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G0A07

ELECTRICAL AND RF SAFETY · - RF safety principles, rules, and guidelines; routine station evaluation

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What is the effect of modulation duty cycle on RF exposure?

  1. AA lower duty cycle permits greater power levels to be transmitted
  2. BA higher duty cycle permits greater power levels to be transmitted
  3. CLow duty cycle transmitters are exempt from RF exposure evaluation requirements
  4. DHigh duty cycle transmitters are exempt from RF exposure requirements
Why RF exposure limits are based on power density averaged over time (6 minutes for controlled exposure, 30 minutes for uncontrolled), not on peak power. Modulation duty cycle is the fraction of transmit time the carrier is actually at full output: SSB voice is roughly 20 percent, CW around 40 percent, while FM, RTTY and many digital modes are 100 percent. Because a low duty cycle mode puts out less average power for the same PEP, you can run a higher peak power and still stay under the same exposure limit.
Watch out The reverse statement, that a higher duty cycle allows more power, has it backwards: a 100 percent duty cycle mode like FM delivers full average power the whole time, so it is the most restrictive. Duty cycle never exempts a station from evaluation; exemptions are based on power and frequency thresholds in 97.13(c).
Exposure is averaged, so less on-time means more allowed power. SSB beats FM for headroom.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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The rest of G0A

  1. G0A01 What is one way that RF energy can affect human body tissue?
  2. G0A02 Which of the following is used to determine RF exposure from a transmitted signal?
  3. G0A03 How can you determine that your station complies with FCC RF exposure regulations?
  4. G0A04 What does "time averaging" mean when evaluating RF radiation exposure?
  5. G0A05 What must you do if an evaluation of your station shows that the RF energy radiated by your station exceeds p…
  6. G0A06 What must you do if your station fails to meet the FCC RF exposure exemption criteria?
  7. G0A07 What is the effect of modulation duty cycle on RF exposure?
  8. G0A08 Which of the following steps must an amateur operator take to ensure compliance with RF safety regulations?
  9. G0A09 What type of instrument can be used to accurately measure an RF field strength?
  10. G0A10 What should be done if evaluation shows that a neighbor might experience more than the allowable limit of RF …
  11. G0A11 What precaution should be taken if you install an indoor transmitting antenna?
  12. G0A12 What stations are subject to the FCC rules on RF exposure?
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