Microwaves are a form of electromagnetic radiation, with frequencies ranging from several hundred megahertz to several hundred gigahertz and wavelengths ranging from approximately 1 to 20 centimeters. Because of their high frequencies, microwaves have the advantage of being able to carry more information than ordinary radio waves and are capable of being beamed directly from one point to another. In addition to their telecommunications applications (which include telephony and computer networking as well as television), microwaves are used in cooking, police radar, and certain military applications.

Microwave is a “line-of-sight” technology (i.e., because a microwave transmission cannot penetrate the Earth’s surface, it will not extend beyond the horizon); therefore, long-distance terrestrial transmission of messages is accomplished via a series of relay points known as “hops.” Each hop consists of a tower (often atop a mountain) with one antenna (typically a parabolic antenna) for receiving and another for retransmitting. Hops typically are spaced at 25-mile intervals.

Prior to the widespread use of communications satellites in television industries, terrestrial microwave relays frequently were used to deliver programming from broadcast networks to their affiliates or to deliver special-event programming, such as sports, to local stations. Beginning in the 1950s, terrestrial microwave relays were employed to supplement expensive telephone land lines for long-distance transmission of programming. Microwave mobile units (vans with microwave transmitters attached) have also been used in television news reporting since the late 1950s.

Microwave technology was critical to the development of the community antenna television (CATV) industry. Before microwave technology became available in the early 1950s, local CATV systems were limited in channel selection to those stations that could be received over the air via tall “master” antennas. In such situations, a CATV system could flourish only within 100 to 150 miles of the nearest broadcast television markets. Microwave relays, however, made it possible for CATV systems to operate many hundreds of miles from television stations. The new technology thus was a boon to remote communities, especially in the western United States, which could not have had television otherwise.

Microwave also introduced the possibility for CATV operators to select which broadcast signals they would carry, sometimes allowing them to bypass closer signals in order to provide their customers with more desirable programming—perhaps from well-funded stations in large cities. For this reason, it was microwave technology above all that prompted the earliest efforts by the Federal Communications Commission (FCC) to regulate CATV. By the late 1950s, some concern had been voiced by broadcasters as to the legality of the retransmission—and, in effect, sale—of their signals by CATV systems and CATV-serving microwave outfits. The most notable of these complaints resulted in the U.S. Supreme Court case Carter Mountain Transmission Co. v. FCC (1962). In 1965 and 1966, respectively, the FCC issued two bodies of regulation to govern the rapidly growing CATV industry. Both of these focused primarily on the legalities of microwave-delivered CATV programming.

The rules did very little, however, to curtail the growth of CATV (more widely known as “cable television” by the late 1960s), and microwave continued to play a key role. Throughout the United States, the signals of several independent television stations, some of which have become cable “superstations,” were delivered to cable systems by microwave. In addition, in late 1972 and early 1973, Home Box Office (HBO) began serving customers in the Northeast via two existing microwave relay networks.

Historically, then, terrestrial microwave technology accomplished many of the television programming tasks for which communication satellites are used today. Terrestrial relays still exist and serve many important functions for television. In recent years, they have also been enlisted for nontelevision applications, such as computer networking and the relaying of long-distance telephone messages. Some companies that began as terrestrial microwave outfits have also diversified into satellite program delivery.