Principles of Shortwave Radio Communication Are Discovered

Date 1919

Theoretical principles proposed and tested by physicists, coupled with technological and empirical experimentation, made possible reliable long-distance radio communication via shortwaves.

Locale Europe; North America

Key Figures

• James Clerk Maxwell (1831-1879), Scottish physicist
• Heinrich Hertz (1857-1894), German physicist
• Sir Oliver Joseph Lodge (1851-1940), English physicist
• Guglielmo Marconi (1874-1937), Italian engineer and inventor

Summary of Event

When James Clerk Maxwell first postulated the theory that the flow of an electric current created an accompanying electromagnetic force, he did so without recourse to empirical experimentation and as an enlargement on his theories of light waves and optics. His mathematical and theoretical suppositions were published in the celebrated 1864 article “A Dynamical Theory of the Electromagnetic Field,” in which he maintained that electromagnetic waves operated under the same laws and in the same manner as light. As a theoretician, Maxwell neither proposed nor foresaw any practical uses for the electromagnetic (radio) waves his formulas described.

Heinrich Hertz, however, was a physicist with a distinctly experimental approach. Basing his work in part on Maxwell’s theories, he designed and built what is generally acknowledged to have been the first practical demonstration of transmitted and received radio waves. His design consisted of a spark gap oscillator and a loop receiving aerial, in which a small opening allowed the received “signal” to spark. Because of the space limitations of his laboratory, the aerials he constructed were relatively small. In fact, his initial experiments, which demonstrated the practical ability to transmit radio waves and detect them at a distance from the transmitter, were conducted in what is now called the very high frequency (VHF) range; that is, the waves oscillated approximately 100 million times per second or at a frequency of 100 megahertz. Because his experiments were carried out at a frequency that operates best in direct, line-of-sight fashion, his apparatus was more important for the physical phenomenon it demonstrated, not as an immediately useful communications device.

In the years after Hertz’s results were published, much work was done in extending the sensitivity of radio wave devices; as a result, the range over which such waves could be sent and received was increased. Particularly important was the work of Sir Oliver Joseph Lodge, who developed and refined the radio wave detector, then called a “coherer,” a device that would complete an electrical circuit (which, in turn, might be used to ring a buzzer) in the presence of radio waves of sufficient intensity. Second, Lodge emphasized the significance of “syntony,” or tuned resonance, in the radio wave spectrum. The design of spark gap transmitters used at the time was such that their radiated energy was spread over a broad number of frequencies. Lodge’s insistence on tuning circuits to “focus” transmitted and received radio wave energy was an important step in making simultaneous communications between many stations practical. In fact, Lodge received patents for both of these achievements.

The progress made by Lodge was the basis on which Guglielmo Marconi (whose name is associated most often with the early days of “wireless” technology) was able to establish a corporation whose sole product was radio equipment and communications services. Marconi relied less on theoretical physics than on empirical research; that is, he tried what worked and refined it as best he could, even if such an approach led him to investigate unproductive ideas. His refinements of the coherer, his use of resonant circuits, and his practical experiments with transmissions on low frequencies enabled him to conduct the first transatlantic radio transmission on December 12, 1901, thus demonstrating the possibilities the medium offered. Despite the distance covered in that demonstration (nearly 2,700 kilometers, or approximately 1,678 miles, from England to Newfoundland), however, the first practical application of early radio was in maritime communications between ships at sea and ship to shore at distances up to 300 kilometers (about 186 miles). Beyond that range, communications were not reliable.

The establishment of reliable long-distance radio communications became Marconi’s goal. He found little guidance from theoretical physics in this area. Nineteenth century physicists had supposed earlier that electromagnetic forces necessarily traveled through some sort of medium, which, although of unknown composition, they called the “ether.” Only at the beginning of the twentieth century did the experience of radio communicators suggest that the particular frequency range of a given radio wave had much to do with the distance it seemed to cover and that no other medium was necessary to account for electromagnetic wave propagation. Today, the radio frequency spectrum is divided into various “bands,” or ranges of frequencies, that share similar propagation characteristics. At the dawn of the radio age, however, it seemed clear only that radio waves with wavelengths greater than 200 meters (656 feet) were the most reliable for communications up to 300 kilometers. Consequently, very low frequency (VLF, 10 to 30 kilohertz), low-frequency (LF, 30 to 300 kilohertz), and medium-frequency (MF, 300 to 3,000 kilohertz) ranges were employed by corporations and by governments for their commercial and military communications. Prior to 1919, the wavelengths below 200 meters (that is, frequencies above 1.5 megahertz) were held to be of no commercial value. This band of frequencies from 3 to 30 megahertz (the high-frequency, or HF, band) is known as the “shortwave” band because their wavelength is, in comparison with wavelengths then generally in use, considerably shorter.

