Why are there separate electric (E) and magnetic (H) MPE limits at frequencies below 300 MHz?
The correct answer is D: All these choices are correct. There are separate electric (E) and magnetic (H) MPE limits at frequencies below 300 MHz because the body reacts to both E and H fields, ground reflections cause field strength to vary, and E and H field peaks can occur at different locations. All these factors require separate limits.
At frequencies below 300 MHz, the wavelength is long compared to typical distances, so E and H fields don't have a fixed relationship (unlike far-field where E and H are proportional). The body can react differently to E and H fields. Ground reflections and scattering cause field strength to vary significantly with location. E field and H field intensity peaks can occur at different places. Therefore, separate E and H field limits are needed to properly protect against exposure. Above 300 MHz, E and H are proportional in the far field, so a single power density limit is sufficient.
Exam Tip
Separate E/H limits below 300 MHz = All factors. Remember: Separate E and H MPE limits exist below 300 MHz because the body reacts to both fields, field strength varies with location, and E and H peaks can occur at different locations. All factors are correct.
Memory Aid
"**S**eparate **E**/**H** **L**imits = **A**ll **F**actors (think 'SEHL = AF')"
Real-World Application
You're evaluating RF exposure for your 40-meter (7 MHz) station. At this frequency (below 300 MHz), you must check both E field and H field limits separately. The E field might peak in one location while the H field peaks elsewhere due to ground reflections. The body can react differently to each field. This is why separate limits are needed - a single power density limit isn't sufficient.
FCC Part 97.13Key Concepts
Why Other Options Are Wrong
This question has no incorrect options since all choices are correct. However, it's important to understand that all three factors (body reaction to both fields, field strength variation, and different peak locations) contribute to the need for separate E and H limits below 300 MHz.
题目解析
The correct answer is D: All these choices are correct. There are separate electric (E) and magnetic (H) MPE limits at frequencies below 300 MHz because the body reacts to both E and H fields, ground reflections cause field strength to vary, and E and H field peaks can occur at different locations. All these factors require separate limits. At frequencies below 300 MHz, the wavelength is long compared to typical distances, so E and H fields don't have a fixed relationship (unlike far-field where E and H are proportional). The body can react differently to E and H fields. Ground reflections and scattering cause field strength to vary significantly with location. E field and H field intensity peaks can occur at different places. Therefore, separate E and H field limits are needed to properly protect against exposure. Above 300 MHz, E and H are proportional in the far field, so a single power density limit is sufficient.
考试技巧
Separate E/H limits below 300 MHz = All factors. Remember: Separate E and H MPE limits exist below 300 MHz because the body reacts to both fields, field strength varies with location, and E and H peaks can occur at different locations. All factors are correct.
记忆口诀
**S**eparate **E**/**H** **L**imits = **A**ll **F**actors (think 'SEHL = AF')
实际应用示例
You're evaluating RF exposure for your 40-meter (7 MHz) station. At this frequency (below 300 MHz), you must check both E field and H field limits separately. The E field might peak in one location while the H field peaks elsewhere due to ground reflections. The body can react differently to each field. This is why separate limits are needed - a single power density limit isn't sufficient.
错误选项分析
This question has no incorrect options since all choices are correct. However, it's important to understand that all three factors (body reaction to both fields, field strength variation, and different peak locations) contribute to the need for separate E and H limits below 300 MHz.
知识点
E and H field limits, Below 300 MHz, RF exposure, MPE limits
Verified Content
Question from official FCC Extra Class question pool. Explanation reviewed by licensed amateur radio operators.