How do directional couplers and hybrids work?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

A directional coupler samples a wave traveling in one direction. It is characterized by coupling C = −20·log10|S₃₁|, isolation I = −20·log10|S₄₁| and directivity D = I − C. A hybrid is a 3 dB coupler whose outputs are 90° (quadrature) or 180° apart.^[1]

What coupler types are common?

Coupled-line: two parallel lines a quarter wavelength long; weak to moderate coupling. Microstrip directivity suffers from unequal even/odd mode velocities.
Lange: interdigitated coupled lines achieving tight 3 dB coupling over an octave.
Branch-line (quadrature) hybrid: four λ/4 lines, two of Z₀/√2 and two of Z₀; outputs 90° apart, about 10–20% bandwidth.
Rat-race (ring) hybrid: a 1.5λ ring of √2·Z₀; sum and difference ports with 0° and 180° outputs.

^[1]

Where are couplers used?

Power monitoring and leveling loops, VNA test sets (directivity sets raw measurement accuracy), balanced amplifiers (two identical amplifiers between quadrature hybrids give good input/output match), image-reject mixers, I/Q generation and antenna feed networks.^[2]

Why does directivity matter in measurement?

Finite directivity lets some forward wave leak into the reflected sample, setting a floor on how small a reflection can be measured uncorrected. VNA calibration removes most of this error term. See VNA calibration.

Frequently asked questions

What is a 20 dB coupler?

One that couples 1% of forward power to its coupled port.

Why use a balanced amplifier?

Reflections from the two amplifiers cancel at the input hybrid's port, giving good match even if each amplifier is mismatched.

Is a coupler reciprocal?

Passive couplers are reciprocal.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
“Understanding the Fundamental Principles of Vector Network Analysis (Application Note 1287-1)”. Keysight Technologies. application note.

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