What is Coupling Factor or Coupling Ratio?

The Coupling factor/Coupling Ratio is the ratio of incident power to the coupled power, measured in dB.

So when we say a coupler has a coupling value of 3 dB, it means that 50% of the power is coupled to the coupled port of the coupler and the rest of the power is routed to the output port of a coupler. If the coupling value is 30 dB, this means that 0.1% of the power is coupled to coupled port and 99.9% of the power is passed through to the output port of a directional coupler.

The table below lists some common coupling values in dB with their equivalent value in terms of linear power split.