FT8 Duty Cycle: Why Full Power Can Overheat Your Radio
A radio that has worked for years on SSB and CW without a problem can become hot, noisy with its fan and unreliable after a few evenings of FT8. The transmitter has not changed. What has changed is the way in which it is used. Voice modes put a fluctuating signal into the amplifier, and the average power is only a fraction of the peak. A digital mode sends a steady signal at the full level for several seconds at a time, and the heat that this creates is much greater. Understanding the FT8 duty cycle, and what it means for the final amplifier, is the key to getting long life from a transceiver that is used for digital modes. This guide explains the numbers, shows how modes compare, tells you how far to turn down the power and gives a simple routine for setting the level correctly.
What the Duty Cycle Means
The duty cycle of a transmitter is the fraction of the time that it is sending. A value of 100 percent means continuous transmission, and 10 percent means that it transmits for a tenth of the time. For heating, the length of each transmission matters as well. A very short burst can be taken up by the heat capacity of the transistors and the heatsink, but a long burst raises the temperature until it reaches a balance with the cooling, and a repeated long burst keeps it at a high level.
FT8 works in fixed periods of 15 seconds. The transmission itself lasts about 12.6 seconds, so in the period where you transmit, the radio sends for roughly 84 percent of the time. The signal is a constant-amplitude tone that jumps between eight frequencies, and the FT8 mode is built so that the amplifier works at its steady full output for the entire message. A station that is transmitting does so in only the even or the odd periods, so in a long run of calls the radio transmits in about every second period. The average is therefore about 42 percent over time, but the heat during each period is the heat of full continuous output.
Comparing the Modes
| Mode | Transmit time per cycle | Average over a long run | Load on the amplifier |
|---|---|---|---|
| SSB voice | Varies with speech | Average power about 10 to 25 percent of peak | Light; peaks are short |
| CW | Key down about 40 to 50 percent of the time | Moderate | Moderate; short bursts |
| FT8 | 12.6 s of a 15 s period | About 42 percent when every second period is used | Heavy; steady full output |
| FT4 | 4.5 s of a 7.5 s period | About 30 percent | Heavy during each burst |
| RTTY and PSK31 | Continuous while sending | Up to 100 percent in a long over | Very heavy |
| WSPR | About 110 s of 120 s | Often 20 percent of the periods | Very heavy, a transmission lasts almost two minutes |
The comparison shows the origin of the trouble. A voice signal at 100 watts peak has an average of perhaps 15 to 25 watts, and the final stage dissipates a small share of that as heat. FT8 at the same setting asks for the full 100 watts for 12 seconds at a time, and the heat generated is several times greater. The rating of the transceiver for continuous operation at full power is the figure that counts, and many radios are specified to run at full power for a limited time only.
Where the Heat Goes
The final amplifier of a transceiver is not perfectly efficient. A typical class AB stage turns about half of the DC power into RF output, and the rest goes to heat. For a radio that produces 100 watts, 80 to 100 watts of heat is created in the transistors, the output filters and the relay contacts, and it has to be taken away by the heatsink and the fan. The heat sink has a limited capacity, and when the temperature rises, the protection circuit lowers the power automatically. That is the reason why your power drops after a few minutes of FT8, and why the radio may show a warning or a message about the temperature.
The bad effects are more than a lower power. The higher temperature shortens the life of the transistors and the capacitors, the noise from the fan grows and a radio that is repeatedly pushed to the thermal limit will fail sooner. If you also have a mismatch at the antenna, the problem gets worse: reflected power adds to the stress in the output stage, as our guide on what an SWR meter tells you shows.
How Much to Turn Down the Power
The best rule is simple and widely used: run FT8 at no more than half of the rated power, and use even less if the radio is small or the weather is hot. A 100 watt transceiver is happy at 30 to 50 watts on FT8, and a 5 watt QRP rig may run at full power if it is built for continuous duty. The reduction in the signal is small: halving the power costs 3 dB, which is half an S unit, and in a mode that decodes below the noise, the difference is hardly noticeable. Our article on FT8 operating etiquette gives the same advice for another reason: a high power increases the chance of distortion and interference to the neighbours on the band.
Turning the power down also has a long-term effect on the station. A transceiver that runs at 40 watts seldom reaches the thermal limit, the fan stays quiet and the amplifier stays in its linear region. If you need more signal for a path, it is usually better to improve the antenna by a few decibels than to push the transmitter harder. The choice is described in more detail in the article on buying an HF amplifier, and the same logic holds for any station: reduce the losses first and increase the power last.
Setting the Level in Four Steps
- Connect a dummy load. A test on a dummy load keeps the signal off the air while you set the levels, and it removes the antenna as a variable. See our guide to the dummy load for the choice.
