Directional Couplers: Operation and Applications

Urvashi Sengal
Mini-Circuits

Kit Cox
Mini-Circuits Japan

Directional couplers are an important type of signal processing device. Their basic function is to sample RF signals at a predetermined degree of coupling, with high isolation between the signal ports and the sampled ports — which supports analysis, measurement and processing for many applications. Since they are passive devices, they can also be used in reverse: a signal fed into the coupled port will be injected onto the mainline, with the coupling factor and directivity determining how the injected signal is distributed. Directional couplers are available in several configurations, each described below.

Definitions

Ideally, a coupler would be lossless, matched and reciprocal. The basic properties of three- and four-port networks are isolation, coupling and directivity, the values of which are used to characterize the couplers. An ideal coupler has infinite directivity and isolation, along with a coupling factor selected for the intended application.

The functional diagram in Figure 1 illustrates the operation of a directional coupler, followed by a description of the related performance parameters. The top diagram is a 4-port coupler, which includes both coupled (forward) and isolated (reverse, or reflected) ports. The lower diagram is a 3-port structure, which eliminates the isolated port. This is used in applications that only need a single forward coupled output. The 3-port coupler can be connected in the reverse direction, where the port that was formerly coupled becomes the isolated port:

Functional block diagram of a 4-port directional coupler showing the Input Port (P1), Transmitted Port (P2), Coupled Port (P3), and Isolated Port (P4), with arrows indicating signal coupling between the mainline and coupled line.

Simplified diagram of a directional coupler showing signal flow from Input Port (P1) to Transmitted Port (P2), with a coupled signal at -10 dB directed to the Coupled Port (P3). — Figure 1: Basic directional coupler configurations

Performance Characteristics

Coupling Factor: The fraction of the input power (at P1) that is delivered to the coupled port, P3
Directivity: A measure of the coupler’s ability to separate waves propagating in forward and reverse directions, as observed at the coupled (P3) and isolated (P4) ports
Isolation: Indicates the ratio of input power (P1) to the power delivered to the isolated port (P4).
Insertion Loss: Indicates the ratio of input power (P1) to the power delivered to the through port (P2). This includes power diverted to the coupled and isolated ports, as well as dissipative losses within the coupler.

The value of the above characteristics are expressed mathematically in dB as:

Coupling\,Factor = C = 10 \log \left( \frac{P_1}{P_3} \right)

Directivity = D = 10 \log \left( \frac{P_3}{P_4} \right)

Isolation= I = 10 \log \left( \frac{P_1}{P_4} \right)

Insertion\, Loss = L = 10 \log \left( \frac{P_1}{P_2} \right)

Types of Couplers

Directional Couplers

This type of coupler has three accessible ports, as shown in Figure 2, where the fourth port is internally terminated to provide maximum directivity. Although it can be connected in reverse, this type of coupler does not perform identically when reversed. Since one of the coupled ports is internally terminated, only one coupled signal is available.

In the forward direction (as shown in Figure 2), the coupled port samples the forward wave, reduced by the coupling factor. This may be used to deliver a portion of the output signal to feedback circuitry or for output power monitoring.

One common use of a directional coupler is to sample reflected signals (or indirectly, VSWR). To achieve this, we can connect the coupler in reverse — with its input port facing the load instead of the source (see Figure 8 below). The forward signal from the source passes through the mainline without coupling to the coupled port (the coupled energy is instead dissipated in the internal termination), while any reflected wave from the load enters the coupler’s input port in the coupler’s forward direction. The coupled port then provides a sample of this reflected wave, reduced by the coupling factor. This configuration may be used for reverse power measurement or VSWR monitoring.

Diagram of a 3-port directional coupler. Shows the RF input, RF output, and coupled port, with the fourth port (isolated port) internally terminated with 50Ω. Arrows indicate forward signal flow and coupling to the coupled port. — Figure 2: 50Ω directional coupler

Advantages

Coupler performance is optimized for only one direction
High directivity and isolation
Since the directivity of a coupler is strongly affected by the impedance match at the isolated port, furnishing that termination internally ensures consistently high performance.

Disadvantages

Coupling is only available in one direction
The coupled port power rating is less than the input port because the power applied to the coupled port is almost entirely dissipated in the internal termination.

Example

Photo of the Mini-Circuits ZCDC20-E18653+ directional coupler, a blue coaxial device with four labeled ports: IN, OUT, CPL (coupled), and TERM (terminated with a built-on epoxied terminator).

