12.1 Introduction

Although linear amplitude modulated fiber-optic links are the favored trunking methodology for most new systems, there are situations where they are simply not cost effective. For example, physical barriers, such as waterways, may make physical cable construction very expensive. Alternatively, the network may need to serve several small outlying communities that are widely separated so that the average home density is very low. In either of these cases, point-to-point or point-to-multipoint broadband amplitude modulated microwave links (AMLs) may offer a superior solution. This chapter will cover the basic operation of this equipment and the essential calculations and methodology required to engineer AMLs. No attempt is made to present a comprehensive manual on either internal designs of microwave equipment or the many subtleties of path engineering. Several standard reference books on the subject are available for those desiring to pursue the subject in greater depth. ^1,2,3,4

Before the commercial development of AM fiber technology, large cable systems were sometimes constructed with specially designed coaxial trunks that were optimized to cover long distances. These were known as supertrunks. Since these are seldom used in modern cable systems, we will discuss them only briefly.

12.2 Coaxial Supertrunks

Chapters 9 and 10 cover coaxial technology in general, along with the inescapable trade-offs among noise, distortion, bandwidth, and signal levels in a cascaded coaxial distribution system. Where cable systems found it necessary to cover large distances to interconnect local distribution cables in large systems, they constructed special supertrunk lines typically using some combination of large coaxial cables (typically 1.00 1.125 inches in diameter); feed-forward amplifiers (with high output power capability and low noise figures); and/or band splitting so that each of multiple parallel amplifiers carried only part of the entire spectrum.

There is no magic to coaxial supertrunks. They are just coaxial distribution lines that are designed to push the limits of performance. Since the loss of even the largest coaxial cables are about 100 times that of fiber and they are more costly to purchase and install, coaxial supertrunks are of interest for historical reasons only.

Occasionally, coaxial supertrunks have been used to carry video channels that were FM rather than AM modulated. Such trunks, however, require FM detection to recover the baseband video, then VSB-AM modulation to convert them into the format for transmission to homes. Thus, the amount of equipment required to convert many channels rivals that in a headend. FM transmission circuits, whether coaxial, fiber-optic, or microwave, are properly considered part of the headend interconnect network, not the broadband linear distribution network, and so will not...