- Set the audio drive so that the ALC reads zero. In WSJT-X use the power slider or the audio level of the computer until the radio’s ALC meter shows no deflection. The ALC is not a power control for digital modes, and a deflection means that the audio is too high and the signal is distorted.
- Set the output with the RF power control of the radio. Choose the value that you want, for example 40 watts, and check it on a wattmeter. The RF power setting is the right tool; the audio level is not.
- Watch the temperature and the SWR. Transmit a few messages in a row and note the temperature indication if your radio shows it. If the fan runs at its maximum or the power falls, reduce the setting further.
The steps are the same for FT4, JS8 and other digital modes, and the settings can be saved in a memory of the radio for digital operation, so that you do not have to repeat the procedure each time.
Keeping the Radio Cool
- Give it air. Leave a free space around the vents, and do not put the radio in a closed shelf or on a thick cloth.
- Add a small fan. A cheap fan that blows across the case or the heatsink lowers the temperature by several degrees.
- Clean the vents. Dust in the fan and on the heatsink reduces the cooling over the months.
- Check the SWR. A good match is part of the cooling. A power supply that sags under load, a point that our guide to choosing a power supply covers, makes the amplifier less efficient and hotter.
- Use less power in summer. A room at 30 degrees leaves much less margin than a room at 18.
- Take breaks in long runs. If you call CQ for hours, stop for a few minutes now and then, or use the modes at a lower duty.
You will also hear that FT8 is gentle on the amplifier because it does not need linearity. It is true that the constant envelope of the signal means that the distortion of the amplifier does not matter, and that you could in principle use a more efficient class of operation. A common transceiver does not switch to that mode, and its heat is the result of the full output. The explanation of the amplifier classes in the article on power amplifier classes gives the background.
An Evening on 20 Meters
An example from a normal evening shows how the heat builds up. You start the session in a cool room, call CQ at 100 watts and work a few stations. During each exchange you transmit about five or six times, each for 12.6 seconds, in alternating periods, so in ten minutes the radio has been active for about four minutes. The heatsink warms up gradually, and after twenty minutes the fan is at full speed. At that point the protection circuit may start to fold back the power by a few watts, and you notice that the reports are slightly lower, though you believe you are transmitting at full power. An hour later the case is hot to the touch. If you had set 40 watts at the beginning, the radio would have stayed at a comfortable temperature for the whole evening, and the signal at the other end would have been only 4 dB weaker, a difference that FT8 can hardly register. The comparison also explains why digital mode operators, who run many hours in a row in a contest or during a good opening, are advised to be careful.
External Amplifiers and Tuners
The same reasoning applies to everything in the transmit chain. An external linear amplifier is usually rated for a particular duty cycle, and many of the popular models allow only a short transmission at the maximum level, because their tubes and transistors, power supplies and cooling are designed for the average of voice work. Running FT8 through such an amplifier at its rated power can overheat the tubes or the power transistors. The correct level is a fraction of the rating, often half or less, and the instruction manual should be consulted for the continuous rating. An automatic antenna tuner has relays and coils that carry the full current, and a mismatch that is acceptable on SSB can overheat a small tuner in a 12 second transmission. A good match at the antenna, with the tuner doing little, is the best way to keep it cool. The advice for the whole station is to treat the digital modes as a steady load.
The Bottom Line
FT8 transmits a constant-amplitude signal for about 12.6 seconds of each 15 second period, which is an 84 percent duty in the periods where you transmit and about 42 percent over a long run. That is far heavier for the final amplifier than the average power of SSB or CW, and the heat is the heat of full output. Run FT8 at half of the rated power or less, set the audio so that the ALC shows nothing and use the RF power control to set the level on a dummy load. Keep the vents clear, add a fan if the radio gets hot, keep the SWR low and take breaks in long sessions. The loss in signal is small, and the radio will last much longer.
Frequently Asked Questions
What is the duty cycle of FT8?
Each transmission lasts about 12.6 seconds of a 15 second period, so it is about 84 percent in the period used. Since a station transmits in every second period at most, the average is about 42 percent.
How much power should I use for FT8?
A good rule is no more than half of the rated power of the transmitter. A 100 watt radio does well at 30 to 50 watts.
Why does my power drop during FT8?
The protection circuit reduces the output when the temperature of the final amplifier rises. It is a sign that the setting is too high for the cooling available.
Should I use ALC to control the power?
No. For digital modes the ALC should show no deflection. Control the power with the RF power setting of the radio, and use the audio level only to keep the ALC at zero.
Is FT4 easier on the radio than FT8?
The transmissions are shorter, with about 30 percent average, but each burst is at full output. The same rule of half power applies.
Will a fan help?
Yes. A small fan across the heat sink or the case can lower the temperature by several degrees and keeps the radio from reducing its power.