Mini-Circuits ZCDC20-E18653+ is a coaxial directional coupler with 20 dB nominal coupling across the 18 to 65 GHz frequency range. This model provides RF input power handling up to 12W and passes DC current up to 0.48A

Insertion loss vs. frequency graph for the ZCDC20-E18653+ directional coupler, showing insertion loss gradually increasing from approximately 0.25 dB at 5000 MHz to 1.2 dB at 65000 MHz.

Coupling vs. frequency graph for the ZCDC20-E18653+ directional coupler, showing coupling remaining flat at approximately 20 dB across the 5000 to 65000 MHz frequency range, with a slight drop-off at the upper end

Directivity vs. frequency graph for the ZCDC20-E18653+ directional coupler, showing directivity varying between approximately 15 and 32 dB across the 5000 to 65000 MHz frequency range, with peaks near 5000, 35000, and 40000 MHz.

Return loss vs. frequency graph for the ZCDC20-E18653+ directional coupler, showing three traces: Return Loss IN (orange), Return Loss OUT (blue), and Return Loss CPL (green dashed). All three ports generally remain between 15 and 35 dB across the 5000 to 65000 MHz frequency range.

Figure 3: Performance curves for Mini-Circuits’ ZCDC20-E18653+ directional coupler

Bi-directional couplers

This coupler type has four ports, all accessible for the customer to use. It has a symmetric design, allowing forward and reverse signals to be sampled simultaneously. It is the user’s responsibility to properly match or terminate both coupled ports.

Advantages

Symmetric design
Input and output ports are interchangeable
There are two transmission lines, coupled line works the same as the mainline
Forward and reverse coupling available

Disadvantages

Design is critical to maintaining good performance in both directions
The directivity of the coupler depends on how well the isolated port is terminated

Example

Mini-Circuits’ ZGBDC35-93HP+ is a coaxial bi-directional coupler with 35 dB nominal coupling across the 900 to 9000 MHz frequency range. This model provides 250W RF input power handling and passes DC current up to 3A.

Photo of the Mini-Circuits ZGBDC35-93HP+ bi-directional coupler, a silver machined metal housing with two N-type mainline connectors on each end and two SMA coupled-port connectors on the side.

Insertion loss (IN-OUT) vs. frequency graph for the ZGBDC35-93HP+ bi-directional coupler, showing very low insertion loss gradually increasing from approximately 0.01 dB at low frequencies to approximately 0.1–0.19 dB at higher frequencies across the 500 to 12500 MHz range.

Coupling vs. frequency graph for the ZGBDC35-93HP+ bi-directional coupler, showing two traces: Coupling IN-FWD (orange) and Coupling OUT-REV (blue dashed). Both traces closely overlap at approximately 35 dB across the 5000 to 12500 MHz range, with a slight rise at the low-frequency end.

Directivity vs. frequency graph for the ZGBDC35-93HP+ bi-directional coupler, showing two traces: Directivity IN-REV (orange) and Directivity OUT-FWD (blue dashed). Both traces vary between approximately 12 and 34 dB across the 500–12500 MHz range, with a general downward trend at higher frequencies.

Return loss vs. frequency graph for the ZGBDC35-93HP+ bi-directional coupler, showing four traces: Return Loss IN (orange), Return Loss OUT (blue dashed), Return Loss FWD (green dashed), and Return Loss REV (purple). The mainline ports (IN, OUT) show higher peaks reaching 40–55 dB, while the coupled ports (FWD, REV) remain more stable around 20–25 dB across the 500–12500 MHz range.

Figure 5: Performance curves for Mini-Circuits ZGBDC35-93HP+ bi-directional coupler

Dual Directional Couplers

This third type of coupler is a combination of two 3-port couplers placed back-to-back. This configuration provides bi-directional coupler action as well as independent forward and reverse coupled ports. The primary advantage is that because each coupled port belongs to a separate internal coupler with its own termination, a mismatch on one coupled port will not affect the other.

Diagram of a dual directional coupler consisting of two 3-port couplers placed back-to-back. The mainline runs from RF Input to RF Output, with two independent coupled ports: Coupled Forward (bottom) samples the forward wave, and Coupled Reverse (top) samples the reverse wave. Each coupler has its own internal 50Ω load terminating the isolated port. — Figure 6: Dual directional coupler schematic

Advantages

Performance can be optimized for both forward and reverse paths
Higher directivity and isolation can be achieved
Provides forward and reverse coupling
Directivity of one path is not affected by mismatch present on the other path
Can be used to simultaneously monitor both the forward and reverse power of a system

Disadvantages

Larger size compared to directional and bi-directional couplers
No through path available between forward and reverse coupled ports
Higher insertion loss than single directional and bi-directional couplers

Example

Photo of the Mini-Circuits DDCH-50-13+ stripline-based surface-mount dual directional coupler, a compact gold-plated package with castellated edge-mount pads and a green orientation marker.

