COAXIAL CABLE VS. MICROWAVE

by

R. J.. Reiman, Historian

The lack of intercity transmission facilities impeded the growth of TV broadcasting after World War II. Networking was essential for intercity video transmission. Video circuits needed MHz bandwidths, compared to KHz for radio. AT&T considered the two technologies available: coaxial and microwave.

Coaxial cable, with a center stranded or solid wire surrounded by a cylindrical conductor with insulation in between, was capable of transmitting broadband video signals over long distances as a replacement to twisted pairs which were used for voice circuits. Amplifiers or repeaters were necessary to compensate for cable losses which attenuated the signal.

Microwave transmission systems had been developed during World War II and operated at frequencies of about 4,000 MHz and these systems could handle the wideband requirements of TV. If the use of microwave became common, however, AT&T could lose its monopoly of intercity connections.

AT&T settled for a system using both methods. Cable became the method for short, high-density routes, and microwave for longer ones and for low-density needs. The cable with repeaters was capable of transmitting monochrome TV and the microwave of transmitting both monochrome and color to the industry's National Television Standards Committee (NTSC) standards. These would later be adopted by the FCC for this purpose.

The AT&T microwave design was extremely conservative and costly, but very reliable. Repeater stations were installed at intervals of about 25 miles and redundant facilities were provided to be single-failure proof. By 1960, the microwave network reached most of the major cities and largely replaced cable for intercity video transmission.

Pipelines, electric utilities and other bulk users of communications services began to install private microwave systems and this first step in competition to AT&T would lead to divesture and deregulation. Later, a brand new technology of fiber optics would become a factor in transmission of color TV.