Duty cycle
The term duty cycle refers to the amount of time a transmitter is transmitting compared to the time that it is not.
Modes with a carrier, such as AM and FM, have a high duty cycle, as the transmitter is operating at its full rated power when transmitting.
For example, a 50,000W broadcast station transmitter has a 100% duty cycle – the transmitter is running at full power all the time.
Similarly, a 50W amateur FM transmitter produces 50W whenever it transmits.
On the other hand, a typical amateur 100W SSB transmitter has a relatively low duty cycle. The transmitter is not designed to transmit 100W continuously, as SSB does not drive a transmitter to continuous full power – the power output is proportional to the modulation (voice) input. The full 100W output power is only reached very occasionally on voice peaks. Typical Duty Cycle for SSB is around 25% and Morse Code is around 40%.
However, digital modes, such as FT8 and RTTY, have a 100% duty cycle, as the digital tones can drive the transmitter to its full rated power. For this reason, it is very important that HF transceivers are not operated at full power when using digital modes. The power must be reduced, otherwise the transmitter will overheat and may produce spurious emissions which could cause interference. You could also damage your transmitter.