Ham Radio: Averaging & Duty Cycle
Since the effects of RF exposure are related to heating and take place over many seconds, the MPE limits are based on averages, not peak exposure. This allows exposure to be averaged over fixed time intervals.
The averaging period is 6 minutes for controlled environments
The averaging period is 30 minutes for uncontrolled environments
The difference in averaging periods reflects the difference in how long people are expected to be present and exposed. People are assumed less likely to stay in an uncontrolled environment receiving continuous exposure, so the averaging period is much longer.
During the averaging period, a transmitter may only be generating RF for a fraction of the time. For most amateur contacts, the transmitter is keyed no more than 50% of the time and usually much less. This pattern lowers the duty cycle of the emissions.
Duty cycle is the ratio of the transmitted signal's on-the-air time to the total operating time during the measurement period.
Duty factor is the same as duty cycle expressed as a fraction, instead of percent such as 0.25 instead of 25%.
Since duty cycle affects the average power level of transmissions, it must be considered when evaluating exposure.
So really, since the time spent transmitting on the air is low, this will reduce average exposure. Some modes also have lower average power than others.
CW - The transmitter is off between the dits and dashes
SSB - Only reach peak power for short periods on voice peaks and have the lowest duty cycle
FM - This is a constant power mode and so the signal is continuously at full power when the transmitter is keyed.
This is the emission duty cycle. For a given PEP, an emission with a lower emission duty cycle produces less RF exposure (that sounds obvious). There are commonly accepted operating duty cycles for different modes used by amateurs (I should mention. I think that duty cycle = emission duty cycle = operating duty cycle in this context. If anyone wants to correct me on that, please let me know!):
Average power = PEP x operating duty cycle x (time transmitting/averaging period)
So let's say your 100watt transmitter was generating conversational SSB without speech processing. The table shows an operating duty cycle of 20% for that mode. During your operating period you transmitted for 1 minute out of every 3. your average power during the evaluation period is:
Average power = 100watts x 20% for conversational SSB x (1min / 1+2 min) = 6.6watts
During a 2 meter net as net control, using your 50 watt VHF FM transmitter, you transmit and listen for equal periods.
Average power = 50 watts x 100% for FM x 50% on/off = 25 watts.
So one other thing you may need to consider. Beam antennas focus radiated power toward one direction, creating gain. Gain has the effect of increasing your average power in the preferred direction. It also decreases your average power in other directions. This means that there are four factors that affect RF exposure: transmitter power and frequency, distance to the antenna, and the radiation pattern of the antenna.
If you use an antenna with gain, you will need to include the effect of gain in your exposure measurements. So if your antenna has 6dBi of gain, corresponding to a four-fold increase in power radiated in the preferred direction, you would multiply your average power by four when calculating RF exposure in the antenna's forward direction.
