17 August 2009

Long-distance radiotelephony

The invention: The first radio transmissions fromthe United States to Europe opened a new era in telecommunications. The people behind the invention: Guglielmo Marconi (1874-1937), Italian inventor of transatlantic telegraphy Reginald Aubrey Fessenden (1866-1932), an American radio engineer Lee de Forest (1873-1961), an American inventor Harold D. Arnold (1883-1933), an American physicist John J. Carty (1861-1932), an American electrical engineer An Accidental Broadcast The idea of commercial transatlantic communication was first conceived by Italian physicist and inventor Guglielmo Marconi, the pioneer of wireless telegraphy. Marconi used a spark transmitter to generate radio waves that were interrupted, or modulated, to form the dots and dashes of Morse code. The rapid generation of sparks created an electromagnetic disturbance that sent radio waves of different frequencies into the air—a broad, noisy transmission that was difficult to tune and detect. The inventor Reginald Aubrey Fessenden produced an alternative method that became the basis of radio technology in the twentieth century. His continuous radio waves kept to one frequency, making them much easier to detect at long distances. Furthermore, the continuous waves could be modulated by an audio signal, making it possible to transmit the sound of speech. Fessenden used an alternator to generate electromagnetic waves at the high frequencies required in radio transmission. It was specially constructed at the laboratories of the General Electric Company. The machine was shipped to Brant Rock, Massachusetts, in 1906 for testing. Radio messages were sent to a boat cruising offshore, and the feasibility of radiotelephony was thus demonstrated. Fessenden followed this success with a broadcast of messages and music between Brant Rock and a receiving station constructed at Plymouth, Massachusetts. The equipment installed at Brant Rock had a range of about 160 kilometers. The transmission distance was determined by the strength of the electric power delivered by the alternator, which was measured in watts. Fessenden’s alternator was rated at 500 watts, but it usually delivered much less power. Yet this was sufficient to send a radio message across the Atlantic. Fessenden had built a receiving station at Machrihanish, Scotland, to test the operation of a large rotary spark transmitter that he had constructed. An operator at this station picked up the voice of an engineer at Brant Rock who was sending instructions to Plymouth. Thus, the first radiotelephone message had been sent across the Atlantic by accident. Fessenden, however, decided not to make this startling development public. The station at Machrihanish was destroyed in a storm, making it impossible to carry out further tests. The successful transmission undoubtedly had been the result of exceptionally clear atmospheric conditions that might never again favor the inventor. One of the parties following the development of the experiments in radio telephony was the American Telephone and Telegraph (AT&T) Company. Fessenden entered into negotiations to sell his system to the telephone company, but, because of the financial panic of 1907, the sale was never made. Virginia to Paris and Hawaii The English physicist John Ambrose Fleming had invented a twoelement (diode) vacuum tube in 1904 that could be used to generate and detect radio waves. Two years later, the American inventor Lee de Forest added a third element to the diode to produce his “audion” (triode), which was a more sensitive detector. John J. Carty, head of a research and development effort at AT&T, examined these new devices carefully. He became convinced that an electronic amplifier, incorporating the triode into its design, could be used to increase the strength of telephone signals and to long distances. On Carty’s advice, AT&T purchased the rights to de Forest’s audion. A team of about twenty-five researchers, under the leadership of physicist Harold D. Arnold, were assigned the job of perfecting the triode and turning it into a reliable amplifier. The improved triode was responsible for the success of transcontinental cable telephone service, which was introduced in January, 1915. The triode was also the basis of AT&T’s foray into radio telephony. Carty’s research plan called for a system with three components: an oscillator to generate the radio waves, a modulator to add the audio signals to the waves, and an amplifier to transmit the radio waves. The total power output of the system was 7,500 watts, enough to send the radio waves over thousands of kilometers.The apparatus was installed in the U.S. Navy’s radio tower in Arlington, Virginia, in 1915. Radio messages from Arlington were picked up at a receiving station in California, a distance of 4,000 kilometers, then at a station in Pearl Harbor, Hawaii, which was 7,200 kilometers from Arlington. AT&T’s engineers had succeeded in joining the company telephone lines with the radio transmitter at Arlington; therefore, the president of AT&T, Theodore Vail, could pick up his telephone and talk directly with someone in California. The next experiment was to send a radio message fromArlington to a receiving station set up in the Eiffel Tower in Paris. After several unsuccessful attempts, the telephone engineers in the Eiffel Tower finally heard Arlington’s messages on October 21, 1915. The AT&T receiving station in Hawaii also picked up the messages. The two receiving stations had to send their reply by telegraph to the United States because both stations were set up to receive only. Two-way radio communication was still years in the future. Impact The announcement that messages had been received in Paris was front-page news and brought about an outburst of national pride in the United States. The demonstration of transatlantic radio telephony was more important as publicity for AT&T than as a scientific advance. All the credit went to AT&T and to Carty’s laboratory. Both Fessenden and de Forest attempted to draw attention to their contributions to long-distance radio telephony, but to no avail. The Arlington-to-Paris transmission was a triumph for corporate public relations and corporate research. The development of the triode had been achieved with large teams of highly trained scientists—in contrast to the small-scale efforts of Fessenden and de Forest, who had little formal scientific training. Carty’s laboratory was an example of the new type of industrial research that was to dominate the twentieth century. The golden days of the lone inventor, in the mold of Thomas Edison or Alexander Graham Bell, were gone. In the years that followed the first transatlantic radio telephone messages, little was done by AT&T to advance the technology or to develop a commercial service. The equipment used in the 1915 demonstration was more a makeshift laboratory apparatus than a prototype for a new radio technology. The messages sent were short and faint. There was a great gulf between hearing “hello” and “goodbye” amid the static. The many predictions of a direct telephone connection between New York and other major cities overseas were premature. It was not until 1927 that a transatlantic radio circuit was opened for public use. By that time, a new technological direction had been taken, and the method used in 1915 had been superseded by shortwave radio communication.


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