Peak Envelope Power (PEP): Wattmeters, SSB and Power Limits

Look at the specification of any HF transceiver and you will see an output power of 100 watts, usually followed by the letters PEP. Look at the licence conditions of your country and you will find limits expressed in the same unit. Yet many operators have never been told what peak envelope power actually measures, and they are puzzled when their wattmeter shows 20 or 30 watts while they are speaking into the microphone at full output. The explanation is simple once you see what a single sideband signal looks like. This guide covers the definition of PEP, how it relates to average power in different modes, how to read and test it, and what it means for your amplifier, your licence and your signal.

What Peak Envelope Power Means

A radio signal has a fast oscillation at the carrier frequency and a slower variation of its amplitude, the envelope, which carries the information. The definition on the peak envelope power page is that PEP is the average power over a single radio frequency cycle at the crest of the modulation. In other words, you wait for the highest peak of the envelope and measure the power during one cycle of the carrier at that instant.

Regulators like this definition because it describes the potential of a transmitter to cause interference. A signal that reaches a high peak, even briefly, can overload a receiver or splatter into neighbouring channels, regardless of what the average power over a minute looks like. For this reason, many regulatory authorities set their maximum power standards in PEP.

PEP and Average Power in Different Modes

The relationship between PEP and average power depends entirely on the mode.

• CW. With properly formed dots and dashes, the PEP is equal to the steady carrier power during a key-down. The average power depends on how much of the time the key is down, which is a fraction of the PEP.
• FM, FSK and RTTY. The envelope is constant, so the PEP equals the average power while the transmitter is on.
• SSB voice. The envelope follows the speech, with frequent peaks and long valleys. The typical average power of an SSB voice transmission is only about 10 to 20 percent of the PEP. With heavy speech processing, the average can rise to around half of the PEP, which is why a processed signal sounds louder and carries further for the same peak power.
• AM. For perfect, symmetrical 100 percent modulation, the PEP is four times the carrier power. A modern 100 watt transceiver is therefore normally limited to a carrier of 25 watts or less on AM, because the peaks of the modulation add to the carrier to reach the full rating.
• Digital modes with constant envelope. FT8 and similar modes produce a constant-amplitude tone, so the average power is the same as the PEP. That is why running the full rated power on FT8 is a strain for the transmitter, the power supply and the amplifier, as discussed in our article on SSB audio settings, which also shows how drive and ALC interact on voice modes.

The practical consequence is that a transmitter rated at 100 watts PEP on SSB is not a 100 watt heater. It delivers 100 watts only for the loudest syllables, and its average output is far smaller. A power supply, a cooling system or an amplifier rated for SSB is not automatically suitable for a continuous carrier at the same number.

Why a Wattmeter Seems to Under-Read

There are two basic types of RF wattmeter.

• Average-reading meters. They respond to the average of the RF over a short time. On SSB the needle bounces at a low level, since the average is a small fraction of the peak. A steady carrier gives a correct reading, so a “whistle” test is the way to calibrate them.
• Peak-reading meters. They hold the highest value for a moment, so the needle or display follows the peaks. A good peak-reading wattmeter with a suitable time constant gives a number that approximates PEP on voice. Some digital wattmeters include a peak-hold function and show both peak and average values.

Many of the built-in power meters on modern transceivers show relative output or ALC, not true power, so they are for tuning and not for measuring. When a book or a dealer says a transmitter produces a given PEP, they may be referring to a laboratory measurement with a two-tone signal or a steady tone, not a typical voice.

The speed of the meter matters too. A meter with a slow response and a long hold time will read higher than the real PEP of a fast burst, while a very fast meter may catch brief overshoots that the transmitter’s own ALC does not report. The result is that two different meters can show different numbers for the same signal, and neither is wrong. Treat the reading as an indication, and use a proper measurement if the number matters for your licence.

Measuring PEP Properly

If you need to know the true PEP of your transmitter, there are two reliable methods.

• The steady tone method. Feed a single audio tone into the microphone input, or use the mode that generates a carrier, and measure the output with a calibrated wattmeter into a dummy load. For a single sinusoidal tone the output is a steady RF carrier, so its average power equals its PEP.
• The two-tone test. Two equal-amplitude audio tones, separated by a modest frequency, give an envelope that rises and falls. The PEP of this signal is twice its average power. You can therefore measure the average power with an ordinary wattmeter and double it, or you can watch the envelope on an oscilloscope. This method is a standard way to check the linearity of SSB transmitters, because distortion shows up as additional products between the tones.

With an oscilloscope connected across a 50 ohm dummy load, use the peak voltage of the RF envelope. For a sine wave across a resistance R the power is the square of the peak voltage divided by twice the resistance. For example, a peak of 200 volts across 50 ohms corresponds to 200 squared, or 40000, divided by 100, which gives 400 watts. A probe or an attenuator suitable for the voltage is essential, because RF at this level can destroy an ordinary scope input.

