Updated: Jul 23
Radio amateurs today use coaxial cable to connect the equipment to an antenna. Prior to its invention, the common transmission line was an open-wire balanced feeder made from wire and insulating spacers. The history of coaxial cable is an interesting topic.
First recorded patent for coaxial line
The first known patent granted for coaxial the transmission line was a British patent in 1880 to Oliver Heaviside, a self-taught British electrical engineer. Those astute in history will recognize the name as the one due to the Kennelly-Heaviside layer; we now call the ionosphere, which Heaviside in conjunction with Kennelly discovered the reflecting medium-high above Earth. Heaviside was the nephew of the British inventor Sir Charles Wheatstone, the co-inventor of the British telegraph system and known for his invention of the Wheatstone Bridge to measure resistance.
Heaviside’s coaxial cable was a copper tube, which formed the outer of the line. The inner concentric (coaxial) conductor was a copper wire which was supported by insulating discs to keep the central wire a constant distance from the inner of the copper tube. The major dielectric was air, giving low loss to signals traveling along with it.
The use of the word “cable” as we know it today for coaxial lines implies a flexible type of conductor. In the old days, the word cable meant anything that contained wires, such as mains cable, or telephone cable with multiple pairs. Similarly, transatlantic cables could be multiple pairs or coaxial lines or a mixture of both types, today we also have fiber optic cable, but the “conductors” are strands of glass or plastic to act as a waveguide for a light source.
Heaviside was a prolific experimenter and he coined the names of several items used today. He invented the word impedance, admittance, conductance, permeability, inductance, reactance, reluctance, and permittance, which we still use today. (He seemed to like the use of the letters ANCE at the end of the item). He also solved Maxwell’s equations and invented differential equations in order to perform this work.
Heaviside’s coaxial transmission lines found many uses; not the least was the transatlantic cables used to carry telegraph and later telephone traffic over vast distances. The first transatlantic telegraph cable was a normal wire cable laid in the late 1800s and was a disaster. The inventor was Lord Kelvin, earlier Sir William Thompson. He, however, did not know about distributed capacitance, inductance and impedance, the signal loading on this cable was so high it limited the signaling speed to about 5wpm. Unfortunately, the cable failed about 2 weeks after it was laid and was never re-established. It was a financial disaster for the sponsors.
A German patent in 1884 was granted to Ernst Werner von Siemens for a concentric coaxial transmission line similar to Heaviside’s design, but little detail is known about this patent or whether the cable was ever put into service.
US patent for coaxial cable
The next known patent for coaxial cable was granted to two American engineers. This patent granted in 1931 was almost the same as Heaviside’s original design but had a subtle difference. It was semi-flexible and could be coiled more easily. The patent diagram clearly shows a similar construction to Heaviside’s design with an outer copper jacket and inner wire supported by insulating discs. U.S. Patent No. 1,835,031 for a “concentric conducting system” was aw