Directional coupler is  considered to be as measurement and 4 port devise that is used to measure the microwave power. Directional Coupler is usually designed to distribute the power. In directional coupler there is 4 port the first one is considered to be as Input port, the second port name is output port, the third port name is coupled port and the fourth port named as Isolated port. In this the input power is divided into two port  output and  coupled port  named as coupling factor. Coupling Factor can be any value like 5,6,7,10,15,20,30,50 db and many more. 

As given below there are some specifications for directional coupler: