• If power is feed into port (1), the power is coupled in port (2) and (3), i.e., power flows in the forward direction of the auxiliary arm port (3), but no power couples in port (4), i.e., in backward direction.
• Similarly, power is fed in port (2) couples to port (1) and (4), and not in port (3).
• All the four ports are matched, i.e., if three of them are terminated in matched loads, the fourth is automatically terminated in a matched load.
• If power couples in reverse direction i.e., power fed in port (1) appears in port (2) and (4), and nothing in port (3), then such type of coupler is known as backward direction coupler.
• The conclusion is that in the auxiliary section the power is coupled only in one direction.

Directionality and coupling of a directional coupler:

• The characteristics of directional coupler are expressed in two parameters:

Directionality (D) of a directional coupler:

• It is defined as the ratio (in dB) of the power output in the coupled auxiliary arm to the power flowing in uncoupled auxiliary arm.
• Theoretically D should be infinite because no power should couple in arm (4).

Coupling factor or coupling (C) of a directional coupler:

• It is defined as the ratio (in dB) of the power entering at input of the main line to the power couples at the output in the auxiliary arm.

• The high value of C is desired.
• Both the parameters are frequency dependent. Hence to have a wide band directional coupler, ideal conditions can’t be achieved.
• However, total directionality of the signal (in addition to frequency) depends on reflections from internal termination, intrinsic directionality and reflections from the output flange.
• Directionality is a measure of how will the forward travelling wave in the first waveguide couples only to a specific port of second waveguide.
• The directionality D is a measure of the discrimination property of a directional coupler between the waves travelling in the two directions in the main line.
• An ideal directional coupler should have infinite directionality.