What usually happens to radio waves with frequencies below the LUF?
The correct answer is C: They are attenuated before reaching the destination. What usually happens to radio waves with frequencies below the LUF is that they are attenuated before reaching the destination. LUF (Lowest Usable Frequency) is the minimum frequency that can propagate - frequencies below LUF are too weak due to absorption. For amateur radio operators, this explains why very low frequencies don't work for long-distance propagation. Understanding this helps explain LUF.
Exam Tip
Frequencies below LUF = attenuated before reaching destination. Think 'B'elow 'L'UF = 'B'elow 'L'owest 'U'sable = 'B'adly attenuated. Frequencies below LUF are too weak due to absorption - attenuated before reaching destination. Not refracted effectively, not pass through, not trapped - just attenuated.
Memory Aid
"Frequencies below LUF = attenuated before reaching destination. Think 'B'elow 'L'UF = 'B'elow 'L'owest 'U'sable = 'B'adly attenuated. Frequencies below LUF are too weak due to absorption. Don't work for long-distance propagation."
Real-World Application
LUF for your path is 7 MHz. You try to operate at 5 MHz (below LUF). The signal is heavily absorbed by the D-layer and attenuated before reaching the destination. The signal may be refracted, but it's too weak to be usable. This is why frequencies below LUF don't work for long-distance propagation.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Frequencies below LUF aren't refracted back effectively - they're too weak due to absorption. Refraction may occur but signals are too weak.
Option B: Incorrect. Frequencies below LUF don't pass through - they're absorbed, not passing through. Absorption is the issue.
Option D: Incorrect. Frequencies below LUF aren't trapped in the ionosphere - they're absorbed. Trapping isn't the mechanism.
题目解析
The correct answer is C: They are attenuated before reaching the destination. What usually happens to radio waves with frequencies below the LUF is that they are attenuated before reaching the destination. LUF (Lowest Usable Frequency) is the minimum frequency that can propagate - frequencies below LUF are too weak due to absorption. For amateur radio operators, this explains why very low frequencies don't work for long-distance propagation. Understanding this helps explain LUF.
考试技巧
Frequencies below LUF = attenuated before reaching destination. Think 'B'elow 'L'UF = 'B'elow 'L'owest 'U'sable = 'B'adly attenuated. Frequencies below LUF are too weak due to absorption - attenuated before reaching destination. Not refracted effectively, not pass through, not trapped - just attenuated.
记忆口诀
Frequencies below LUF = attenuated before reaching destination. Think 'B'elow 'L'UF = 'B'elow 'L'owest 'U'sable = 'B'adly attenuated. Frequencies below LUF are too weak due to absorption. Don't work for long-distance propagation.
实际应用示例
LUF for your path is 7 MHz. You try to operate at 5 MHz (below LUF). The signal is heavily absorbed by the D-layer and attenuated before reaching the destination. The signal may be refracted, but it's too weak to be usable. This is why frequencies below LUF don't work for long-distance propagation.
错误选项分析
Option A: Incorrect. Frequencies below LUF aren't refracted back effectively - they're too weak due to absorption. Refraction may occur but signals are too weak. Option B: Incorrect. Frequencies below LUF don't pass through - they're absorbed, not passing through. Absorption is the issue. Option D: Incorrect. Frequencies below LUF aren't trapped in the ionosphere - they're absorbed. Trapping isn't the mechanism.
知识点
LUF, Lowest Usable Frequency, Attenuation, Below LUF
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.