Shortwave Radio
On December 11, 1901, Guglielmo Marconi amazed the world when he successfully transmitted a Morse code signal 2,137 miles (3,440 km) from England to Canada using radio waves. The feat was not only remarkable but, according to the experts, impossible.
Scientists had been dabbling with radio waves ever since 1885, when German physicist Heinrich Rudolf Hertz (1857-1894) used a spark-gap to create them. Nine years later English physicist Oliver Lodge invented the coherer, a device to detect Hertzian waves, and in 1895 Russian physicist Aleksandr Popov invented an antenna to send and receive signals.
It was known that radio waves were a part of the electromagnetic spectrum, located below the wavelengths of visible light. Although radio waves cannot be detected with the eye, they were expected to behave in the same manner as light waves; that is, they should move in straight lines and not bend around corners. It was believed that the curvature of the earth would limit radio transmissions to a distance of 200 miles (300 km). Marconi's feat sent the scientists scrambling for an explanation.
Arthur Kennelly (1861-1939) in the United States and Oliver Heaviside (1850-1925) in England independently suggested there might be a layer of charged particles in the upper atmosphere that were responsible for reflecting the radio waves. This layer became known as the Kennelly-Heaviside layer. In 1924 Edward Appleton (1892-1965) discovered the existence of the Kennelly-Heaviside layer, now called the ionosphere, about 60 miles (100 km) high. In 1926 Breit and Tuve discovered there were four individual layers which were named (from bottom to top) D, E, F1 and F2.
Meanwhile scientists had been studying radio waves from the turn of the nineteenth century. It had become accepted that those radio wavelengths that were in the Very Low Frequency, Low Frequency, and Medium Frequency ranges were best suited for communication. The High Frequency waves, know as "short waves" were of no commercial value and were ignored.
In addition to the scientists, many amateurs had become interested in radio communication. There were no government regulations and anyone with the interest (and money) could build their own radio sets. With the start of World War I, however, amateurs in Europe and the United States were abruptly prohibited from using their equipment. Restrictions in the United States were finally lifted on October 1, 1919, thanks to the efforts of amateurs such as Hiram Maxim. The amateurs discovered that as the radio wavelengths became shorter (increased in frequency), they traveled farther, even using low power. Obviously the ionosphere was very efficient at reflecting these wavelengths, making communication on a global scale possible. By the 1930s, world-wide shortwave communication had been established.
Shortwave radio networks, such as the United StatesÕ Voice of America, air in numerous languages, reaching areas not able to be served by amplitude modulation (AM) frequency modulation (FM) radio broadcasts. Though shortwave is basically limited to voice (and not music) transmissions due to its low-fidelity nature, it performs an important international information dissemination service. For example, during the events surrounding the end of communism in the Soviet Union in the early 1990s, shortwave was one way of hearing news from the country itself. Sometimes news can be learned before other media picked up on the story. In the 1990s, shortwave radioÕs future seemed threatened by digital satellite delivery of radio on the L band, but if such a network develops, its transmissions can be easily blocked, while shortwave radio cannot. Between 10 and 20 million shortwave radio sets were in use in the United States in 1995.
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