Why a radio signal carrying Morse Code is called Continuous Wave even though it's turned on and off

(Originally published on Google+.)

As you may know, lately I'm into amateur radio. In this world, Morse code is still alive and well, though it is not necessary to learn it to get a license. When two operators use Morse code to communicate, quite often they use a mode called "Continuous Wave", or CW for short.

For quite a while I thought that CW was quite an odd name for a way to transmit Morse code. There's certainly a wave: that's the radio wave on which the Morse code is modulated. What I didn't see so clearly was the reason for the "continuous" adjective. After all, the wave is being turned on and off all the time: that's precisely how you can send Morse. If it's being turned on and off, it's not continuous. What's the deal?

Well, the deal is that before we had continuous waves, we already had Morse code on the radio, transmitted with a different kind of radio wave: the Damped Wave.

A Continuous Wave is a sinusoidal wave with a precise frequency. Nowadays it's very easy for us to produce precise and stable sinusoidal waves using pretty cheap electronics. However, in the early days of radio it wasn't so: there weren't good enough electronic oscillator circuits that could produce a quality continuous wave. So radio stations used a different mechanism to produce a different kind of radio waves.

This mechanism was the spark-gap transmitter. The general idea is that a high voltage across a gap produces an electric arc (a spark). The transmitter contains a circuit that, when an arc starts, produces a "ringing" oscillation, like the sound of a bell being struck once by a hammer. This oscillation is fed to an antenna to transmit it as a radio wave, which is called a "damped wave" because it loses amplitude with time, just like the sound of a bell stroke.

As the damped wave only lasts for a tiny fraction of a second, the spark gap is set up so that those sparks are extinguished almost as soon as they start, and a new one starts almost immediately, which produces another damped wave. In this way, lots of damped waves are produced and transmitted every second, like a school bell ringing seemingly continuously because its hammer strikes the bell several times per second.

The problem with spark-gap transmitters is that they are very inefficient and produce a prodigious amount of interference, so a lot of effort was spent in discovering a good way to generate a "continuous wave" that doesn't lose strength with time so you only need to produce the one wave and turn it on and off as needed.

Eventually, several systems were developed, like high-frequency electric generators, electronic oscillators, etc. As those became commonplace, the old spark-gap transmitters and the damped waves they produced were retired and then banned worldwide (so big was the interference problem).

And that's why a radio signal carrying Morse Code is called Continuous Wave even though it's turned on and off.