TABLE OF CONTENTS To understand the operation of a directional coupler, you must understand the terms directional and coupler.
First, let us look at the term coupler. If two transmission lines are placed close together, energy will be "coupled" from one line to the other, as shown in Figure 4.1. In the Figure 4.1, two transmission lines (A and B) are spaced a certain distance apart ( S). There is no direct connection between the two lines. If the spacing is small enough, some of the energy applied to transmission line A will be seen on transmission line B. The amount of energy on transmission line B depends on how close the lines are to each another. The closer they are, the more energy that is coupled to the second line, transmission line B. The farther away they are, the less energy that is coupled. Thus, we can say that a coupler is a device that consists of two transmission lines with no direct connection, placed very close to each other so that a portion of the energy in one line is present in the second line. For most applications, that is not really what you want in an RF or microwave circuit. Usually you make every effort to keep transmission lines away from one another so that coupling does not occur. However, when you want to sample energy to check the power level or frequency of a specific signal, a coupler is an ideal...
