Pre-Post World War I
Next to aviation, radio was considered to be the wonder of the age during the early twentieth century. Initially known as wireless telegraphy, it freed long distance communication from the constraints of wires. Wireless telegraphy meant exactly that Morse code transmitted by electromagnetic waves instead of wires. The discharge of a spark across a gap caused by the pressing of a telegraph key generated the electromagnetic waves that relayed the message. The years 1900 to 1915 constituted "the golden age of the spark transmitter," with the names of Guglielmo Marconi, Reginald Fessenden, and Lee de Forest the most prominent in the early development of radio.
Spark-gap technology possessed several important drawbacks. From a security standpoint, a spark transmission could not be tuned; it covered a span of frequencies and could be intercepted by anyone with a receiver. Moreover, the signals of all stations within range of each other caused mutual interference. Not only did the noisy spark create a great deal of distortion, the consequent dissipation of energy over the broad band of frequencies lessened the distance over which the signals could travel. Only with advances in continuous wave technology would wireless telegraphy evolve into wireless telephony, or radio broadcasting.
The Signal Corps began investigation into radio with its own electrical expert, 1Lt George O. Squire. He had received a Ph.D. from Johns Hopkins in 1893, one of the first signal soldiers to get one of these advanced degrees. Squire developed a wireless system that was first used in 1899, between Fire Island and the Fire Island Lightship at the north approach to New York Harbor. This was the first wireless system ever placed in service in the Western Hemisphere.
In 1910 Major Squire demonstrated his multiplex telegraphy system. With this system many messages could be sent simultaneously along the same wire. Voice signals could be sent by radio along telephone lines. Radio signals could travel along the wires without interference with the regular telephone traffic. This "wired wireless" provided greater secrecy than broad band and made more efficient use of existing wires.
Mainly through the efforts of Squire and young engineers such as Edwin H. Armstrong, and through cooperation with American industry. Armstrong had discovered the capabilities of Lee DeForest’s Audion and developed the super heterodyne radio receiver which greatly amplified weak signals and enabled precise tuning. Other the important developments were the perfection and mass production of the vacuum tube and, in cooperation with the Western Electric Company, of the radio telephone, or voice radio, which went into production in 1918. But, it was too late to use in the war. There was also some experimentation that looked toward radar.
Find out more in "Getting the Message Through" at http://www.history.army.mil/books/30-17/Front.htm#toc