In the early 20th century, the United States became increasingly “wired.” Telephone and electrical wires began to crisscross the landscape, connecting homes and businesses. As telephone traffic increased, better wires to carry the calls were needed. AT&T, an American Bell subsidiary formed to provide telephone and long-distance service, constructed its first long-distance network in the late 1800s from copper wiring mounted on telephone poles. Calls passed through the wiring in the form of analog signals. For a single call to be made, two wires were necessary, the first carried the signal one way, the second carried the signal in the opposite direction. Due to the electrical resistance that the signals encountered, early telephone wires, especially those connecting distant locations, were relatively thick.
Advances such as loading coils and vacuum-tube repeaters reduced resistance and allowed for thinner and longer telephone wires. But there was still a limit to how many calls you could send down a wire at one time: All human voices share the same basic frequency range, and two concurrent signals would overlap and cause distortion.
By 1918, engineers realized you could fit multiple calls on a single wire if you could change their frequencies. They did this using a modulator, then superimposed, or “stacked,” multiple signals on a single wire. Since there was no overlap in frequencies, the calls remained distinct. At the receiving end, the signals were demodulated back to their natural frequencies so the voices could be heard. This was known as the carrier-current system.
Despite these improvements, a more significant breakthrough was needed. By the 1920s, one in three Americans had a phone. In order to meet the ever-increasing demand for telephone service and to have wiring suitable for the developing technology of television, AT&T engineers began searching for a better kind of wire. Working in an AT&T telephone laboratory, Lloyd Espenschied and Herman Affel found exactly what they were looking for. The pair invented coaxial cable, a form of wire capable of efficiently carrying the high-frequency signals required for broadband communications. Coaxial cable consists of an inner conductive core made of copper surrounded by an insulator, which in turn is sheathed in another conductive layer, usually formed from braided copper. An outer jacket surrounds the outer conductive layer. Many such cables can be grouped in yet another casing. The name “coaxial” stems from the fact that both conductors share a single axis.
Espenschied and Affel filed a patent for coaxial cable in 1929, which was granted in 1931. The design was a great improvement over other types of wire available at the time. Coax, as techies often call it, does not experience interference from other electromagnetic fields, nor does it interfere with them. This is because the signal carried by coaxial cable is restricted to an electromagnetic field between the cable’s inner and outer conductive layers.
In the mid 1930s, AT&T established an experimental coaxial cable line between New York and Philadelphia. It was used to transmit the human voice, but moving pictures were not far behind. In 1941, the company put the new wire into commercial use. The initial line stretched from Minneapolis, Minnesota, to Stevens Point, Wisconsin. It could accommodate 480 telephone calls or a single TV channel. With further improvements, call and channel capacity increased considerably. In 1956, coaxial cable made the first transatlantic telephone system possible. Dubbed the TAT-1, the submarine cable connected Scotland to Newfoundland.
In 2006, Espenschied and Affel were inducted into the U.S. National Inventors Hall of Fame for their invention. The use of coax has declined, especially since fiber optics has become affordable. It is still preferred, however, for applications such as bringing cable television into homes and connecting home video equipment.