Question
Over what range of frequencies are the FCC human body RF exposure limits most restrictive?
Answer Options
- A) 300 kHz - 3 MHz
- B) 3 - 30 MHz
- C) 30 - 300 MHz
- D) 300 - 3000 MHz
Correct Answer: C
Explanation
The FCC’s Maximum Permissible Exposure (MPE) limits are not constant across the entire radio spectrum; they vary based on the efficiency of the human body to absorb RF energy at different frequencies. The primary hazard is thermal (heating of tissue), and the human body acts most efficiently as an antenna at certain wavelengths.
The human body absorbs \text{RF} energy most efficiently when its length (or parts of its length) approaches rac{1}{4} to rac{1}{2} wavelength of the radio signal. This resonance effect means the \text{FCC} human body \text{RF} exposure limits are most restrictive over the 30 \text{ MHz} - 300 \text{ MHz} range (which includes the \text{VHF} and some \text{UHF} amateur bands). In this range, even relatively low power levels can potentially exceed MPE limits if the distance is too short.
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