Directional coupler is something like a snifer – how much power is transferred from Port1 to Port2.
On Port 3 you can observe how many power is transferred from port 1 to port2. On Port4 you can observe how many power is reflected from side port2 to port1 .
The main parameter is coupling factor – this is parameter which can show you how many power you should have on coupled port.
Second parameter is bandwidth of this coupler, and this parameter is strongly corelated with type of coupler(for example pcb coupler in FR4 substrate)
For design you can use Qucs – freeware simulator.
First – calculate your data
use internal calculator for coupled lines and click synthesize and copy to clipboard: