What is the principle of Coupler?

The coupler is a radio frequency device that extracts a small part of the signal from the main channel of the wireless signal. Like the power divider, it is a power distribution device. The difference is that the coupler is a distribution device with unequal power. The coupler is used in conjunction with the power divider, mainly to achieve one goal - to make the transmit power of the signal source as evenly distributed as possible to each antenna port of the indoor distribution system, so that the transmit power of each antenna port is basically the same.