Directional coupler performance terminology – What two parameters are standard? In microwave engineering, which pair of terms are universally used to characterize a directional coupler's performance?

Introduction:
Directional couplers are four-port passive devices that sample a defined fraction of power traveling in one direction on a transmission line or waveguide. The two most widely cited figures of merit are how much power is sampled and how well the device discriminates between forward and reverse waves.

Given Data / Assumptions:

• Lossless/low-loss, reciprocal passive directional coupler.
• Ports: input, through, coupled, and isolated.
• Single-frequency or narrowband characterization.

Concept / Approach:

Coupling (in dB) quantifies the intended fraction of forward power appearing at the coupled port, typically defined as 10 * log10(P_in / P_coupled). Directivity (in dB) measures the device’s ability to distinguish direction, defined as the ratio of power coupled from the forward wave to that from the reverse wave at the same coupled port. High directivity means minimal contamination from reflections, enabling accurate VSWR and return-loss measurements.

Step-by-Step Solution:

Verification / Alternative check:

Datasheets specify “Coupling: 10 dB, 20 dB, …” and “Directivity: ≥ 25 dB, ≥ 30 dB,” confirming the standard terminology.

Why Other Options Are Wrong:

“Gain” applies to active devices, not passive couplers. Isolation is related but is not paired as the defining duo in specifications; noise figure is irrelevant for a passive component in this context.

Common Pitfalls:

Confusing directivity with isolation; directivity compares forward vs reverse coupling at the same port, whereas isolation is port-to-port leakage.

Final Answer:

coupling and directivity.