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Saturday, January 19, 2013
What does 'CW' really mean?
From Jim Wades, WB8SIW come the following inforation regarding the origins of the term "continuous wave." Here is his abridged version.
The first radio transmitters generated RF by discharging a high voltage spark across an L-C resonant circuit, which was coupled to the antenna. Each spark discharge across the gap would "ring" the L-C tuned circuit, which would then oscillate at its resonant frequency with decreasing amplitude until the decay was such that oscillation ceased. This created a "damped" oscillation analogous to ringing a bell or plucking a guitar string.
The biggest problems with spark transmitters were occupied bandwidth, resulting in relatively poor efficiency, and the fact that they could not be modulated. This problem was solved through a number of approaches, which produced "undamped oscillations," also called "continuous waves." The earliest methods of producing an undamped oscillation were the arc transmitter and the radio frequency alternator. Both were usuable only at very low frequencies, and both produced RF directly at high power levels, which were difficult to modulate. However, such systems remained in service well into the mid 1940s, and one Alexanderson long-wave alternator remains intact and operational in Sweden as a museum piece.
With the development of stable, good quality vacuum tubes in the 'teens, it became possible to develop a "modern" RF oscillator. Better yet, a RF oscillator could operate at low levels and be buffered and amplified in stages to produce reasonably high power levels. Furthermore, such vacuum tube oscillators could be conveniently modulated. Therefore, immediately after World War One and through the 1920s, we begin to see a variety of applications arise centered around voice communications including radio broadcasting, police radio, and point-to-point SSB circuits for international telephone service.
The term "continuous wave" during this early period emerged as a tool to differentiate a modern radiotelegraph transmitter generating undamped oscillations from its antecedent, the older spark transmitter producing damped oscillations. Over time, as the spark technology receded into the past, the term "CW" became somewhat idiomatic; a term used by radio operators and engineers in reference to all radiotelegraph communications.
In reality, all modern communications systems use "CW," from your ham radio CW equipment to the latest cellular telephone or wireless router! I hope that explains things! - 73, WB8SIW