A like sample of the power flowing from port 2 to port 1 will be dissipated across R1, but will be out of phase and cancelled at port 3. The directional coupler may be operated in the opposite sense with input on port 2 and output on port 1 and port 3 used to combine power from another source with loss equal to the value of the directional coupler. In this case, most of the power coming in on port 3 that portion that doesn't flow to port 1 is dissipated in resistor R1. By varying the turns ratio of both T1 and T2, various values of coupling may be achieved. Table 1 lists some of the commonly used values in cable work.
Losses exist in directional couplers, as they do in splitters, and return loss is worse than shown.
The basis of the theoretical insertion loss is illustrated in Figure 2 . If the tap value is X, then the power coupled from port 1 to port 3 is X dB lower than the input power. This much of the power supplied to port 1 is not available to go to port 2. The transformers used in the directional coupler are critical to its performance. If the transformers are wound on shotgun cores, usually only one core is used.