In RF applications Directional couplers are used to separate signals based on propagation direction. The wave induced in the auxiliary arm (coupling line) is proportional to the forward wave traveling in the transmission system (main line). The ratio of the induced power to the input power is known as the coupling factor.

Directivity is defined as the difference in dB of the power output at a coupled port, when power is transmitted in the desired direction, to the power output at the same coupled port when the same amount of power is transmitted in the opposite direction.

Power meters measure power in the transmission line by measuring the output of a directional coupler at the coupling port. This output is affected by the coupling factor.

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