A directional coupler is a low-loss device that accepts one input signal and outputs two signals with the following characteristics in theory.

1. The output amplitudes are not equal.

The output end of the main line is a larger signal, which can basically be regarded as a straight-through, and the output end of the coupling line is a smaller signal. The ratio of the smaller signal on the coupling line to the amplitude of the main line signal is called "coupling" and expressed in dB.

Coupling port power = input power-coupling degree

For example, a 10dB directional coupler, the input power is 30dBm (1W), then the output power of its output end is 30dBm, and the output power of the coupling end is 20dBm. On the surface, it seems to violate the principle of power conservation, but in fact, power is conserved. The misunderstanding is caused by the sometimes unclear conversion between power watts and decibels.

2. The theoretical loss on the main line is determined by the signal level of the coupled line, that is, the degree of coupling.

3. The main line and the coupled line are highly isolated.