Next time you browse a ham radio catalog, compare the relative power ratings of some of the accessories. I was interested in replacing some antenna switches, and here’s what I found listed under the product specs:
- Alpha Delta 4B Coax Switch: 1,500W PEP/1,500 CW
- DX Engineering RR8B-HP Remote Antenna Switch: 7kW ICAS all modes, under 2:1 SWR, 5kW CCS all modes, under 2:1 SWR
- Hamplus Desktop Antenna Switch AS-603A: 2000W power handling at SWR of 1.5:1 or les
PEP (Peak Envelope Power) is often the default rating for switches, antennas, baluns, amplifiers, and other ham radio accessories. But you’ll notice others on the list above, like ICAS (Intermittent Commercial and Amateur Service) or just watts (W).
What do these mean to you and station operation?
Explaining Duty Cycle
Before we tackle the alphabet soup of power ratings, understand they’re only part of the picture.
To understand power ratings, you also need to consider the duty cycle—the time your radio or amp delivers power. Each mode has its own duty cycle, expressed in percent. For example, a 100% duty cycle continuously delivers power.
Typical Duty Cycle Percentages
| Mode | Duty Cycle |
|---|---|
| SSB | 10 – 20%* |
| CW | 50% |
| FM/AM | 100% |
| Digital/RTTY | Up to 100% |
| * Duty cycle varies with compression/ALC/mic gain settings. | |
The higher the duty cycle, the more potential for stress on your finals. That’s why radio manufacturers recommend 25W AM, for example.
The popular FT8 digital mode doesn’t require a lot of power—many users find 20 to 30W sufficient to make contacts and only increase power as needed to make the QSO.
Peak Envelope Power (PEP)
PEP is used when output power varies, which is typical for modes like SSB. It’s the highest envelope power supplied to the antenna by a transmitter during any full RF cycle or series of complete radio frequency cycles.
A transmitter delivering 100W PEP can momentarily produce 100W power. Similarly, coaxial switches or other radio accessories rated for 100W PEP power can momentarily handle that amount of power.
We usually associate PEP with SSB transmissions, but it can also describe other modes. For a CW signal, PEP is measured during the key-down period when the transmitter is on.
PEP is equal to the steady carrier power in a CW transmission. PEP is also equal to the average power in a steady FM, FSK, or RTTY signal.
Let’s not forget AM.
Assuming a linear, perfectly symmetrical, 100% modulation of a carrier, PEP output of an AM transmitter is four times its carrier PEP. A typical 100W amateur transceiver is rated for no more than 25W of carrier output when operating in AM mode.
Continuous/Intermittent Commercial Service (CCS & ICAS)
Other ratings include CCS and ICAS. CCS stands for Continuous Commercial Service. This rating means the amplifier can put out its rated power nonstop, 24 hours a day, 7 days a week. That would equate to a 100% duty cycle. TV and radio broadcast stations look for these ratings when choosing transmitters.
Generally, amateur HF radio transmitter power ratings are defined as ICAS (Intermittent Commercial and Amateur Service). Intermittent operation implies that no operating period exceeds five minutes and is followed by a standby period of at least five minutes (or longer when tubes are used). In a solid-state environment, ICAS adjusts the rating to keep the device junction and case temperatures within their maximum ratings while also incorporating operating/standby periods.
ICAS appears to leave some room for interpretation by manufacturers. The following is found in the Palstar LA-1K Amplifier manual:
“The power output of the LA-1K is 1,000 Watts PEP CW ICAS. Intermittent operation of the LA-1K implies that no operating or ON period of 1000W of Continuous Carrier Power will exceed approximately 1(ONE) minute. On Single Side Band (SSB) voice duty there is no limit on transmit time at full power of 1000W PEP.”
Two ratings—one amp? You can rag-chew as long as you want (PEP rating), but watch it with RTTY and digital (ICAS).
Does Your Meter Measure Up?
An oscilloscope can be one of the best ways to measure RF power in your shack because you can see the waveform directly on the screen. However, most people don’t own one.
Many of us do own ham-grade add-on meters. They are good indicators of whether things are working normally and will give a ballpark reading, but don’t expect perfection. Generally speaking, most of these meters are rated at 10% full-scale accuracy. At full deflection (200 or 300W, for example), they will read within 10% of the actual output.
For those who want to read more than watts, the following provide average/peak readings and reasonably high accuracy ($250+): Palstar PM2000AM, Elecraft W2, Ameritron AWM-30B, and the Monitor Sensors Power and SWR Meter.
Your transceiver or amp may also have a built-in power meter. Depending on the display size, it may be an easy or difficult read. Accuracy can also vary among models and brands. I’m familiar with the Icom IC-7300 meters, and they appear to be close to the readings on my external meter.
Whatever you use, connect your radio or radio/external meter to a 50-ohm dummy load rated at least 100W–not the antenna–to get the most accurate reading. That way, you’ll avoid the effect of reflected power and other antenna system anomalies.
Are you looking for that 100W promised by the manufacturer?
The easiest way is to choose one of the modes with a 100% duty cycle–my go-to is RTTY–and briefly crank the power output to 100W. Be sure your radio is connected to a 50-ohm dummy load as described in the above paragraph. Allow some leeway for meter accuracy.
