RF and Microwave Circuit Design

1RF Transmission Lines

1.1 Introduction

Transmission lines, in the form of cable and circuit interconnects, are essential components in RF and microwave systems. Furthermore, many distributed planar components rely on transmission line principles for their operation. This chapter will introduce the concepts of RF transmission along guided structures, and provide the foundations for the development of distributed components in subsequent chapters.

Four of the most common forms of RF and microwave transmission line are shown in Figure 1.1.

1. Coaxial cable is an example of a shielded transmission line, in which the signal conductor is at the centre of a cylindrical conducting tube, with the intervening space filled with lossless dielectric. The dielectric is normally solid, although for higher-frequency applications it is often in the form of dielectric vanes so as to create a semi-air-spaced medium with lower transmission losses. A typical coaxial cable is flexible with an outer diameter around 5 mm, although much smaller diameters are available with 1 mm diameter cable being used for interconnections within millimetre-wave equipment. Also, for very high-frequency applications, the cable may have a rigid or semi-rigid construction. Further data on coaxial cables are provided in Appendix 1.A.
2. Coplanar waveguide (CPW), in which all the conductors are on the same side of the substrate, is also shown in Figure 1.1. This type of structure is very convenient for the mounting of active ...