Samuel Morse
Samuel Morse
Morse code was originally used for communication over telegraph circuits in the 19th century. It predates radio communications by a long time and was invented by Samuel Morse (see illustration) in the 1830s.
Morse, more properly known as continuous wave (CW) modulation or on-off keying, is the simplest form of modulation. It simply involves interrupting a constant amplitude radio frequency signal in the transmitter so that patterns of dots and dashes are formed into characters of an alphabet.
At the receiver, a local oscillator is used to mix with the incoming radio frequency signal to produce an audio tone that can be fed to a loudspeaker or headphones. The frequency of the audio tone is normally around 800 Hz for ease of listening, but any comfortable audio frequency can be used.
This can be seen in this spectrogram where the transmitter signal appears as an audio tone because of the action of the receiver. You can pick out the characters being sent by comparing the patterns of dots and dashes with that shown in the table below.
Spectrogram of a morse signal
transmitted using on-off keying (CW)
Spectrogram obtained using software by Richard
S.Horne and AOR AR7030 Communications receiver.
The advantage of using morse code is that the bandwidth required is very low (300 Hz or less) and that consequently very low power levels are required. Worldwide communications using a few hundred watts are possible using morse; this contrasts sharply with the hundreds of kilowatts that are used by AM broadcasters. The disadvantage of the code is that it requires skilled operators to manually decode the transmissions and for this it reason has begun to fall into disuse. Morse code is still commonly used by radio amateurs in the shortwave bands.
International Morse Alphabet (with equivalent sound)
| A di-dah | S di-di-dit |
| B dah-di-di-dit | T dah |
| C dah-di-dah-dit | U di-di-dah |
| D dah-di-dit | V di-di-di-dah |
| E dit | W di-dah-dah |
| F di-di-dah-dit | X dah-di-di-dah |
| G dah-dah-dit | Y dah-di-dah-dah |
| H di-di-di-dit | Z dah-di-dit |
| I di-dit | 1 di-dah-dah-dah-dah |
| J di-dah-dah-dah | 2 di-di-dah-dah-dah |
| K dah-di-dah | 3 di-di-di-dah-dah |
| L di-dah-di-dit | 4 di-di-di-di-dah |
| M dah-dah | 5 di-di-di-di-dit |
| N dah-dit | 6 dah-di-di-di-dit |
| O dah-dah-dah | 7 dah-dah-di-di-dit |
| P di-dah-dah-dit | 8 dah-dah-dah-di-dit |
| Q dah-dah-di-dah | 9 dah-dah-dah-dah-dit |
| R di-dah-dit | 0 dah-dah-dah-dah-dah |
| ? di-di-dah-dah-di-dit | . di-dah-di-dah-di-dah |
| , dah-dah-di-di-dah-dah | / dah-di-di-dah-dit |
| = dah-di-di-di-dah (break symbol) | AR di-dah-di-dah-dit (end of message) |
| VA di-di-di-dah-di-dah (end of working) | AS di-dah-di-di-dit (wait) |
| CT dah-di-dah-di-dah (start of transmission) | error di-di-di-di-di-di-di-dit |
| over dah-di-dah (K) | KN dah-di-dah-dah-dit (over restricted to one specific station) |
Each dash is 1/3 of the length of each dot and there is a gap of one dot between each dot or dash. Each character is separated by three dots and each word is separated by five dots.
Morse (CW) can be heard in these frequency bands: 7000-7100 kHz, 14000-14070 kHz, 21000-21080 kHz and throughout the HF band.
