October 13, 2013

FCC Technician Exam Question Of The Day (T0C11)

Q) What is meant by "duty cycle" when referring to RF exposure?

A) The ratio of on-air time to total operating time of a transmitted signal

NJ2X Notes:
A duty cycle is the time that a device spends in its active state as a fraction of the total time under consideration. In an amateur radio and RF safety context, the term is used with reference to transmitters.  So a transmitter's duty cycle is the time a transmitter is functionally transmitting in a specified period of time.  Different operating modes have different duty cycle characteristics since the RF being radiated is often intermittent. A lower duty cycle reduces the average exposure to the RF radiation.

Examples of operating mode typical duty cycles:

  • CW = 40%
  • SSB voice = 20%
  • FM = 100%
Duty cycle is used to determine average power. Here is the formula:

Average power = PEP x operating cycle x (transmitting time / averaging period)