What Is a Directional Coupler?
A directional coupler is a 4-port passive device that samples a portion of RF power travelling in one direction while providing high isolation to the reverse direction. It is essential in power monitoring, signal sampling, VSWR bridges, and balanced amplifier designs.
Port Definitions
| Port | Name | Function |
|---|---|---|
| 1 | Input | Main signal source |
| 2 | Through | Main transmission path |
| 3 | Coupled | Sampled forward power |
| 4 | Isolated | Terminated, samples reverse |
Key Performance Parameters
| Parameter | Formula | Typical Value |
|---|---|---|
| Coupling factor C | −20·log|S31| dB | 6, 10, 20 dB |
| Insertion loss IL | −20·log|S21| dB | 0.1–0.5 dB (+ coupling) |
| Directivity D | −20·log|S41/S31| dB | 20–40 dB |
| Isolation I | −20·log|S41| dB | C + D |
| Return loss | −20·log|S11| dB | >20 dB |
Example: 10 dB coupler with 30 dB directivity → Isolation = 40 dB
Coupling Mechanism Types
- Branchline (90° hybrid): Equal split, 90° phase difference, narrowband
- Lange coupler: Tight coupling (3 dB) using interdigitated lines, broadband
- Microstrip edge-coupled: Loose coupling (10–20 dB), limited directivity on single-layer PCB
- Stripline broadside-coupled: High directivity, multilayer PCB required
- Lumped element: <1 GHz, compact but frequency-dependent
Analyzing a Coupler with RF View
Load the coupler's .s4p file in RF View to inspect all 16 S-parameters. Key checks:
- S21 magnitude vs. frequency → through-path insertion loss and flatness
- S31 magnitude → coupling factor, verify it matches spec at center frequency
- S41 magnitude → isolation, should be > (coupling + 20 dB)
- S11/S22 → input/output match, should be <−20 dB in band
- Phase(S21) − Phase(S31) → should be 90° for quadrature coupler
Directivity Limitation on Microstrip
Edge-coupled microstrip couplers suffer from poor directivity (10–15 dB) because the even- and odd-mode velocities differ on inhomogeneous substrates. Solutions include:
- Wiggly-line coupling to equalize velocities
- Capacitive compensation at coupler ends
- Use of stripline (homogeneous medium) for >30 dB directivity
RF View's port analysis tools help you measure actual directivity from the .s4p file and compare against design targets.