**3. Coupled-Line Couplers (CLCs)**

#### **3.1 Conventional CLCs**

Coupled line couplers are indispensable components in radio frequency (RF)/microwave communication systems. In these structures two unshielded transmission lines are close together, as indicated typically in Fig. 3, and power can be coupled between the lines. Such lines are referred to as coupled transmission lines (Mongia et al., 1999). The coupler is frequently utilized in a variety of circuits including modulators, balanced amplifiers, balanced mixers, and phase shifters. Rapidly expanding applications such as modern wireless technology continue to challenge couplers with extremely stringent requirements high performance, broad bandwidth, and small size (Pozar, 2004).

In general, two types of CLCs have been proposed; backward and forward CLCs. When the coupled port is located on the same side of the structure as the input port and power is subsequently coupled backward to the direction of the source, this coupler is conventionally called a backward coupler and otherwise the CLC is called forward coupler (Mongia et al., 1999).

On the other hand, two types of edge-coupled backward CLCs have been presented. The first is the symmetrical coupler. When the two lines constituting a CLC are the same, the structure is called symmetric. In the symmetric structures, coupling mechanism is based on the difference between the characteristic impedances of the even and odd modes. The second one is the asymmetrical coupler. This coupler is asymmetrical as it is constituted of two different transmission lines. In this case, decomposition in even and odd modes is not possible anymore. The analysis becomes more difficult and the even/odd modes have to be replaced by the more general c and π modes, which are two fundamental independent modes, as described in (Mongia et al., 1999).

Fig. 3. Typical structure of a coupled-line coupler (CLC).

Symmetrical coupled lines represent a very useful but restricted class of couplers. In many practical cases, it might be more useful or even necessary to design components using asymmetrical coupled lines. For example, in some situations, the terminal impedance of one of the coupled lines may be different from those of the other. It may then be more useful to choose two coupled lines with different characteristic impedances. Also, an asymmetrical coupled-line coupler has usually broader bandwidth than symmetrical one (Mongia et al., 1999).
