CHAPTER FOUR
Transmission Lines and Components
In this chapter, basic concepts and design equations for microstrip lines, coupled microstrip lines, discontinuities, and components useful for design of filters are briefly described. Another two types of planar transmission lines, that is, coplanar waveguide (CPW) and slotlines are also included in this chapter as there is an increasing interest in developing planar filters using mixed planar transmission-line components. Although comprehensive treatments of these topics can be found in the open literature, they are summarized here for easy reference.
4.1 MICROSTRIP LINES
4.1.1 Microstrip Structure
The general structure of a microstrip is illustrated in Figure 4.1. A conducting strip (microstrip line) with a width W and a thickness t is on the top of a dielectric substrate that has a relative dielectric constant εr and a thickness h, and the bottom of the substrate is a ground (conducting) plane.
FIGURE 4.1 General microstrip structure.
4.1.2 Waves in Microstrip
The fields in the microstrip extend within two media — air above and dielectric below — so that the structure is inhomogeneous. Because of this inhomogeneous nature, the microstrip does not support a pure TEM wave. This is because a pure TEM wave has only transverse components and its propagation velocity depends only on the material properties, namely, the permittivity ...