You may have heard of directional couplers but not really known what is meant by the term. This is true for most people, so it is nothing to really be alarmed about. Here is a basic breakdown of everything you need to know about directional couplers.
Essentially, the couplers are circuits consisting of four-ports where a single port is completely isolated from the input port. By consisting of these attributes, they are defined as networks of passive reciprocity. Each of the four ports is matched to the other three and the circuit is completely lossless.
There are several ways in which the realization of the directional coupler applications can be solidified. These ways include coaxial, stripline, wavelength, and microstrip. They can be used in this way to sample the signal. This sampling technique can include both the wave reflections and the actual incident itself. The couplers also generally work by using the individual properties distributed by microwave circuits. This property of coupling tends to exclusively work within a distance of a quarter-wavelength or in portions of multiple quarter-wavelengths of the device doing the distribution.
The four ports of the directional coupler each have individual names and attributes. The first port is known as the through port. This port is where the signal of incidence mainly exists. The second port is known as the coupled port. This port is where a portion of the input signal will appear and represent itself in the form of decibels, in most cases. The third port is called the isolated port which is where the signal will be terminated. The fourth port, obviously, is called the input. This port is where the signal originates. In some instances, the end user may choose to reverse the direction of the signal so that it actually exits through the input port but this is not a common occurrence.
