UNDERGRADUATE ELECTRICAL ENGINEERING

Directional Couplers

Master the fundamentals of passive microwave components. Explore coupling, directivity, and scattering parameters through interactive simulations.

Launch Virtual Lab Study Theory

What is a Directional Coupler?

A directional coupler is a four-port passive device used in radio frequency (RF) and microwave circuits. It is designed to sample a small amount of microwave power for measurement purposes without interrupting the main power flow.

It consists of an auxiliary line coupled to a main transmission line in such a way that a wave traveling in a specific direction in the main line couples to a specific port in the auxiliary line, while a wave traveling in the opposite direction couples to a different port.

// KEY PRINCIPLE

If power enters Port 1 (Input), it exits Port 2 (Through).
A fraction of this power leaks to Port 3 (Coupled).
Ideally, NO power goes to Port 4 (Isolated).

FIG 1.1: QUADRATURE HYBRID
1 2 3 4
PORT 1
Input
PORT 2
Through
PORT 3
Coupled
PORT 4
Isolated

Virtual Laboratory: Power Distribution

Adjust the Coupling Factor (C) and Input Power to observe signal flow.

Parameters

10 dBm
20 dB

Lower dB = Stronger Coupling (More power to Port 3)

40 dB

Higher dB = Better Isolation (Less power to Port 4)

~0.04 dB

Power Distribution Analysis

LIVE
1
10 dBm
2
9.96 dBm
3
-10 dBm
4
-30 dBm
Port 1 (Input)
10 dBm
Port 2 (Through)
9.96 dBm
Port 3 (Coupled)
-10 dBm
Port 4 (Isolated)
-30 dBm

Critical Parameters

Coupling Factor (C)

The ratio of input power (P1) to coupled power (P3).

C(dB) = 10 log(P1/P3)

Directivity (D)

The ratio of coupled power (P3) to isolated power (P4). Measures isolation quality.

D(dB) = 10 log(P3/P4)

Isolation (I)

The ratio of input power (P1) to isolated power (P4).

I(dB) = 10 log(P1/P4)

Insertion Loss

Power lost in transmission from Port 1 to Port 2 (excluding coupled power).

IL(dB) = -10 log(P2/P1)

Relationship Between Parameters

Fundamental Identity
I = C + D
+
Insertion Loss (Ideal)
IL = -10 log(1 - 10-C/10)

Note: In practice, Insertion Loss also includes dielectric and conductor losses.

Common Types

01

Microstrip Coupler

Parallel lines on a PCB. Easy to fabricate, but limited directivity due to unequal even and odd mode phase velocities.

02

Waveguide Coupler

Two waveguides sharing a common wall with holes (apertures). High power handling and excellent directivity.

03

Quadrature Hybrid (90°)

Equal power split (-3dB) with a 90° phase shift between outputs. Crucial for mixers and amplifiers.

04

Rat-Race Hybrid (180°)

Ring-shaped structure. Provides 180° phase shift. Used for combining power and creating sum/difference ports.

Applications

  • Reflectometer: Measuring VSWR and reflection coefficients.
  • Power Monitoring: Sampling signal power without interrupting the main path.
  • Balanced Amplifiers: Combining two amplifiers to improve bandwidth and VSWR.
  • Antenna Beamforming: Distributing signals to antenna arrays with specific phase relationships.

Study Tip

Remember: A perfect coupler has infinite directivity (Port 4 receives 0 power). In real designs, improving directivity often involves adding compensation capacitors or using dielectric overlays to equalize phase velocities.