A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off. Information is carried in the varying duration of theon and off periods of the signal, for example by Morse code in early radio. In early wireless telegraphy radio transmission, CW waves were also known as "undamped waves", to distinguish this method from damped wave transmission.
Very early radio transmitters used a spark gap to produce radio-frequency oscillations in the transmitting antenna. The signals produced by these spark-gap transmitters consisted of brief pulses of radio frequency oscillations which died out rapidly to zero, called damped waves. The disadvantage of damped waves was that they produced electromagnetic interference that spread over the transmissions of stations at other frequencies. Mathematically, the extremely wideband excitation provided by the spark gap was bandpass filtered by the self-oscillating antenna side circuit, which, because of its simple construction, also had a relatively broad and poorly controlled filter characteristic.
This motivated efforts to produce radio frequency oscillations that decayed more slowly. Strictly speaking, an unmodulated continuous carrier has no bandwidth and cannot interfere with signals at other frequencies, but conveys no information either. Thus it is commonly understood that the act of keying the carrier on and off is necessary. However, in order to bring the bandwidth of the resulting signal under control, the buildup and decay of the radio frequency envelope needs to be slower than that of the early spark gap implementations.
When this is done, the spectrum of the signal approaches that of a continuous sinusoidal oscillation, while temporally its amplitude varies between zero and full carrier strength. As such, the resulting narrower bandwidth mode of operation is to this day described as "continuous wave". The resulting signal allows many radio stations to share a given band of frequencies without noticeable mutual interference.
In on-off carrier keying, if the carrier wave is turned on or off abruptly, communications theory can show that the bandwidth will be large; if the carrier turns on and off more gradually, the bandwidth will be smaller. The bandwidth of an on-off keyed signal is related to the data transmission rate as: Bn = BK where Bn is the necessary bandwidth in hertz, B is the keying rate in signal changes per second baud, and K is a constant related to the expected radio propagation conditions; K=1 is difficult for a human ear to decode, K=3 or K=5 is used when fading or multipath propagation is expected. What is transmitted in the extra bandwidth used by a transmitter that turns on/off more abruptly is known as key clicks. Certain types of power amplifiers used in transmission may increase the effect of key clicks.
The first transmitters capable of producing continuous wave, the Alexanderson alternator and vacuum tube oscillators, became widely available after World War I.
Early radio transmitters could not be modulated to transmit speech, and so CW radio telegraphy was the only form of communication available. CW still remained a viable form of radio communication for many years after voice transmission was perfected, because simple transmitters could be used. The low bandwidth of the code signal, due in part to low information transmission rate, allowed very selective filters to be used in the receiver which blocked out much of the atmospheric noise that would otherwise reduce the intelligibility of the signal.
Continuous-wave radio was called radiotelegraphy because like the telegraph, it worked by means of a simple switch to transmit Morse code. However, instead of controlling the electricity in a cross-country wire, the switch controlled the power sent to a radio transmitter. This mode is still in common use by amateur radio operators.
Morse code is a method of transmitting text information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment. The International Morse Code encodes the ISO basic Latin alphabet, some extra Latin letters, the Arabic numerals and a small set of punctuation and procedural signals as standardized sequences of short and long signals called "dots" and "dashes", or "dits" and "dahs". Because many non-English natural languages use more than the 26 Roman letters, extensions to the Morse alphabet exist for those languages.
Each character (letter or numeral) is represented by a unique sequence of dots and dashes. The duration of a dash is three times the duration of a dot. Each dot or dash is followed by a short silence, equal to the dot duration. The letters of a word are separated by a space equal to three dots (one dash), and the words are separated by a space equal to seven dots. The dot duration is the basic unit of time measurement in code transmission. For efficiency, the length of each character in Morse is approximately inversely proportional to its frequency of occurrence in English. Thus, the most common letter in English, the letter "E," has the shortest code, a single dot.
Morse code is most popular among amateur radio operators, although it is no longer required for licensing in most countries. Pilots and air traffic controllers usually need only a cursory understanding. Aeronautical navigational aids, such as VORs and NDBs, constantly identify in Morse code. Compared to voice, Morse code is less sensitive to poor signal conditions, yet still comprehensible to humans without a decoding device. Morse is therefore a useful alternative to synthesized speech for sending automated data to skilled listeners on voice channels. Many amateur radio repeaters, for example, identify with Morse, even though they are used for voice communications.
For emergency signals, Morse code can be sent by way of improvised sources that can be easily "keyed" on and off, making it one of the simplest and most versatile methods of telecommunication. The most common distress signal is SOS or three dots, three dashes and three dots, internationally recognized by treaty.
International Morse Code Table
Another software tool for decoding Morse Code is called CW Skimmer. This is a cool program in that it actually puts the dots and dashes on the graphical display. You can download a trial version that is good for 30 days. A registered version costs around $75
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