9.1 Introduction

Cable television s technical roots are the distribution of analog television signals to the antenna terminals of customers receivers. The least expensive way to accomplish that was to avoid the need for in-home equipment by carrying each video stream on a different, standard television channel so that subscribers could use their existing TV sets to select and view programs. Although cable systems have evolved since then into sophisticated multiservice networks using combinations of linear fiber optics and coaxial cable for transmission, the essential characteristics have remained unchanged. In particular, the last portion of the network is still a linear, broadband coaxial network that simultaneously carries many modulated RF signals, each occupying a different band within the spectrum frequency division multiplexing (FDM) as discussed in several previous chapters. In modern systems, many of the modulating signals may be digital rather than analog; however, the network must still be linear to avoid generation of unwanted additional signals due to mixing among the desired carriers. This is discussed in detail in Section 9.3.3.

This chapter will treat coaxial network technology in detail, including cable, amplifiers, passive components, and powering systems. Basic linear network concepts will be introduced that will also apply to the linear fiber-optic links whose unique characteristics are discussed in Chapter 11 and to the microwave links covered in Chapter 12. Chapter 10 will discuss coaxial design practices and cascaded performance.

9.2 Coaxial Cable

Coaxial cable is not the only option for transmitting broadband RF signals. Indeed, many early systems were built using open, parallel-wire balanced transmission lines, and a few even used an ingenious single-wire cable known as G-line, which had only a center conductor and dielectric. Coaxial cable, however, offers the advantages of a high degree of shielding, coupled with relatively low cost and moderate signal loss. For that reason, most modern RF systems use coaxial cable as at least one of the physical media through which signals are transported.