At which of the following frequencies does maximum permissible exposure have the lowest value?
The correct answer is B: 50 MHz. Maximum permissible exposure (MPE) has the lowest value at 50 MHz. RF exposure limits vary with frequency because the human body absorbs RF energy differently at different frequencies. Around 30-300 MHz (including 50 MHz), the body is most efficient at absorbing RF energy, so exposure limits are lowest (most restrictive) in this range. For amateur radio operators, this means you need to be especially careful with RF exposure on 6 meters (50-54 MHz) and nearby frequencies. Understanding this helps ensure compliance with RF safety regulations.
Exam Tip
Lowest MPE = 50 MHz. Think '5'0 MHz = '5'trictest limits. Around 30-300 MHz, body absorbs RF most efficiently, so exposure limits are lowest (most restrictive). HF and UHF have higher (less restrictive) limits.
Memory Aid
"Lowest MPE = 50 MHz. Think '5'0 MHz = '5'trictest limits. Body absorbs RF most efficiently around 30-300 MHz, so exposure limits are lowest. Be especially careful on 6 meters."
Real-World Application
You operate on 6 meters (50-54 MHz) with 100 watts. The exposure limits are very restrictive at this frequency because your body efficiently absorbs RF energy in this range. You might need to maintain greater distance from your antenna or reduce power compared to operating on 20 meters or 70 cm, where limits are less restrictive.
Key Concepts
Why Other Options Are Wrong
Option A (3.5 MHz): Incorrect. Lower frequencies (HF) have higher exposure limits because the body doesn't absorb HF energy as efficiently. Limits are less restrictive at HF.
Option C (440 MHz): Incorrect. Higher frequencies (UHF) have higher exposure limits than the 30-300 MHz range. The body is less efficient at absorbing UHF energy.
Option D (1296 MHz): Incorrect. Much higher frequencies (microwave) have even higher exposure limits. The most restrictive limits are in the 30-300 MHz range.
题目解析
The correct answer is B: 50 MHz. Maximum permissible exposure (MPE) has the lowest value at 50 MHz. RF exposure limits vary with frequency because the human body absorbs RF energy differently at different frequencies. Around 30-300 MHz (including 50 MHz), the body is most efficient at absorbing RF energy, so exposure limits are lowest (most restrictive) in this range. For amateur radio operators, this means you need to be especially careful with RF exposure on 6 meters (50-54 MHz) and nearby frequencies. Understanding this helps ensure compliance with RF safety regulations.
考试技巧
Lowest MPE = 50 MHz. Think '5'0 MHz = '5'trictest limits. Around 30-300 MHz, body absorbs RF most efficiently, so exposure limits are lowest (most restrictive). HF and UHF have higher (less restrictive) limits.
记忆口诀
Lowest MPE = 50 MHz. Think '5'0 MHz = '5'trictest limits. Body absorbs RF most efficiently around 30-300 MHz, so exposure limits are lowest. Be especially careful on 6 meters.
实际应用示例
You operate on 6 meters (50-54 MHz) with 100 watts. The exposure limits are very restrictive at this frequency because your body efficiently absorbs RF energy in this range. You might need to maintain greater distance from your antenna or reduce power compared to operating on 20 meters or 70 cm, where limits are less restrictive.
错误选项分析
Option A (3.5 MHz): Incorrect. Lower frequencies (HF) have higher exposure limits because the body doesn't absorb HF energy as efficiently. Limits are less restrictive at HF. Option C (440 MHz): Incorrect. Higher frequencies (UHF) have higher exposure limits than the 30-300 MHz range. The body is less efficient at absorbing UHF energy. Option D (1296 MHz): Incorrect. Much higher frequencies (microwave) have even higher exposure limits. The most restrictive limits are in the 30-300 MHz range.
知识点
Maximum permissible exposure, RF exposure limits, Frequency-dependent limits, RF safety
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.