Mini-Circuits DDCH-50-13+ is a stripline-based surface-mount dual-directional coupler with a 50 dB nominal coupling ratio across the 20 to 1000 MHz frequency range. This model provides up to 120W RF input power handling and DC current passing up to 4A.

Insertion loss vs. frequency graph for the DDCH-50-13+ dual directional coupler at three temperatures: -55°C (red), +25°C (blue), and +105°C (green dashed). All three traces closely overlap, increasing gradually from approximately 0.03 dB at low frequencies to 0.38 dB at 1500 MHz, indicating minimal temperature dependence.

Coupling vs. frequency graph for the DDCH-50-13+ dual directional coupler, showing two traces: In-Cpl Fwd (red) and Out-Cpl Rev (blue dashed). Both traces remain flat at approximately 49–50 dB from 0 to 1200 MHz, then drop sharply to approximately 43–44 dB at 1500 MHz.

Directivity vs. frequency graph for the DDCH-50-13+ dual directional coupler, showing six traces at three temperatures (-55°C, +25°C, +105°C) for both coupled ports: In-Cpl Rev (red, blue, green dashed) and Out-Cpl Fwd (purple, black dashed, red dashed). Directivity generally ranges from 22 to 30 dB across the 0–1200 MHz range, then drops sharply to approximately 15–17 dB at 1500 MHz.

Return loss vs. frequency graph for the DDCH-50-13+ dual directional coupler, showing four traces: In (red), Out (purple), Cpl Fwd (blue), and Cpl Rev (black dashed). The mainline ports (In, Out) show high return loss of approximately 32–50 dB, while the coupled ports (Cpl Fwd, Cpl Rev) closely overlap at a significantly lower 7–14 dB across the 0–1500 MHz range.

Figure 7: Performance curves for Mini-Circuits DDCH-50-13+ dual directional coupler

Directional Coupler Applications

Reflectometer

When connected as shown in Figure 8, the coupled port provides a sample of the wave reflected by the load. This allows measurement of reflected power, representing the degree of mismatch of the load. When placed at the transmitter output, this configuration can monitor the VSWR of the antenna system, both for measurement and monitoring. Many RF systems include adjustments for minimum VSWR, while others include detection of excessive VSWR for circuit protection, usually by either reducing power or shutting down.

Forward Sampling

When incident power is applied to the input port, the coupled port provides a sample of the output (forward signal) attenuated by the coupling factor. This sample can be used for waveform monitoring, spectrum analysis, and other test and measurement functions.

Leveled Generator

The sample may also be used to drive feedback circuitry. One important application of this type is leveling the amplitude of a signal generator, providing a constant signal source for a test system.

Schematic of a 3-port directional coupler in a levelled generator setup. The Signal Generator's RF Output passes through the coupler in the forward direction to deliver a Levelled Output to Load. The coupled port provides a sample of the forward signal to a Power Detector, which feeds back to the Signal Generator to maintain a constant output level. — Figure 9: Schematic of a 3-port directional coupler in a leveled generator setup

Receiver Intermodulation Test Setup

The test signals for 2-tone testing may be combined in either a directional coupler or a power combiner. Both methods will provide the necessary isolation between the signal sources.

Bi-directional Coupler Applications

Forward and Reverse Sampling

Although measurement of reflected power/VSWR is important, it may be more useful to simultaneously sample both the forward and reflected signals. This functionality can be achieved using a bi-directional coupler, which allows monitoring and measurement of output (forward) power and reflected (reverse) power. Built-in test (BIT) systems, production testing, and routine operational monitoring all benefit from bi-directional coupling.

Dual Directional Coupler Applications

Forward and Reverse Sampling

As noted above, and in Figure 6, the dual directional coupler acts as a bi-directional coupler, but with separate forward and reverse coupling paths. This provides isolation that eliminates the effects of mismatch of one path on the other path. Accuracy is thus improved, especially under conditions where one coupled port or the other may have a significant mismatched load.

Summary

Directional couplers are important devices in RF systems. Their ability to sample the forward and/or reverse direction of signal propagation allows a wide range of applications in test, measurement, monitoring, feedback, and control. This note should help system designers understand the function, architecture, and performance of the coupler so he or she may select a suitable type for their particular application.

Find the right directional, bi-directional, or dual directional coupler for your application from the hundreds in the Mini-Circuits catalog.

Directional Coupler Catalog