5G communication testing, wireless terminal automated testing
Aerospace communication systems
Precision measurement equipment, radio frequency signal control of radar transmitters
Communication and navigation systems
Optical communication module-related testing
Interconnection of wireless communication base stations
A directional coupler is a four-port device with input, output, coupled, and isolated ports. Power entering the input is split: most continues to the output, while a known fraction appears at the coupled port. Power traveling in the reverse direction ideally exits at the isolated port and not at the coupled port. By combining two couplers back-to-back, a dual directional coupler simultaneously measures forward and reflected power — the basis of VSWR and return-loss measurement.
A directional coupler is a passive RF component that performs signal splitting with directionality. It is described by four ports:
The relationship between the four ports defines the coupler's behavior and is captured in three key parameters: coupling, insertion loss, and directivity.
Forward power enters Port 1, exits Port 2 mostly intact, and is sampled at Port 3. Reverse power ideally exits at Port 4 (isolated) — directivity measures how well Port 3 rejects this reverse power.
| Specification | Definition |
|---|---|
| Coupling Factor | Ratio of input to coupled power (e.g., 20 dB, 30 dB) |
| Insertion Loss | Main-line loss from input to output |
| Directivity | Coupled port's ability to reject reverse-wave leakage = Isolation − Coupling |
| Isolation | Leakage from input to isolated port |
| VSWR | Match of each port, usually < 1.2:1 |
| Frequency Range | Operating band |
| Power Handling | Maximum CW and peak power at the main line |
| Phase Balance | Tracking between forward/reverse sampled signals (dual couplers) |
| Amplitude Balance | Tracking of forward/reverse amplitudes (dual couplers) |
The relationship between three core metrics is:
Directivity (dB) = Isolation (dB) − Coupling (dB)
Two parallel waveguides share multiple coupling holes for high directivity. Standard above 2 GHz for radar and satellite systems.
Two waveguides crossing at 90° with coupling apertures. Very high directivity in a compact package.
Stripline sections with parallel coupled lines. Broadband, easy to integrate. Common up to 40 GHz.
Two coaxial lines share a coupling aperture. Very high directivity (40 – 50 dB), broadband, used in precision reflectometry.
Quarter-wave branches forming a 90° hybrid. Used in feed-forward amplifiers and balanced topologies.
Interdigitated microstrip lines for tight coupling (3 dB) over wide bandwidth. Common in MMIC design.
Resistive sheets sample the main-line voltage. Cheap and broadband, but lossy — used for low-power monitoring.
Two single couplers back-to-back, providing simultaneous forward and reflected power samples. Essential for VSWR monitoring.
| Aspect | Single | Dual |
|---|---|---|
| Ports | 3 (input, output, coupled) | 4 (input, output, Fwd, Rev) |
| Measures | Forward only (or reverse only) | Both forward and reverse |
| VSWR Monitoring | Not directly | Yes, by comparing forward and reverse |
| Cost | Lower | Higher |
| Use Case | Signal sampling, monitoring | Radar ALC, broadcast foldback, TWT protection |
The coupling factor sets how much of the main-line power reaches the coupled port:
The most common use: a dual directional coupler samples forward and reflected power on a transmitter feed line. The ratio gives VSWR or return loss, used for foldback protection and compliance verification.
The forward-coupled port feeds back into the transmitter's gain-control loop, maintaining constant output power despite supply, temperature, or load variations.
Precision coax couplers (40 – 50 dB directivity) measure antenna return loss with high accuracy across wide bandwidths.
Couplers tap the source and load sides of a device under test so that vector network analyzers can extract impedance information.
Dual directional couplers sample the transmitted pulse and the reflected return, providing both ALC and built-in test capability.
Two couplers — one at input, one at output — drive a cancellation loop that suppresses intermodulation distortion in power amplifiers.
A loosely-coupled sample feeds a spectrum analyzer or monitoring receiver without disturbing the main traffic.
Directional couplers split and combine signals on multi-tap distribution networks.
A directional coupler is a four-port RF component that samples power traveling in one direction along a transmission line while isolating the opposite direction.
Directivity (dB) = Isolation − Coupling. It measures how well the coupled port rejects the reverse-traveling wave, enabling accurate forward-power measurement.
A dual directional coupler simultaneously samples forward and reflected power. It is the standard building block for VSWR monitoring, ALC loops, and feed-forward amplifiers.
Waveguide couplers are preferred above 2 – 3 GHz and at very high power. Coaxial couplers are more convenient, broadband, and easier to install but limited by connectors.
Choose coupling that places the sampled signal 20 dB or more above your detector's noise floor. Common values are 20 dB for instrumentation and 30 – 40 dB for high-power monitoring.
High-tech Enterprise, Feifeng Talent
Chief Engineer Wang graduated with a master's degree in high-power microwave from the Institute of Electronics, University of Chinese Academy of Sciences.
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