Also known as: duty cycle, duty ratio, transmit duty cycle
Duty cycle is the fraction of a repeating cycle during which a transmitter is actively emitting — D = t_on / (t_on + t_off), expressed as a percentage.1 It directly sets the ratio of average power to peak power, which in turn governs how much heat a power amplifier must dissipate and therefore how the radio is cooled and rated.
How it works
Peak (envelope) power is set by how hard the amplifier is driven during a transmit burst; average power is that peak scaled by the duty cycle. A 50 W transmitter keyed 10 % of the time delivers only 5 W average. Because heating in the final stage tracks average dissipated power, a low duty cycle lets a small amplifier and heatsink handle bursts that would overheat them if sustained continuously.
Duty cycle spans a wide range in practice:
- Continuous / 100 % — analog FM broadcast, an unmodulated carrier wave, and data modes that hold the transmitter keyed for the whole message.
- High — a busy repeater or a control channel that transmits almost continuously.
- Low / bursty — push-to-talk voice (keyed only while someone speaks) and TDMA systems where each subscriber transmits in assigned time slots and is off between them.
Duty cycle is about time on versus off; it is distinct from crest factor / PAPR, which describes the amplitude statistics within a transmission. A signal can have 100 % duty cycle yet a high PAPR, or a low duty cycle yet a nearly constant envelope while keyed.
In practice
Component and radio datasheets quote power ratings against a stated duty cycle. A dummy load rated “100 W continuous, 300 W intermittent” survives higher peaks only if the average — after applying the duty cycle — stays within its thermal limit. Amateur and commercial data modes that approach 100 % duty (FT8, RTTY, digital voice) commonly force operators to reduce output power below the rig’s SSB rating so the finals do not overheat. Antenna and feedline power handling, and the sizing of cooling, all flow from the same average-power calculation.
Relevance to SDR
Duty cycle explains a lot about how trunked and time-division systems behave on the air. A TDMA protocol like P25 Phase 2 or DMR runs each logical channel at roughly 50 % duty because two conversations share one RF carrier in alternating slots; the transmitter’s average power — and its battery drain in a portable — falls accordingly. A control channel, by contrast, runs at very high duty because it must broadcast signalling continuously for subscribers to find it.
GopherTrunk is a receiver and never keys a transmitter, so it has no duty-cycle rating of its own. The concept is still useful for interpreting captures: the bursty, slotted structure of a TDMA signal in a spectrogram is a direct picture of its duty cycle, and recognising a near-continuous versus a slotted emission helps identify which protocol and channel type is present before decoding.
Sources
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Duty cycle — Wikipedia, definition of the on-time fraction and its relation to average power. ↩