Steady carrier
A signal with nothing on it: a steady, unchanging tone.
What does it sound like?
One constant whistle or tone that doesn't change.
Who uses it?
Transmitters being tested or tuned, idle stations, and some beacons between messages.
Where and when can I hear it?
Anywhere.
The science
Every radio transmitter starts with a carrier, a single steady radio frequency; voice or data is added by changing it. With nothing added, the regulations classify the emission as an unmodulated carrier carrying no information (emission code N0N). On an AM receiver it sounds like silence with the noise suddenly quieting, while a receiver in SSB or CW mode mixes it with its own internal tone, a process called heterodyning, so it comes out as a steady whistle whose pitch changes as you tune.
Why it happens
Operators send a plain carrier while tuning an antenna or testing a transmitter, and a station can sit on the air with its carrier on and no audio between programs or messages. For a listener, a steady carrier is useful mainly as a marker that a transmitter is present and where exactly it is.
Lore
Early wireless used spark transmitters that made bursts of damped waves; Reginald Fessenden began sending true continuous waves in the fall of 1906 from Brant Rock, Massachusetts, using a high-frequency alternator designed by Ernst Alexanderson and built by General Electric. That idea of a continuous wave is where Morse operators' term CW comes from.
Easily confused with
- Interference or noise, from the same family