Also known as: continuous wave, CW
Continuous wave (CW) is a steady, single-frequency carrier that is switched on and off by hand or machine to send Morse code.1 It is the oldest practical radio mode and, despite the name, the transmitter is not continuous in use — the carrier is keyed into the dots and dashes of Morse. CW is essentially on-off keying of a pure tone, which makes it the narrowest and most power-efficient way to move information over the air.
How it works
A CW transmitter generates a stable carrier and gates it with a key. To avoid a harsh click that splatters energy across the band, the on/off transitions are shaped with a gentle rise and fall time, keeping the occupied bandwidth just a few tens of hertz wide. Because there is no sideband information beyond the keying, all of the transmitter’s power sits in one tone, so CW reaches far on very little power — the reason it remains popular for weak-signal work and beacons.
A CW signal is, strictly, unmodulated when the key is down, so a plain amplitude receiver would only hear silence and clicks. Receivers therefore use a beat-frequency oscillator (BFO): they mix the received carrier against a local oscillator offset by a few hundred hertz, turning the on/off carrier into an audible tone that pulses in time with the Morse. In single-sideband receivers this is the same product-detector path used for voice, simply tuned so the carrier lands at an audio pitch.
Relevance to SDR
CW is trivially handled in software radio: tune near the carrier, take a narrow slice of spectrum, and either listen to the beat tone or detect the on/off envelope to decode Morse automatically. Because the mode is so narrow, an SDR can pack many CW signals into a small span, and a waterfall of the amateur CW sub-bands shows dozens of thin dashed vertical lines, each a separate operator. Amateur radio, navigational and propagation beacons, and some legacy maritime identifiers still use CW.
For GopherTrunk this is background rather than a decode target: GopherTrunk is a trunked land-mobile scanner (P25, DMR, NXDN, TETRA), and CW telegraphy belongs to the HF/amateur world served by general-purpose SDR tools. CW is included here because it is the historical root of digital keying — the direct ancestor of ASK and OOK — and remains the canonical example of trading data rate for range and bandwidth.
Sources
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Continuous wave — Wikipedia, for the definition of CW as a keyed carrier for Morse telegraphy and its bandwidth/efficiency properties. ↩