Understanding PEP power and Duty Cycle in amateur radio

Upon request, I build 1:1 baluns (aka common mode chokes) for other amateur radio operators. I often receive questions about the maximum power at which the balun can be used. In response, I provide the maximum load for use in SSB mode. I do this for a reason, as there is much confusion about the extent to which equipment can be loaded.

Confusion about PEP

For many amateur radio equipment, manufacturers specify a maximum power. For example coaxial switches, antennas, baluns, tuners, etc. Often, the PEP (Peak Envelope Power) is specified. There is considerable confusion about what PEP exactly entails. We’ll clarify that now.

Where things can go wrong

The term “PEP power” is used when power varies, typical for modes like SSB. A transmitter delivering 100 W PEP can momentarily produce 100 W power. Similarly, a coaxial switch rated for 100 W PEP power can handle that power momentarily. This is where things can go wrong, and we’ll address that shortly.

Something called “duty cycle”

There is also something called “duty cycle,” expressed in a percentage. Duty cycle is essentially the on-time duration. A transmitter at 100% duty cycle continuously delivers its power. At 25% duty cycle, the transmitter delivers a quarter of the time it is operational, its power.

Each mode can be associated with a duty cycle

Duty cycle is also applied in amateur radio, and each mode can be associated with a duty cycle as a guideline. ARRL has a good definition. The benchmark is the time a transmitter operates at full power during a single transmission. Here is my personal duty cycle guideline:

Transmission modeDuty cycle
FM / AM100%
SSB20% *
Digital modes (FT8, RTTY)100%
* = In competitive conditions such as contesting and DXCC hunting or with extreme speech processing, this can double to more than 50%.

The FT8 paradox

Since the introduction of FT8, quite some equipment has been damaged. That is not surprising. You only need to look at the duty cycle to see where things goes wrong. The problem lies in the fact that FT8 uses SSB as transmission mode (specifically USB). However, the 20% duty cycle for SSB is not applicable to a digital mode like FT8. When working with FT8, you quickly notice that the transmitted power is almost continuous. The duty cycle for FT8 is, in fact, 100% instead of 20%.

A 1000 watts in FT8

I recently built a 1:1 balun and told the user that the maximum load was 1000 watts in SSB. Shortly thereafter, a message came that the SWR of the balun kept increasing during transmission, indicating that the toroid core was overheating due to excessive power. It turned out that the operator regularly transmitted with 1000 watts in FT8. For my baluns, I specify, in the tips for use, that for digital modes, the maximum power is around 20% of the maximum SSB power. In the case of FT8, this is 200 watts.

Back to PEP power

Now, back to that PEP power and the example of the coaxial switch. When a manufacturer specifies the maximum power as PEP, you can assume that this applies to a mode like SSB. If your coaxial switch can handle a maximum of 1000 watts PEP, you can use 200 watts as a guideline for FT8.

Power in watt ICAS

Now, some manufacturers use the power term watt ICAS. ICAS stands for ‘Intermittent Commercial and Amateur Service’. The term supposedly represents the power when in use by an amateur radio operator. My personal opinion is that the term has little value and is rather confusing. Because one amateur may only chat a bit in an SSB, while another amateur in RTTY contest puts much more load on his equipment.
ICAS = don’t take it too serious.

FT8 can put a lot of stress on transceivers

It’s also noteworthy that an increasing number of manufacturers, both of transceivers and peripheral equipment, include disclaimers about the maximum power in FT8. Operating a transceiver for long periods at maximum power in digimodes like FT8 can put a lot of stress on transceivers, potentially leading to damage and expensive repairs.

Running full power in digital modes like FT8 puts a lot of stress on your transceiver

The best approach

What I actually want to convey is that you should pay close attention to the maximum power ratings for equipment. There are quite a few manufacturers which are quite good at exaggerating. But the best approach is simply not to use more power than necessary. If you come in with an S8 at 25 watts somewhere, 100 watts brings only 1 S-point more, but much more load on your equipment.