Coupler
Understanding Directional Couplers
Directional couplers are among the most important passive components in RF systems. They allow non-destructive monitoring of power levels, enable network analyzer measurements, and serve as building blocks for balanced mixers and beam-forming networks.
Key Parameters
- Coupling factor: The fraction of input power delivered to the coupled port, in dB. Common values: 3, 6, 10, 20, 30 dB.
- Directivity: The ratio of coupled power to isolated port power. Measures how well the coupler distinguishes forward from reverse waves. Higher is better; typically 15-40 dB.
- Isolation: Attenuation from input to isolated port. Isolation = Coupling + Directivity.
- Insertion loss: Loss in the main line due to coupling. For a 10 dB coupler, theoretical insertion loss is 0.46 dB.
Types
- Waveguide coupler: Bethe-hole, multi-hole, or branch-line. Very high directivity.
- Stripline/microstrip coupler: Edge-coupled or broadside-coupled lines. Compact.
- Hybrid coupler (3 dB): Equal power split with 90 or 180 degree phase difference between outputs.
Isolation (dB) = -10 log10(P_isolated/P_input)
Directivity (dB) = Isolation - Coupling
Insertion loss = -10 log10(1 - P_coupled/P_input)
For 10 dB coupler: couples 10% of power
Insertion loss = -10 log10(0.9) = 0.46 dB
Frequently Asked Questions
What is a directional coupler used for?
Directional couplers sample a fraction of RF power without significantly disturbing the main signal. They are used for power monitoring, VSWR measurement, signal leveling, and as building blocks in balanced mixers and antenna feed networks.
What does coupling factor mean?
Coupling factor is the fraction of input power delivered to the coupled port, expressed in dB. A 10 dB coupler delivers 10% of the input power to the coupled port. A 20 dB coupler delivers 1%. Higher coupling values (numerically) mean less power is sampled.
What is directivity in a coupler?
Directivity measures how well the coupler distinguishes forward from reverse traveling waves. High directivity (30-40 dB) means the coupled port output accurately represents only the forward wave. Low directivity causes measurement errors when monitoring VSWR or reflected power.