As shown in the given diagram, the directional coupler consists of an auxiliary piece of transmission line of which one end is connected with the detection meter and the opposite end is connected with the pure resistor known as non-reflecting end. There are two slots in the directional coupler through which the probes are coming out for the measurement of the power. The distance between the probes or slots is kept λ/4 of the operating frequency.
Working of Directional Coupler
The directional coupler is used to measure the true power provided by the source to the load. In this case if there is a matched load, then the standing wave along the transmission line will be the same as the incident wave, if the load is miss-matched than the standing wave or the power present at the transmission lines will not be the same to the power provided by the source.
The arrangement of directional coupler enables us to measure the true power provided by the source to the load looking towards the diagram. If we consider the travelling from source to load we can see that at point A, the forward wave is provided to the directional coupler with the help of probe at point B the wave has travelled the distance of λ/4 both at the main transmission line and at the auxiliary line of the directional coupler.
The wave travelling through opposite to the direction of detection meter from the probe will find itself at the non-reflecting termination where its whole energy will be observed. The wave travelling towards point B will become stranger due to additive effect of energy provided by the main line to the probe at point B.
Now in case of reflected energy at point B travelling towards point A will find itself out of phase at point B with the incident energy but in phase at point A with same incident energy. It is due to the reason that the behavior of the reflected wave will be vice versa at point A with respect to point B. Because the distance between these two points is λ/4.
In this way we come to the conclusion that in case of miss matched load the power present over the transmission line and the reflected power present at point A of the directional coupler will be added up which is the result of true power produced to the detection meter for measurement.