideal or well-designed directional coupler with specified coupling (for example, 10 dB, 20 dB) and high directivity.
• Coupler placed inline with a source and load to pick off samples of forward and reflected waves.
• Appropriate detectors or a vector network analyzer to read amplitude and phase.

Concept / Approach:
The coupler provides two auxiliary ports: a “coupled” port that samples the forward-travelling wave and an “isolated” port that ideally rejects it (and vice versa for the reverse direction). By measuring voltages at the sampled ports with phase-sensitive instruments, one can determine the magnitude and phase of travelling waves, enabling calculation of reflection coefficient, VSWR, and S-parameters.

Step-by-Step Solution:

Verification / Alternative check:
With a known calibration standard (open/short/match), readings at the coupler ports replicate expected Γ magnitudes and phases, verifying the device’s intended measurement role.

Why Other Options Are Wrong:

• Transmit microwave signals: transmission occurs via the main line; the coupler is a sampler, not a transmitter.
• Generate microwave signals: generation is the role of oscillators, klystrons, Gunn sources, etc.
• Both (a) and (b): conflates measurement hardware with sources/lines.

Common Pitfalls:
Confusing coupling (fixed fraction) with insertion loss; ignoring directivity limits which affect accuracy of reflected-wave measurement; using the wrong coupling value, leading to either overload or too little signal at the detector.

Final Answer:
to measure amplitude and phase of a travelling wave