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Modulated continuous wave is Morse code telegraphy, transmitted using an audio tone to modulate a carrier wave.
The Federal Communications Commission defines modulated continuous wave in 47 CFR §97.3 as "Tone-modulated international Morse code telegraphy emissions having designators with A, C, D, F, G, H or R as the first symbol; 2 as the second symbol; A or B as the third symbol." See Types of radio emissions for a general explanation of these symbols.
Types of Morse code radio transmissions discussed in this article include:
Unlike A1A CW transmissions, A2A MCW will produce an audible audio tone from an AM radio receiver that is not equipped with a beat oscillator. MCW is commonly used by RDF beacons to transmit the station identifier.