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The Quality that Made Radio Popular
Although it was the late 1920's before vacuum tube shortwave transmitters began to replace Alexanderson's mighty alternators, exploratory work using vacuum tubes as amplifiers in radio receiving equipment began around 1900.Lee DeForest, an Iowa preacher's son who earned a Yale PhD, announced his Audion vacuum tube in a Scientific American article in 1906. Although he acknowledged in this article that he didn't have a "completely satisfactory theory" as to why the tube amplified the reception of radio signals, understanding this curious tube led other researchers, such as Edwin Armstrong, to significant breakthroughs in amplifying both radio transmissions and reception before, during, and after WWI.
Armstrong was 11 years old when Marconi's trans-Atlantic transmission occurred. It fired his imagination and he became a collector and creator of homemade wireless equipment. As a teenager his patient parents allowed him to build a 125 foot antenna in the yard so he could further his studies on radio. He was 16 when DeForest announced his Audion tube and one of these fragile, expensive tubes was added to his study equipment.
In 1912, as a junior at Columbia University he continued his interest in radio and the Audion tube by inventing a regenerative circuit that fed part of the current back to the grid in the tube. This strengthened the incoming signal. In fact, Armstong received distant stations so loudly that he could hear them without headphones - something unheard of at that time.
Further experiments led him to discover that by increasing the feedback into the tube even more he could produce rapid enough oscillations for the tube to act as a transmitter as well as a receiver. From this work Armstrong's regenerative circuit became the basis for continuous wave transmitters that are still at the heart of radio operations today.
When Armstrong entered the Army Signal Corp in WWI he did not leave the development of radio behind. Instead, as in so many areas of technology, work done for the U. S. military during times of war led to significant breakthroughs for civilian industry once the war was completed. So it was with vacuum tubes and radios during and after WWI.
In 1917, when the U.S. entered WWI, as a result of powers given to it by the Radio Act of 1912 (a law motivated in part by the Titanic disaster), the federal government shut down all private radio operations in the United States. This was not as drastic a measure as it might seem today since the commercial broadcasting we now know did not begin until 1920. But it was major blow to the thousands of amateur or "ham" radio operators who had discovered and begun to popularize the new medium of radio. Many of these men, like Armstrong, joined the Army, Navy, or Merchant Marine in order to put their now precious skills to work on behalf of the United States.
Whereas communication in previous wars had been dependent on runners, flags, carrier pigeons, smoke signals, and other methods, WWI's commanders wanted quicker, more reliable communication with the soldier in the field. And radio had advanced enough to believe this a feasible objective if the Army Signal Corp, working with General Electric/DeForest Radio and Telephone and Western Electric, could devise a way to go from the pre-War situation in which about 400 vacuum tubes were manufactured per week to making about 20,000 reliable, powerful tubes a week.
As often happens in times of war, the impossible was achieved and General George Squier, Chief Signal Officer of the Army, reported in 1919 that:
Thus, by the end of WWI, vacuum tubes were developed to the point where they were used for "electric-wave detection, radio-frequency, and audio-frequency amplification, radiotelephony, particularly in the airplane radiophone, continuous-wave radiotelegraphy, voltage and current regulators on generators, and for other miscellaneous purposes."
Armstrong's work for the Army signal corp fell into another area. His task was to develop a way to detect enemy shortwave communications. In the process of meeting this objective, in 1918 he developed an eight-tube receiver that could amplify radio signals to a degree never known before. He named this receiver the superheterodyne circuit and it remains the basic circuit used in nearly 100% of radio and television receivers today.
Armstrong had one other great invention up his sleeve - FM radio - which both greatly improved the quality of broadcasting and played a major role in making today's cellular and PCS phones possible.
In 1935 Armstrong revealed his final great work, motivated by his own dislike of the static he constantly heard on the radio. His original paper on frequency modulation was entitled "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation." Likely he did not imagine that this advance would be resisted. But, afraid that FM would make AM radio obsolete and slow down new developments in television, Armstrong's major financial backer withdrew its financial support.
So Armstrong established his own distribution channel by building a demonstration inter-city FM relay for New England's Yankee Network. A shift in the location of the FM radio frequency, to accommodate the spectrum needs of the new television industry, made all Armstrong's FM equipment obsolete. It was not until the 1960's, after Armstrong's death, that the quality advantage of FM combined with stereo was enjoyed by most Americans.
But, beyond the quality that FM brought to radio broadcasting, it also played a role in development of Motorola's 1973 DynaTAC - the first cellular phone - invented by a Martin Cooper and his team.
Although mobile telephones had been around since 1946, it wasn't until the 1980's that the quality of frequency modulated sound, combined with reasonably priced microprocessors, digital switching, and a final decision on celluar system spectrum combined to make it feasible to offer the first commercial cellular phone services in the United States.
Today, an unbounded future for wireless radio transmissions remains as much an article of faith in innovative science as it was for Marconi and Fessenden over a century ago. Bluetooth, Wi-Fi, 3G phones, and cognitive radio are just a few of the technologies that will carry wireless transmissions successfully through radio's second century.
|last reviewed/updated on 11/21/05|
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