====== Continuous wave ======

**Continuous wave (CW)** literally describes a sustained radio-frequency carrier. In amateur operating, CW usually means radio telegraphy made by switching a carrier on and off to send [[morse_code|Morse code]]. Morse is the character code; keyed CW is a way of carrying it over radio.

===== Keying and reception =====

A straight key lets the operator control every element. A paddle with an electronic keyer generates timed dits and dahs; other systems send stored or computer-generated text.

An unmodulated RF carrier is outside human hearing. A receiver produces an audible beat note by mixing it with a nearby oscillator. In a simplified model:

<code>
Audio beat frequency = |f_signal - f_oscillator|
</code>

A 700 Hz difference produces a 700 Hz tone while the carrier is present. Modern receivers may implement this conversion digitally. Changing the receive tone does not inherently change the transmitted frequency; the transceiver's CW offset and tuning conventions determine that relationship.

===== Bandwidth and signal quality =====

A carrier keyed with abrupt edges produces unwanted sidebands, heard as **key clicks**. Controlled rise and fall times shape the envelope and reduce this unwanted energy. CW therefore has a finite occupied bandwidth that depends on speed and keying shape.

A narrow receive filter can reduce broadband noise and nearby interference. For flat noise spectral density:

<code>
Noise power ratio_dB = 10 log10(B_new / B_old)
</code>

Reducing a filter from 2500 Hz to 250 Hz reduces admitted white-noise power by 10 dB. This calculation does not guarantee a 10 dB improvement for every signal: the filter must still pass the wanted signal, and interference and fading may dominate.

===== Operating practice =====

Readable spacing and a stable tone are more useful than speed alone. Operators normally answer at a speed they can copy, identify their station and leave room for replies. A short exchange may include callsigns, a signal report, name and location; contests use their own defined exchanges.

CW remains useful for compact equipment, manual weak-signal communication and training in listening and timing.

===== References =====

  * [[https://www.itu.int/rec/R-REC-M.1677-1-200910-I|ITU: Morse code and procedural signals]].
  * [[https://www.arrl.org/learning-morse-code|ARRL: learning Morse code]].
  * [[https://www.arrl.org/contestupdateissue?issue=2019-02-20|ARRL: keying waveform and key clicks]].
  * [[https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-8|NIST: logarithmic power ratios]].