Always do such tests into a proper load, not an antenna. Our guide to the dummy load shows what rating and construction you need, and why a load must be able to handle the power for the duration of the test.

PEP in the Rules

Because the rules use PEP, it matters which power the limit refers to. In the United States, the Part 97 transmitter power standards, set out in 47 CFR 97.313, say that an amateur station must use the minimum transmitter power necessary to carry out the desired communications, and that no station may transmit with a transmitter power exceeding 1.5 kW PEP. There are lower limits for some segments. For example, the 30 metre band segment from 10.10 to 10.15 MHz is limited to 200 W PEP. On the 60 metre channels, the rules use a different form, a radiated power limit referenced to a half-wave dipole, so the antenna gain counts.

Three details of this wording are worth noticing. First, the limit is a transmitter power, which in US rules is measured at the transmitter output, not at the antenna after feedline losses. Second, the “minimum power necessary” principle applies at all times, no matter how large the legal maximum is. Third, the number is PEP, so the limit is about the peaks and not about the average. A station running a linear amplifier with a lot of speech processing could keep the average well below the limit and still exceed the PEP limit if the amplifier is overdriven.

Other countries set their own limits, sometimes in PEP at the transmitter output and sometimes at the antenna, and these limits can depend on licence class and band. Always check the exact wording from your own regulator before you rely on a number you found in a forum.

Implications for Amplifiers and Overdrive

A linear amplifier is rated for a given PEP output. If the drive is increased beyond what the amplifier needs, the output stage saturates and the peaks are clipped. The result is a distorted waveform, a wide signal and splatter on adjacent frequencies, even when the wattmeter does not show a large number. Our overview of the HF linear amplifier covers drive levels, cooling and the differences between SSB and continuous modes. Some practical points apply here:

• Adjust the ALC so that it barely moves on voice peaks. A high ALC reading is a warning that the signal is being compressed unnaturally.
• For digital modes, reduce the drive and the output well below the SSB rating, since the duty cycle is high.
• Do not use the peak-reading function as proof that you are legal. Verify the actual output of the transmitter and the amplifier with a calibrated meter.
• Remember that speech processing raises the average power. A processed signal that reaches the same PEP has a much higher average and heats the amplifier more.

Reference Table of Common Modes

Common Misunderstandings

The first misunderstanding is “my wattmeter shows 25 watts on SSB, so my radio is broken.” The needle on an average-reading meter is moving with your voice, and the peaks are higher than it shows. Check with a steady tone or a peak-reading meter. The second is “100 watts PEP is the same as 100 watts on FT8.” It is the same peak, but FT8 holds that power continuously. The third is “a linear amplifier doubles my signal.” An amplifier changes the power, but whether the other station hears the change depends on the ratio between the new and the old levels, and the power needs to increase a lot for a visible effect on the S-meter.

A fourth, subtler mistake is to measure the output at the transmitter and then forget the feedline. Our guide to the SWR meter explains that power delivered to the antenna is less than power leaving the transmitter by the loss of the line. A 100 watt transmitter feeding a lossy cable may put out half that power at the antenna. The rules typically refer to the transmitter power, but the effect on the other station depends on what reaches the antenna.

The Bottom Line

Peak envelope power is the highest power of the modulation envelope, and it is the number used in transceiver specifications and in most licence conditions. On SSB, the average power is only a fraction of it, which explains why wattmeters appear to under-read and why a transmitter that is comfortable on voice may struggle on a continuous digital mode. Use a steady tone or a two-tone test into a dummy load to measure it, keep the drive below the point where the ALC works hard and remember that the legal limit refers to the peaks. Understanding PEP makes your signal cleaner, your equipment safer and your reading of the meter more realistic.

Frequently Asked Questions

What does PEP stand for?

PEP stands for peak envelope power. It is the average power over one radio frequency cycle at the crest of the modulation envelope.

Why does my wattmeter read only 20 watts on SSB with a 100 watt radio?

An average-reading meter responds to the average of a speech signal, which is typically 10 to 20 percent of the PEP. A peak-reading meter or a steady tone shows the full output.

Is PEP the same as average power on CW?

During key-down the PEP equals the steady carrier power. The average over a longer time is lower, because the key is up between the dots and dashes.

How do I measure PEP at home?

Feed a steady tone into a dummy load and read a calibrated wattmeter, or use a two-tone signal, where the PEP is twice the average power, or measure the peak voltage with an oscilloscope.

What is the US power limit for amateur stations?

47 CFR 97.313 sets the limit at 1.5 kW PEP, with lower limits on some segments, and requires the minimum power necessary. Other countries have their own rules, so check your regulator.

Can I run my radio at full PEP on FT8?

It is not wise. FT8 is a constant envelope mode, so the transmitter works at its full power continuously. Reduce the output well below the SSB rating to protect the equipment and keep the signal clean.