Prior to World War I, there was virtually no governmental regulation of radio transmissions, and many individuals with sufficient inclination, money, and interest in this new technology built their own transmitting and receiving apparatuses. When the war broke out, amateurs in Europe and the United States were forbidden to use their equipment. It was only with the active lobbying of amateur investigators such as Hiram Percy Maxim that, on October 1, 1919, hobbyist radio experimenters in the United States were allowed once again to make their broadcasts to one another.

Amateur operators undertook the same types of communications experiments on the higher frequencies as Marconi’s corporation was conducting. It soon became clear that as one went higher in frequency (that is, as the wavelength became shorter), greater distances could be covered. A combination of improved antennas, more powerful transmitters, and more sensitive and selective receiving equipment proved that transatlantic communications were possible. As continuing experiments were to reveal, there is a region of electrically charged ions and electrons located about 50 to 400 kilometers (about 31 to 249 miles) above and encircling the earth. This region is called the ionosphere and it acts to refract or bend radio waves below 30 megahertz. (The ionosphere is a complex series of differing layers whose electrical charges vary on a daily basis as well as in response to the eleven-year cycle of sunspot activity.) Whereas VLF, LF, and MF waves tend to radiate along the ground and will follow the curvature of the earth, HF waves—shortwaves—are bounced back by the ionosphere, generally at distances quite far from the transmitting station. If an operator chooses carefully a frequency of operation within the shortwave band, even low-power transmitters can be heard around the world as a consequence of waves reflected multiple times (or “hops”) by the ionosphere.

Significance

Throughout history, humanity has continually sought ever faster and more reliable means of communication. With the rise in scientific and engineering research that preceded and accompanied the industrial age in Europe and North America came not only the ability but also the perceived need to send messages over great distances. Samuel F. B. Morse’s invention of the telegraph system in the 1860’s thus met with ready acceptance and, by the beginning of the twentieth century, undersea cables linking the United States to Europe were already in operation. Radio communications were seen initially as little more than an oddity inasmuch as early radio systems had limited range, were not always easy to operate, and were not secure (because radio waves were transmitted through open air, anyone within range of a transmitter and inclined to build a receiver could intercept those radiotelegraphic signals).

The business of radio communications was first limited primarily to maritime use. The sinking of the Titanic in 1912 provided dramatic evidence of how useful such communications could be and impetus toward further refinement of the technology. As the Titanic began to sink, its wireless operator transmitted an emergency call for aid. A ship about 130 kilometers (81 miles) distant happened to be equipped with radio as well and, although it took hours, came to assist the ship, saving hundreds of lives. As a result of this incident, in both the United States and Europe, laws were passed to equip ships with operating networks of shore stations. Nevertheless, such communications were limited to relatively short distances. It was to be the role of the shortwave band to encourage long-distance radio communications.

Among the “amateur” experimenters in this field were most of the noted engineers and scientists of radio, who communicated their findings in their amateur radio journal, QST. In the 1920’s, AM (amplitude modulation) radio broadcasting became an important part of everyday life. Broadcasters were quick to realize that the shortwave frequencies offered them the chance to transmit not only to a local audience in a given city but also to a large area. Because it has always been impractical for one transmitter on any one shortwave frequency to service the entire world adequately, both nations and individual corporations soon erected numerous transmitting sites and appropriate directional antennas operating on different frequencies.

In order to increase the amount of information that could be sent by shortwave radio and to “encode” those data to keep all but the most determined from listening in on transmissions, alternate forms of communication were developed. Radioteletype, for example, uses mixtures of two different tones in patterns not at all like the Morse code to transmit data quickly; it soon became standard for international news services, telegraph companies, and governments (who further encrypted their messages using ciphers). The airline industry found shortwave radio communications to be of considerable value in keeping in touch with airplanes en route and in communicating with foreign airports to learn of weather conditions. In the 1930’s, facsimile devices were invented that allowed international transmission of pictures and diagrams. In short, the shortwave band was exploited quickly for its ability to provide long-distance radio communication.

