Two hole directional coupler
A two-hole directional coupler with traveling waves propagating in it is illustrated in Figure The spacing between the centers of two holes must be :
where n is any positive integer
Figure : Two-hole directional coupler
A fraction of the wave energy entered into port 1 passes through the holes and is radiated into the secondary guide as the holes act as slot antennas. The forward waves in the secondary guide are in the same phase, regardless of the hole space, and are added at port 4. The backward waves in the secondary guide (waves are progressing from right to left) are out of phase by (2L/ λg)2π rad and are canceled at port 3.