Directional Coupler Symbol
Definition: The Directional Coupler symbol represents a passive four-port RF and microwave network component that samples a small, fixed fraction of the signal power travelling in one direction along a transmission line—without significantly disturbing the main signal—depicted in circuit diagrams as a rectangle with four labelled ports: In, Thru, Coupled, and Isolated, and referenced in IEEE 315 / ANSI Y32.2 and IEC 60617.
Also known as: RF coupler, microwave coupler, 10 dB coupler, hybrid coupler, branch-line coupler, coupled-line coupler, directional power sampler.
What the Directional Coupler symbol means
The Directional Coupler symbol denotes a four-port passive device that splits RF or microwave power in a direction-sensitive manner. The main signal path flows from the In port to the Thru port with very low insertion loss (typically 0.1–0.5 dB), while a precisely attenuated sample of that forward-travelling power exits the Coupled port; the Isolated port receives virtually none of the forward power (high isolation, typically 20–40 dB).
In RF circuit diagrams the directional coupler symbol is used wherever a small signal sample must be extracted for measurement, monitoring, or feedback without loading the primary transmission path. The coupling value in decibels (e.g. −10 dB) is usually annotated next to the symbol to specify the fraction of power diverted to the Coupled port.
How to identify the Directional Coupler symbol
The Directional Coupler glyph is drawn as a rectangle with four external connection points: In and Thru on opposite ends of one axis (the main through-line), and Coupled and Isolated on opposite ends of a perpendicular axis. An internal diagonal line or a pair of parallel lines (representing the coupled transmission-line sections) is often shown inside the rectangle to indicate the electromagnetic coupling mechanism. The symbol is clearly distinguished from a simple power splitter (which is symmetric) by the directional asymmetry implied by the four distinct port labels.
Function in a circuit
A directional coupler samples forward-travelling RF power from the In-to-Thru main line and routes a known fraction (the coupling factor, in dB) to the Coupled port while presenting high isolation at the Isolated port. This directional selectivity allows the device to distinguish between forward and reflected waves on the main line. Typical applications include power monitoring, automatic level control (ALC), reflected-power detection (SWR measurement), and signal injection for test purposes. The coupling factor and directivity (isolation minus coupling, in dB) are the key performance parameters.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 includes a directional coupler symbol as a four-terminal passive network element; it is represented by a rectangle with two input/output ports on one axis and two coupled/isolated ports on the perpendicular axis, consistent with the IEC graphical symbol conventions for RF and microwave components. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 defines the directional coupler symbol in the RF/microwave component category as a rectangular block with four labeled ports: input, through (Thru), coupled output, and isolated port. The coupling value in dB is typically noted inside or adjacent to the symbol. |
| Key difference | IEC and ANSI/IEEE symbols for the directional coupler are functionally identical four-port rectangles. Minor differences exist in port label conventions: IEC documentation may use 'input', 'output', 'coupled', and 'isolated' while IEEE 315 uses 'In', 'Thru', 'Coupled', and 'Isolated'. The internal coupling-line representation is optional in both standards. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| thru | Thru |
| coupled | Coupled |
| isolated | Isolated |
Typical values
Coupling factor: −3 dB (hybrid), −6 dB, −10 dB, −20 dB (typical values). Insertion loss: 0.1–0.5 dB. Directivity: 20–40 dB (higher is better). Frequency range: 1 MHz to 100 GHz+ depending on construction. Impedance: typically 50 Ω (RF) or 75 Ω (cable TV). Power handling: milliwatts to kilowatts depending on design.
Where the Directional Coupler symbol is used
- RF transmitter automatic level control (ALC) loops sampling forward power for feedback to a variable-gain amplifier
- Standing wave ratio (SWR) meters and network analysers measuring forward and reflected power simultaneously
- Cellular base station power amplifier output stages monitoring transmitted power for regulatory compliance
- Radar and electronic warfare systems splitting a reference signal for I/Q demodulation
- Cable television (CATV) signal distribution networks tapping main trunk lines without disrupting downstream signal levels
- Microwave test benches injecting a calibrated signal into a device under test while monitoring incident power
- Satellite uplink and downlink systems splitting a local oscillator signal to multiple mixer stages
Example
In a VHF transmitter block diagram, a Directional Coupler symbol is placed immediately after the 50 W power amplifier; the In pin receives the amplified signal, the Thru pin delivers the signal to the antenna through a low-pass filter, the Coupled pin (−20 dB, giving 5 mW from 50 W) connects to an RF power detector diode for ALC feedback, and the Isolated pin is terminated in a 50 Ω load.