By the mid-1930’s, it became clear that uncontrolled radio use, especially in the shortwave band, was growing in an alarming fashion, and nations enacted laws to regulate the limited radio spectrum at home. Because radio waves—especially the far-reaching shortwaves—do not respect borders, however, an international organization was formed to divide the shortwave spectrum into various subbands for different uses (commercial, amateur, governmental, international maritime, and the like) in different regions. Today, the International Telecommunication Union’s Radiocommunication Sector holds a World Radiocommunication Conference every two or three years to review the international treaty governing radio spectrum use.

By the late twentieth century, an alternative technology had been developed that somewhat reduced the importance of the shortwave band. Geosynchronous satellites, which reliably carry the broad signals necessary for television transmissions and for multiplexed voice and computer data transmissions, gained widespread use. Nevertheless, demands made on the shortwave spectrum scarcely diminished. Whereas international telephone and television transmissions shifted to satellite systems, the shortwave band continued to provide a reliable and inexpensive medium for individuals, small organizations, and nations with limited financial resources. Indeed, the fact that there are so many international shortwave broadcasts intended for individual listeners around the world demonstrates the interest and use this technology still garners in the twenty-first century.

Bibliography

Aitken, Hugh G. J. The Continuous Wave: Technology and American Radio, 1900-1932. Princeton, N.J.: Princeton University Press, 1985. Continuation of Aitken’s earlier book, cited below, concentrates on the establishment of broadcast services and the development of radio technologies employed in the late 1980’s. Accounts for the economics of radio as well by following the rise of large American technology and broadcasting corporations. Well illustrated and indexed.

‗‗‗‗‗‗‗. Syntony and Spark: The Origins of Radio. New York: John Wiley & Sons, 1976. Discusses both the physical and the experimental bases of the discovery of spark gap radio communications, the earliest phase of radio. Emphasizes the processes by which physics at first anticipated the discovery of radio waves and, later, was surprised by the discovery of the ionosphere. Includes numerous diagrams, illustrations, and index.

Hale, Bruce S., ed. The ARRL Handbook for Radio Communications, 2006. Newington, Conn.: American Radio Relay League, 2005. Massive volume, updated annually since its first appearance in 1926, covers virtually all aspects of radio (and television) communication for the enthusiast. Includes heavy emphasis on shortwave frequency communications, construction and alignment of equipment, and erection of various antennas. Presents fundamentals of electricity, waves and their propagation, and electronics as well as amateur operating principles. Well illustrated and indexed.

Head, Sydney W., Thomas Spann, and Michael A. McGregor. Broadcasting in America: A Survey of Electronic Media. 9th ed. Boston: Houghton Mifflin, 2000. The standard introduction to the institutions of radio and television in the United States. Begins with an analysis of the invention of wireless radio broadcasting.

Maclaurin, W. Rupert, and R. Joyce Harman. Invention and Innovation in the Radio Industry. 1949. Reprint. New York: Arno Press, 1976. Readable account of the early history of radio communications. Includes discussion of individuals not mentioned elsewhere. Illustrated and indexed.

O’Hara, J. G., and W. Priche. Hertz and the Maxwellians. London: Peter Peregrinus, 1987. Brief, eminently readable history of the discovery of electromagnetic waves. Introduction is followed by excerpts from the correspondence of Hertz and the scientists with whom he discussed and evaluated his findings. Includes illustrations and photographs.

Terman, Frederick E. Radio Engineering. 3d ed. New York: McGraw-Hill, 1947. Explanations of radio technology of historical interest only; discussions of radio wave propagation and antennas as valid in the twenty-first century as at the time of publication. Well illustrated.

Weightman, Gavin. Signor Marconi’s Magic Box: The Most Remarkable Invention of the Nineteenth Century and the Amateur Inventor Whose Genius Sparked a Revolution. Cambridge, Mass.: Da Capo Press, 2003. In addition to describing Marconi’s experiments, focuses in large part on the competition that existed among the various inventors who were pursuing the goal of wireless communications at the same time as Marconi. Includes photographs.