What do coupling, directivity, and isolation actually mean?

Coupling is the fraction of input power that appears at the coupled port, expressed in dB (e.g., -20 dB means 1% of power is coupled). Isolation is the fraction of input power that appears at the isolated port, which should be as small as possible (e.g., -50 dB). Directivity is the difference between isolation and coupling: D = Isolation - Coupling. A 20 dB coupler with 50 dB isolation has 30 dB directivity. Directivity determines how accurately you can measure the reflected power: with 30 dB directivity, the minimum measurable return loss is about 25 dB.

Why does directivity matter for VSWR measurement?

When measuring reflected power, the coupler's isolated port ideally sees only the reverse wave. But finite directivity means a fraction of the forward wave leaks through. If the forward leakage is comparable to the actual reflected power, the measurement is meaningless. For a load with 20 dB return loss measured through a coupler with 15 dB directivity, the leakage is 5 dB stronger than the reflection, making the measurement unreliable. You need directivity at least 10 dB greater than the return loss you want to measure.

What coupler topology should I use for my frequency?

Below 1 GHz, use lumped-element directional couplers or ferrite-coupled designs. From 1 to 6 GHz, edge-coupled microstrip couplers work well for 10 to 20 dB coupling, but achieving tight coupling (3 dB) requires interdigitated Lange couplers because edge-coupled microstrip cannot achieve the required even/odd mode impedance ratio. Above 6 GHz through mmWave, use waveguide multi-hole couplers or broadside-coupled stripline. Branch-line couplers provide 3 dB coupling with 90-degree phase difference over about 10% bandwidth, suitable for balanced amplifiers and image-reject mixers.

RF Design

Directional Coupler

Every RF transmitter needs to know how much power is reaching the antenna and how much is bouncing back. A directional coupler answers both questions simultaneously by sampling a small fraction of the forward wave at one port and the reverse wave at another, while passing the main signal through with minimal loss. The key performance metric is directivity: the coupler's ability to distinguish forward from reverse power. A coupler with poor directivity confuses the two, reporting phantom reflections that do not exist.

Category: RF Design

Ports: Input, Through, Coupled, Isolated

Key Spec: Directivity (dB)

Four Ports, Three Numbers

Port definitions (power entering Port 1):
Coupling (C) = −10·log₁₀(P₃/P₁) dB
Isolation (I) = −10·log₁₀(P₄/P₁) dB
Directivity (D) = I − C dB
Insertion Loss = −10·log₁₀(P₂/P₁) dB ≈ 10·log₁₀(1 − 10^−C/10)

Example: 20 dB coupler with 50 dB isolation:
Coupling = 20 dB (1% of power sampled)
Directivity = 50 − 20 = 30 dB
Insertion loss ≈ 0.044 dB (99% of power passes through)
Minimum measurable RL ≈ 30 − 6 = 24 dB

Coupler Topology Selection

Topology	Coupling Range	Bandwidth	Directivity	Frequency Range
Edge-coupled microstrip	10 to 30 dB	Octave+	15 to 25 dB	1 to 20 GHz
Lange (interdigitated)	3 to 6 dB	Octave+	15 to 20 dB	2 to 40 GHz
Branch-line hybrid	3 dB	~10%	20 to 30 dB	0.5 to 30 GHz
Broadside stripline	3 to 20 dB	Decade	25 to 40 dB	0.5 to 18 GHz
Multi-hole waveguide	10 to 40 dB	Full WG band	30 to 50 dB	1 to 110 GHz
Lumped-element	3 to 20 dB	20 to 50%	15 to 25 dB	DC to 1 GHz

Common Questions

Frequently Asked Questions

What do coupling, directivity, and isolation mean?

Coupling: fraction of power at the coupled port (e.g., −20 dB = 1%). Isolation: leakage to the isolated port (e.g., −50 dB). Directivity = Isolation − Coupling (30 dB in this example). Directivity sets the accuracy floor for reflected power measurements.

Why does directivity matter for VSWR?

Finite directivity leaks forward power into the reverse measurement port. You need directivity at least 10 dB greater than the return loss you want to measure. A 15 dB directivity coupler cannot reliably measure a 20 dB return loss.

Which topology for my frequency?

<1 GHz: lumped-element. 1 to 6 GHz: edge-coupled microstrip (10 to 30 dB) or Lange (3 dB). >6 GHz: waveguide multi-hole or broadside stripline. Branch-line for 3 dB with 90° phase (balanced amps, image-reject mixers).

Related Terms

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Passive Design

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