Key facts
- A Directional Coupler is a passive four-port RF/microwave device with ports: In, Thru, Coupled, and Isolated; it has no polarity, no DC power requirement, and is characterised by coupling factor (dB), insertion loss (dB), and directivity (dB).
- The coupling factor specifies the fraction of forward power routed to the Coupled port: a −10 dB coupler delivers one-tenth of the input power (10%) to the Coupled port and approximately nine-tenths to the Thru port.
- Directivity is the difference (in dB) between coupling factor and isolation; high directivity (>30 dB) means the Coupled port responds almost exclusively to forward-travelling waves and rejects reflected waves.
- The Isolated port must be terminated in the system characteristic impedance (typically 50 Ω) to achieve specified performance; an open or short-circuited Isolated port degrades directivity and increases VSWR on the main line.
- Directional couplers are realised in multiple technologies: coupled microstrip or stripline (PCB), waveguide, coaxial, and hybrid transformer types; the schematic symbol is the same regardless of construction technology.
- A −3 dB (hybrid) directional coupler splits power equally between the Thru and Coupled ports and is used as a power splitter/combiner or 90°/180° hybrid; it is a special case of the general directional coupler.
- The schematic symbol is referenced in IEEE 315-1975 and IEC 60617; the coupling value in dB is always annotated next to the symbol in circuit diagrams to fully specify the device.
Frequently asked questions
What does the directional coupler symbol mean in a circuit diagram?
The Directional Coupler symbol represents a passive four-port RF device that samples a fixed fraction of forward-travelling signal power from the main transmission path (In to Thru) and routes it to the Coupled port, while the Isolated port receives negligible power. The coupling factor in dB annotated on the symbol specifies how much power is diverted.
What does the directional coupler symbol look like?
The Directional Coupler symbol is a rectangle with four labelled ports: In and Thru on opposite ends of the horizontal axis (the main signal path) and Coupled and Isolated on opposite ends of the vertical axis. An internal diagonal or pair of parallel lines is sometimes drawn inside the rectangle to suggest the coupled transmission-line mechanism.
What is the difference between the In, Thru, Coupled, and Isolated ports on a directional coupler?
In is the signal input port. Thru is the main output port, passing the majority of input power with minimal loss. Coupled is the output port that receives a small, fixed fraction of the forward power (specified by the coupling factor in dB). Isolated is the fourth port that receives virtually none of the forward power and must be terminated in the system impedance.
What does a −10 dB directional coupler do?
A −10 dB directional coupler routes exactly one-tenth of the input power (10%) to the Coupled port and approximately nine-tenths (90%, minus insertion loss) to the Thru port. A −10 dB level is a common choice for power monitoring because it provides a usable sample level without significantly reducing the main-line power.
What is directivity in a directional coupler?
Directivity is the difference in decibels between the coupling factor and the isolation of a directional coupler; it quantifies how well the coupler distinguishes between forward and reverse-travelling signals. High directivity (>30 dB) means the Coupled port responds almost entirely to forward waves; low directivity causes the reflected signal to contaminate the forward-power measurement.
What standard defines the directional coupler symbol?
The Directional Coupler symbol is defined in ANSI Y32.2 / IEEE 315-1975 for North American practice and in IEC 60617 for international practice. Both standards represent the device as a four-port rectangle with In, Thru, Coupled, and Isolated port labels. The coupling value in dB must be annotated alongside the symbol to fully specify the component.
Why must the Isolated port of a directional coupler be terminated?
The Isolated port must be terminated in the characteristic impedance (typically 50 Ω) because an unterminated port creates impedance mismatches that reflect energy back into the coupler, degrading directivity, increasing VSWR on the main line, and causing measurement errors at the Coupled port. A precision 50 Ω termination absorbs any residual signal reaching the Isolated port.
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