2.9 Modulation, Duty Cycle and Rated Transmitter Power

2.9 Modulation, Duty Cycle and Rated Transmitter Power

ACMA Exam Focus: Syllabus item 5.7 requires you to understand the implications of the different types of modulation on the rated output power of a power amplifier (PA), and to understand the term 'duty cycle' in relation to transmitters. In Australia this topic connects directly to the two power figures in the class licence: 100 watts pX and 30 watts pY.

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1. Three Different Ways to Measure Transmitter Power

A single transmitter can be honestly described by three quite different power figures, and choosing the wrong one is the classic exam trap.

Peak envelope power (PEP, symbol pX) is the average power delivered by the transmitter during one radio-frequency cycle at the crest of the modulation envelope. It is an instantaneous-peak measure: it tells you the hardest the amplifier is ever pushed, but nothing about how often it gets pushed that hard.

Mean power (symbol pY) is the average power delivered over a period long compared with the lowest modulating frequency. It is the figure that determines heating, both in your amplifier and in anything the signal passes through.

Carrier power is the power in the unmodulated carrier alone. It only means something for modes that have a carrier - A3E full-carrier AM, F3E frequency modulation, and the carrier of an unkeyed CW transmitter.

For a constant-envelope mode the three converge. An FM transmitter puts out exactly the same amplitude whether you are speaking or silent, because information is carried in frequency, not amplitude. For FM, RTTY sent as FSK, and key-down CW, PEP equals mean power.

For single sideband (J3E) they diverge sharply. J3E has no carrier at all: with no speech there is no output. The envelope follows the syllables, the pauses and the natural peaks and troughs of the voice, so the mean power of a normal SSB voice signal sits far below its peak envelope power. That is why SSB transmitters are rated and regulated in PEP - a mean-power rating would tell you almost nothing about how hard the finals are working on voice peaks.

Full-carrier AM (A3E) is a useful contrast. At 100% modulation the envelope peaks at twice the carrier voltage, so the peak envelope power is four times the carrier power. A transmitter with a 25 W carrier is a 100 W PEP transmitter on AM.

2. What 'Duty Cycle' Actually Means

Duty cycle is the proportion of time that the transmitter delivers full output power. It is used in two related senses, and both matter:

1. Within a transmission (mode duty cycle). During a single over, how much of the time is the transmitter at full output? SSB voice spends most of its time between syllables at low output; RTTY never drops below full output at all.
2. Across the transmit/receive cycle (operating duty cycle). Over a longer period, how much of the time are you transmitting at all? Alternating 15-second FT8 slots means you transmit half the time; a rag chew where you listen for three minutes and talk for one is a 25% operating duty cycle.

The total heating effect is the product of the two. A contest station calling CQ continuously on RTTY has a mode duty cycle of 100% and a high operating duty cycle, which is the hardest possible life for an amplifier.

3. Why a PA Must Be Derated for Digital Modes

A transceiver advertised as 100 W PEP was designed, heatsinked and power-supplied on the assumption that you would use it for speech. Switch it to a 100% duty-cycle digital mode at the same power setting and you have changed the thermal problem completely.

Worked example. Take a standard station running J3E voice at 100 W pX with an unprocessed microphone, giving roughly a 25% duty cycle.

$P_{\text{mean}} = 0.25 \times 100 = 25\ \text{W}$

If the power amplifier is about 50% efficient, the DC input needed to produce 25 W of average RF is about 50 W, so roughly 25 W is dissipated as heat in the final devices.

Now switch to a constant-envelope digital mode without touching the power control. The duty cycle becomes 100%:

$P_{\text{mean}} = 1.00 \times 100 = 100\ \text{W}$

At the same 50% efficiency the DC input is about 200 W and roughly 100 W is dissipated as heat - four times as much, continuously, with no syllable pauses to let the heatsink recover. Manufacturers routinely specify a reduced output for RTTY and digital operation for exactly this reason, and the sensible operating habit is to back the power control down to somewhere between a third and a half of the SSB rating before you transmit a digital signal.

What actually overheats is not only the finals:

• Final devices and heatsink - junction temperature rises, gain falls, and thermal protection either shuts the radio down or, in older designs, does not save it at all.
• Power supply - a supply sized for the intermittent draw of speech may sag or trip on continuous key-down current.
• Antenna tuner - roller inductors, toroid cores and variable-capacitor plates are often rated for SSB duty and will get hot or arc under continuous carrier.
• Coaxial cable - the continuous power rating of thin coax at HF is far below its peak rating, especially with a high standing wave ratio.
• Dummy load - most amateur dummy loads carry two ratings, a short-term peak and a much lower continuous figure.
• Baluns and chokes - core losses become continuous heating rather than brief pulses.

ALC does not save you. Automatic level control limits drive so that the amplifier stays within its linear region and does not splatter. It is a linearity protection, not a thermal protection. An amplifier can run perfectly linearly all the way to destruction.

4. The Australian Power Limits Make the Same Point

Under the Radiocommunications (Amateur Stations) Class Licence 2023, which commenced on 19 February 2024, an amateur standard station is limited to:

• 100 watts pX (peak envelope power) if the station uses emission mode J3E or R3E - that is, single sideband suppressed-carrier or reduced-carrier telephony; and
• 30 watts pY (mean power) in any other case.

Compare this with the other qualification levels: an amateur foundation station is limited to 10 watts pX, and an amateur advanced station to 400 watts pX for C3F, J3E or R3E, or 120 watts pY otherwise.

Read the standard-station numbers alongside the duty-cycle table and the logic is obvious. Single sideband is allowed a peak figure of 100 W because its mean power on speech is only about a quarter of that. Every other mode is measured as mean power and capped at 30 W, because for FM, RTTY, FT8 and packet the mean power is the peak power. The two limits are far closer in real heating terms than the raw numbers suggest - and that is precisely why the syllabus pairs modulation type with rated PA power.

5. Reading the Power Meter Correctly

A moving-coil power meter without peak-hold responds to average power. Point it at a 100 W PEP SSB voice signal and it will indicate somewhere around 20 to 30 W, swinging with your voice. That is not a fault - it is the mean power. To verify compliance with a 100 W pX limit on SSB you need a peak-reading (PEP) meter, ideally checked with a steady two-tone test signal rather than speech.

For the pY modes the problem disappears: since FM, CW key-down and digital modes are constant-envelope, a peak-reading meter and an average-reading meter show the same number, and either will tell you whether you are within 30 watts pY.

Second worked example. An operator runs 30 W pY of FT8 into a dummy load. Each transmission lasts about 12.6 seconds within a 15-second slot, and the operator transmits in alternate slots, so the cycle is 30 seconds long.

$E = 30\ \text{W} \times 12.6\ \text{s} \approx 378\ \text{J per transmission}$

$P_{\text{average}} = \frac{378\ \text{J}}{30\ \text{s}} \approx 12.6\ \text{W}$

A dummy load rated for 50 W continuous copes easily; a small load rated "20 W intermittent" is already marginal, and would fail quickly if the same operator switched to continuous RTTY where the average would jump to the full 30 W.