Question T0C05
From subelement T0 - T0C
Why do exposure limits vary with frequency?
Why is this correct?
The correct answer is D because the human body's RF energy absorption varies dramatically with frequency. Around 50 MHz (6-meter band), Maximum Permissible Exposure limits are most restrictive because the human body absorbs RF energy most efficiently at this frequency. The 1/4 wavelength at 6 meters (about 5 feet) matches typical human height, making our bodies effective antennas. Choice A is wrong—energy per photon increases with frequency, but absorption depends on body resonance. B is incorrect as lower frequencies actually penetrate better. C is false—higher frequencies aren't inherently transient.
Memory tip
Look for frequency-specific safety questions by remembering that biological systems have resonant frequencies just like antennas. When a question asks 'why do limits vary,' think about how the human body interacts differently with different wavelengths, not just the energy content of the RF.
Learn more
In practical station design, this principle explains why VHF operators need more careful RF exposure evaluation than HF or UHF operators. The 6-meter band requires the lowest power levels for compliance because of peak human body absorption. When planning antenna installations, consider that 50 MHz presents the greatest exposure risk per watt, influencing antenna height and proximity decisions. This frequency-dependent absorption also affects which bands benefit most from duty cycle reductions and power management for RF safety compliance.
Think about it
Why do you think amateur radio operators working primarily on 6 meters might need different RF safety precautions compared to those operating mainly on 20 meters or 70 centimeters?