Passive Components

Coupled Port

Q: How do the coupled port and the isolated port differ in a directional coupler?

In a four-port directional coupler the coupled port samples a fixed fraction of the power flowing in the forward direction on the mainline, while the isolated port is the diagonally opposite port that ideally receives nothing from that same forward wave. The ratio between the wanted signal at the coupled port and the leakage at the isolated port is the directivity. If you reverse the wave direction on the mainline, the roles swap: the former isolated port now becomes the coupled port for the reverse wave. This directional behavior is exactly what lets a reflectometer separate forward and reflected power. A typical sidewall waveguide coupler at 10 GHz delivers 20 dB coupling at the coupled port with 35 to 40 dB directivity, so by definition the isolated port leakage sits 35 to 40 dB below the coupled sample.

Q: How do you calculate the power level at the coupled port?

The coupled port level equals the incident forward power minus the coupling factor. Coupling factor C is defined as C = 10 log (P_incident / P_coupled) in dB, so a coupler stamped 20 dB delivers a sample 20 dB below the mainline power. For 40 dBm (10 W) entering the input with a 20 dB coupler, the coupled port outputs 40 minus 20 equals 20 dBm (100 mW). You must also subtract the small mainline insertion loss and account for coupling flatness over frequency, which for a broadband coupler may vary plus or minus 0.5 to 1 dB. Engineers size the coupling factor so the sample lands within the linear range of the power sensor or detector connected to the coupled port, typically minus 20 to plus 10 dBm.

Q: Why must the coupled port be terminated when it is not in use?

Any unused coupler port, including the coupled port, must see a good match because reflections off an open or shorted port re-enter the coupler and degrade both the effective directivity and the mainline return loss. A reflection at the coupled port couples back into the mainline and corrupts the sample seen at the other ports, which is fatal for reflectometry accuracy. In a precision dual-directional coupler used for forward and reflected power measurement, leaving a coupled port open can drop usable directivity from 40 dB to under 25 dB. Standard practice is to fit a 50 ohm load with a return loss better than 20 dB, or to connect a matched detector or power sensor that presents a similar match across the band.

/KUH-puld port/

One of the four ports of a directional coupler, the coupled port delivers a fixed, known fraction of the power traveling forward through the mainline. That fraction is set by the coupling factor C in dB; a coupler marked 20 dB taps a sample 20 dB below the mainline level, so 10 W in yields 100 mW out of the coupled port. The accuracy of that sample is bounded by directivity, the leakage ratio between the coupled port and the diagonally opposite isolated port. Because the coupled port responds only to one direction of travel, it is the working element behind power monitoring, automatic level control loops, and reflectometers that separate forward from reflected energy.

Category: Passive Components

Typical Coupling: 6 to 30 dB

Directivity: 25 to 45 dB

How the Coupled Port Samples Forward Power

A directional coupler is a four-port network: the input and the through (or mainline) port carry the bulk of the signal, while the coupled port extracts a small, calibrated replica of the wave traveling in one specific direction. In the ideal device the coupled port responds only to the forward wave on the mainline and ignores any reverse wave, which is the property that distinguishes a directional coupler from a simple power tap. The amount tapped is governed by the geometry of the coupling region, whether that is the spacing between two parallel transmission lines, the slot pattern in a waveguide common wall, or the lumped reactances of a printed coupled-line section.

The coupled signal is described directly by S-parameters. For a coupler with the input at port 1, through at port 2, isolated at port 3, and the coupled port at port 4, the coupling factor is C = -20 log|S₄₁| and the directivity is D = 20 log(|S₄₁| / |S₃₁|). Coupling sets how much you get; directivity sets how trustworthy it is. A weak 30 dB coupled port disturbs the mainline very little (about 0.004 dB of theoretical coupling loss) but produces a small sample, whereas a 6 dB coupled port pulls a large fraction of the power and costs roughly 1.25 dB of mainline insertion loss.

In practical bench and production use the coupled port feeds a power sensor, a diode detector, or a receiver. Engineers choose the coupling factor so the sample lands inside the linear range of that instrument, usually somewhere between -20 and +10 dBm. Dual-directional couplers place two coupled ports back to back so that both forward and reflected power can be sampled at once, which is the heart of a scalar or vector reflectometer.

Coupling Factor and Directivity Equations

Coupling Factor:
C = 10 log (P_in / P_coupled) = −20 log|S₄₁| dB

Coupled Port Power Level:
P_coupled(dBm) ≈ P_in(dBm) − C(dB)

Directivity:
D = 10 log (P_coupled / P_isolated) = 20 log(|S₄₁| / |S₃₁|) dB

Isolation:
I = C + D = −20 log|S₃₁| dB

Where P_in = forward incident power, P_coupled = power at the coupled port, P_isolated = leakage at the isolated port. Example: P_in = 40 dBm into a 20 dB coupler with 38 dB directivity → P_coupled ≈ 20 dBm, isolation ≈ 58 dB.

Coupling Factor Selection by Application

Coupling Factor	Coupled Port Sample (P_in = 40 dBm)	Mainline Coupling Loss	Typical Directivity	Best Application
3 dB (hybrid)	37 dBm	3.0 dB	20 to 30 dB	Equal power split, balanced mixers
6 dB	34 dBm	1.25 dB	25 to 35 dB	Antenna feed sampling
10 dB	30 dBm	0.46 dB	30 to 40 dB	Power amplifier load monitoring
20 dB	20 dBm	0.044 dB	35 to 45 dB	Transmitter power metering
30 dB	10 dBm	0.004 dB	30 to 40 dB	Low-disturbance reflectometry

Common Questions

Frequently Asked Questions

How do the coupled port and the isolated port differ in a directional coupler?

The coupled port samples a fixed fraction of the forward wave on the mainline; the diagonally opposite isolated port ideally receives nothing from that same forward wave. The ratio between the two is the directivity. Reverse the mainline wave and the roles swap. A sidewall waveguide coupler at 10 GHz might give 20 dB coupling with 35 to 40 dB directivity, which by definition puts isolated-port leakage 35 to 40 dB below the coupled sample.

How do you calculate the power level at the coupled port?

Subtract the coupling factor from the incident power: P_coupled(dBm) ≈ P_in(dBm) − C(dB). For 40 dBm (10 W) into a 20 dB coupler the coupled port delivers about 20 dBm (100 mW). Also subtract the small mainline insertion loss and allow for coupling flatness, which on a broadband part may vary ±0.5 to 1 dB. Size C so the sample stays in the detector's linear range, typically −20 to +10 dBm.

Why must the coupled port be terminated when it is not in use?

A reflection off an open or shorted coupled port re-enters the coupler, corrupts the directional sample, and degrades mainline return loss. On a precision dual-directional coupler an open coupled port can drop usable directivity from 40 dB to under 25 dB. Fit a 50 ohm load with return loss better than 20 dB, or connect a matched detector or power sensor that maintains that match across the band.

Related Terms

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