Amateurs are required to perform a routine evaluation of the strength of the RF fields around their stations, subject to certain exemptions based on peak envelope power (PEP) levels at the various amateur bands. However, the FCC regulations on permissible RF exposure are not based on peak envelope power (PEP), but on average power over a 30-minute time period for uncontrolled environments, or a 6-minute time period for controlled environments. The part of the regulations that determine whether a station operator must perform a periodic evaluation, however, is based on PEP (see categorical exemptions.)

Operating Mode Duty Factor
Morse code (CW) 40%
SSB phone 20%
FM 100%
RTTY/Digital 100%
AM 100%

To estimate your average power, first start with your Peak Envelope Power (PEP.) Multiply that by the duty factor for the mode you are using, then by the maximum percentage of time you expect to operate within the averaging period.

For example, if you operate a 1500 watt PEP SSB phone station that is on for 10 minutes, off for 10 minutes and on for 10 minutes, you are operating with 200 watts average power (1500 watts PEP * 20% * 67% = 200 watts average power) over a 30 minute period. If you operate a 1500 watt Morse Code (CW) station over the same time period, you have 1500 watts PEP * 40% * 67%, or 400 watts average power.

In most cases for the 6-minute controlled environment exposure estimation, you should probably assume that it is possible to operate over the entire 6-minute period, so the 1500 watt PEP SSB phone station would be 300 watts average power for controlled-exposure calculations. An RTTY or digital bulletin station, or FM repeater transmitter, would probably be on for the full 30 minute time period, so the RTTY bulletin station or FM repeater would be 1500 watts average power. If it operated 10 minutes on, 10 minutes off and 10 minutes on, it would have 1000 watts average power over 30 minutes.

Our thanks to the ARRL for this information.