RFUnderstanding Directional Couplers: Principles and Applications
A directional coupler is a passive microwave device that samples or couples a portion of power traveling in one transmission line to another. This is typically done to measure or monitor signals within a microwave system.
The main job of a directional coupler is to allow power to flow from one port to another in a specific direction. It does this while minimizing the power coupled in the reverse direction.
Directional couplers have four ports:
- Port 1: Input Port
- Port 2: Output Port
- Port 3: Coupled Port
- Port 4: Isolated or Terminated Port
The basic structure often includes a main transmission line (the coupled line) and a secondary transmission line (the isolated line or coupled line) that runs parallel to the main line for a certain distance. The coupling between these lines allows a fraction of the power to be coupled from the input to the coupled port, all while maintaining isolation between the input and isolated ports.
Directional Coupler Parameters
Directivity (D)
Directivity measures how well the directional coupler separates the coupled and isolated ports. It’s the ratio of power coupled to the coupled port to the power coupled to the isolated port. Higher directivity means better performance.
The formula for Directivity (D) is:
D = 10 * log10 (Pf/Pb)
Where:
- Pf = Power at the coupled port
- Pb = Power at the isolated port
Coupling Factor (C)
The Coupling Factor defines the ratio of power coupled to the coupled port to the total power entering the coupler. It is expressed in decibels (dB).
C = 10 * log10 (Pi/Pf)
Where:
- Pi = Input Power
- Pf = Power at the coupled port
Isolation (I)
Isolation refers to the ability of the directional coupler to prevent power from traveling in the reverse direction, from the output port to the input port. High isolation ensures that the signals on one path don’t interfere with signals on the other path.
I = 10 * log10 (Pi/Pb)
Where:
- Pi = Input Power
- Pb = Power at the isolated port
Insertion Loss (I_L)
Insertion Loss represents the amount of power lost as the signal passes through the directional coupler. Lower insertion loss is desirable for preserving signal strength.
I_L = -10 * log10 [(Pf/Pi) + (Pr/Pi) + (Pb/Pi)]
Where:
- Pi = Input Power
- Pf = Power at the coupled port
- Pr = Power at the output port
- Pb = Power at the isolated port
Applications
Directional couplers are used in various microwave and RF systems:
- Power Monitoring: They are commonly used to monitor the power levels of signals in a transmission line without significantly affecting the main signal path.
- Signal Sampling: They are used to sample a portion of the signal for measurement or feedback purposes while allowing the majority of the signal to continue along the main transmission path.
- Antenna Systems: Directional couplers can be employed in antenna systems for signal monitoring, power distribution, and feedback control.
- Signal Combining and Splitting: In some cases, directional couplers are used to combine or split signals in RF systems.
Directional couplers come in various configurations and types, including stripline, microstrip, and waveguide directional couplers, each suitable for different applications and frequency ranges.
