Why is morse code called CW?
CW stands for continuous wave, which is the kind of radio signal that carries it. The first wireless transmitters were spark gaps, whose damped waves were broad and wasteful. Vacuum tube oscillators replaced them with a steady, pure note that could be switched on and off by a key, and operators have called keyed morse CW ever since.
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The two letters themselves. The tone you hear is the continuous wave, and the key is doing nothing but interrupting it.
The name is about the transmitter
CW is not a description of the code. It is a description of what the transmitter is doing, and the name stuck to the code by association.
The earliest wireless transmitters worked by making a spark. Each spark produced a burst of radio energy that died away quickly, a damped wave, and the ARRL’s account of the mode is blunt about the result: those waves were very broad and inefficient for communication. A broad signal spreads across a wide stretch of the band, so it interferes with everything near it and wastes most of its power.
Vacuum tube oscillators changed that. They could hold a steady, unbroken signal at one frequency, a continuous wave, with a very pure note. Interrupting it with a key gave you morse on a signal that occupied almost no room on the band.
What you are actually hearing
A continuous wave on its own carries nothing. It is one frequency, held steady, with no variation for a receiver to read. All the information comes from the key that switches it on and off, and from the two lengths ITU-R M.1677-1 allows: a dit of one unit and a dah of three.
The tone you hear in a receiver is not even the transmitted signal. The radio mixes the incoming carrier with an oscillator of its own to bring it down into the range of hearing, which is why the pitch of a CW signal changes as you tune across it while the rhythm stays the same.
Why the mode survived
A signal that occupies very little bandwidth is easy to filter, and a narrow filter keeps out most of the noise. That is why CW gets through on weak paths and cheap equipment where voice does not, and why it is still a normal mode on the amateur bands long after it stopped being a licensing requirement.
More on where morse is still doing real work is in is morse code still used today. To hear a keyed tone for yourself, type anything into the morse code translator and press Play.
Questions people ask next
What does CW stand for?
Continuous wave. It names the signal rather than the code, but on the air it is used to mean morse.
Is CW the same as morse code?
Not quite. Morse is the code, CW is the radio mode that usually carries it, and you can send morse with a lamp or a buzzer without any continuous wave involved.
What came before CW?
Spark gap transmitters. The ARRL describes their damped waves as very broad and inefficient, and vacuum tube oscillators replaced them with a much purer note.
Why is a pure tone better?
Because it takes up less of the band and a receiver can filter tightly around it, so a weak signal still gets through where a broad one would not.
Do amateurs still call it CW?
Yes. It is the normal name for keyed morse on the amateur bands and the label used for the mode on a radio.
Related answers
- Is morse code still used today?
Yes, in the places where a plain on and off tone still beats the alternatives.
- What does morse code sound like?
Like a single steady tone switched on and off.
- Is morse code hard to learn?
The characters are easy; hearing them at speed is the slow part.
Sources
- CW mode (American Radio Relay League).
- Codes and timing follow Recommendation ITU-R M.1677-1 (10/2009), International Morse code (International Telecommunication Union, 2009).