Modulated continuous wave
Morse code telegraphy using an audio tone to modulate a carrier wave.
Modulated continuous wave (MCW) is a method of sending Morse code by using an audio tone to modulate a radio carrier wave. Unlike continuous wave (CW), which has no subcarrier and emits nothing between code symbols, MCW can be produced by any AM or FM transmitter that receives audio from an oscillator or similar source. When an SSB transmitter is keyed with a single audio frequency, the resulting emission is classified as J2A or J2B—considered CW, not MCW.
In AM systems, MCW is typically sent as double-sideband amplitude modulation with one channel carrying digital information via a subcarrier, designated A2A by the FCC. This mode is used, for example, when a commercial AM station transmits Morse code. The FCC symbol A2A stands for amplitude modulation, one channel with digital data using a subcarrier, and aural telegraphy. Single-sideband modulation with full carrier (H2A/H2B) is another MCW form. Amateur radio operators may use A2A, but they more often send Morse as CW using suppressed-carrier single-sideband (J2A or J2B) to reduce bandwidth and power. Unlike A1A CW (double sideband, no subcarrier), A2A MCW produces an audible tone on an AM receiver without a beat oscillator, making it common for RDF beacon station identifiers.
For FM, Morse code sent via frequency modulation is also MCW, designated F2A by the FCC: frequency modulation, one channel with digital data using a subcarrier, aural telegraphy. F2A MCW can be heard on a standard FM receiver and is widely used by commercial and amateur repeater stations for identification, as well as by automatically controlled stations like telemetry transmitters or remote bases.
The FCC defines MCW in 47 CFR §97.3(c)(4) as tone-modulated International Morse code telegraphy emissions with designators where the first symbol is A, C, D, F, G, H, or R; the second symbol is 2; and the third is A or B. These symbols follow ITU definitions, and the FCC classifies all such emissions as digital information. Listed MCW types (all one channel, using a subcarrier) include: double-sideband AM (A2A, A2B), vestigial sideband (C2A, C2B), combined AM and FM/PM (D2A, D2B), FM (F2A, F2B), phase modulation (G2B), single-sideband with full carrier (H2A, H2B), and single-sideband with reduced or variable carrier (R2A, R2B).
- Fcc definition
- 47 CFR §97.3(c)(4) defines MCW 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
- Fcc classification
- International Morse code telegraphy emissions are classified as 'digital information'.
- Mcw designators
- A2A, A2B, C2A, C2B, D2A, D2B, F2A, F2B, G2B, H2A, H2B, R2A, R2B
- Cw designators not mcw
- A1A, A1B, C1A, C1B, H1A, H1B, J1A, J1B, J2A, J2B, R1A, R1B
Lore & Background
MCW can be generated by any AM or FM radio transmitter with audio input from an audio oscillator or equivalent audio source. When an SSB transmitter is modulated by Morse code of only a single audio frequency, the resulting radio frequency emission is J2A or J2B and therefore is CW by definition, not MCW. MCW on AM would typically be sent as double-sideband amplitude modulation, with one channel containing digital information, and using a subcarrier, also known as A2A. For example, Morse code sent in an AM broadcast by a commercial radio station would be modulated CW as it contains a subcarrier. Single-sideband modulation with full carrier (H2A/H2B) is another form of modulated CW (MCW). Amateur radio operators may also use A2A to transmit Morse code, however it would be more typical to send it as CW, rather than MCW, using single-sideband modulation with a suppressed carrier, which reduces the bandwidth and power requirements. The FCC designates this mode as either aural telegraphy (J2A) if intended to be decoded by ear, or otherwise as electronic telegraphy (J2B), if it's intended to be decoded by machine. Unlike A1A CW transmissions (double sideband, with no subcarrier), 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. When Morse code is transmitted using frequency modulation (FM), it is a form of MCW. The FCC designates this mode as F2A: F (frequency modulation), 2 (one channel containing digital information, using a subcarrier), A (aural telegraphy). F2A MCW Morse can be heard on a normal FM radio receiver, and it is commonly used by both commercial and amateur repeater stations for identification. Also, F2A is sometimes used by other types of stations operating under automatic control, such as a telemetry transmitter or a remote base station.
Reader's Guide
MCW's significance lies in its compatibility with standard AM and FM receivers, allowing Morse code to be heard without specialized equipment like a beat oscillator. This makes it practical for applications such as RDF beacons, commercial and amateur repeater station identification, and telemetry transmitters operating under automatic control. The FCC defines MCW through a specific set of emission designators (e.g., A2A, F2A) that all share a second symbol of '2', indicating one channel containing digital information using a subcarrier. This contrasts with CW emissions (e.g., A1A, J2A) which either lack a subcarrier or, in the case of J2A/J2B from SSB transmitters, are classified as CW despite using a subcarrier. The legacy of MCW is that it bridges traditional Morse code telegraphy with modern broadcast and amateur radio infrastructure, enabling identification and beacon signals to be transmitted over existing AM and FM networks without requiring receiver modifications. Its use persists in automatic station identification and direction-finding beacons, where the audible tone simplifies decoding by human operators or simple receivers.
Frequently Asked Questions
Who is Modulated continuous wave?
MCW is a radio emission technique that sends Morse code by layering an audible audio tone onto a carrier wave instead of simply switching the carrier on and off. Think of it as CW wearing a voice: the code is still there, but an oscillator-generated tone rides along with it the whole time.
What is Modulated continuous wave known for?
Because any standard AM or FM transmitter that accepts an audio input can produce MCW, it lets operators send code without a dedicated CW rig. Its official emission designators—A2A, A2B, C2A, C2B, D2A, D2B, F2A, F2B, G2B, H2A, H2B, R2A, and R2B—all share the digit '2' in the middle, signaling that a subcarrier is present.
Why is Modulated continuous wave important?
MCW gives operators a straightforward path to digital Morse code using gear they already own—any AM or FM set with an oscillator or tone generator plugged in. The FCC treats its emissions as 'digital information' under 47 CFR §97.3(c)(4), placing it firmly in the telegraphy family rather than the voice family.
What's the difference between Modulated continuous wave and plain Continuous Wave?
CW (designators like A1A, C1A, H1A) transmits the carrier directly with no subcarrier, so the radio is silent between dots and dashes. MCW, by contrast, always carries an audible tone on the carrier, and its designators carry a '2' in the second position to flag that extra audio layer.
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