RF and Microwave Circuit and Component Design for Wireless Systems

CHAPTER FIVE

Transmission Lines and Impedance Matching Techniques

5.1 INTRODUCTION

At low RF frequencies, a wire or a line on a printed circuit board can be used to connect two electronic components. At high RF or microwave frequencies, the wire becomes lossy, radiates power, and has significant reactance and is difficult to model accurately. Special transmission lines and waveguides with well-defined characteristics are required to connect microwave components and circuit elements. Sections of these transmission lines constitute the basic building blocks of microwave circuits and microwave integrated circuits. Many transmission lines and waveguides have been proposed and used. They can be classified into two categories: conventional and integrated circuit. Figure 5.1 shows cross-sectional views of different transmission lines and waveguides. A qualitative comparison of some of these structures is given in Table 5.1.

The choice of a suitable transmission medium for constructing microwave circuits, components, and subsystems is dictated by electrical and mechanical trade-offs. Electrical trade-offs involve such parameters as transmission line loss, dispersion, higher-order modes, range of impedance levels, power levels, maximum operating frequency, and suitability for component and device implementation. Mechanical trade-offs include ease of fabrication, tolerance, reliability, flexibility, weight, and size. In many applications, cost is an important consideration.

In this chapter, ...