What is meant by the term “critical frequency” at a given incidence angle?
The correct answer is A: The highest frequency which is refracted back to Earth. What is meant by the term 'critical frequency' at a given incidence angle is the highest frequency which is refracted back to Earth. Critical frequency is the maximum frequency that can be refracted (reflected) by an ionospheric layer at vertical incidence. For amateur radio operators, critical frequency helps determine layer characteristics. Understanding this helps explain ionospheric physics.
Exam Tip
Critical frequency = highest frequency refracted back to Earth. Think 'C'ritical 'F'requency = 'C'utoff 'F'requency (highest that refracts). Highest frequency that can be refracted back to Earth at vertical incidence. Not lowest, not SNR - just highest refracted frequency.
Memory Aid
"Critical frequency = highest frequency refracted back to Earth. Think 'C'ritical 'F'requency = 'C'utoff 'F'requency. Highest frequency that can be refracted back to Earth at vertical incidence. Indicates maximum layer ionization."
Real-World Application
The F2 layer has a critical frequency of 8 MHz. This means 8 MHz is the highest frequency that can be refracted back to Earth at vertical incidence. Frequencies above 8 MHz would pass through the layer. Critical frequency indicates the maximum ionization of the layer.
Key Concepts
Why Other Options Are Wrong
Option B: Incorrect. Critical frequency isn't the lowest frequency - it's the highest frequency that can be refracted. Lowest usable frequency is LUF, not critical frequency.
Option C: Incorrect. Critical frequency isn't about signal-to-noise ratio approaching unity - that's not what it measures. It's about frequency that can be refracted.
Option D: Incorrect. Critical frequency isn't about 6 dB signal-to-noise ratio - that's not what it measures. It's about the highest frequency that can be refracted.
题目解析
The correct answer is A: The highest frequency which is refracted back to Earth. What is meant by the term 'critical frequency' at a given incidence angle is the highest frequency which is refracted back to Earth. Critical frequency is the maximum frequency that can be refracted (reflected) by an ionospheric layer at vertical incidence. For amateur radio operators, critical frequency helps determine layer characteristics. Understanding this helps explain ionospheric physics.
考试技巧
Critical frequency = highest frequency refracted back to Earth. Think 'C'ritical 'F'requency = 'C'utoff 'F'requency (highest that refracts). Highest frequency that can be refracted back to Earth at vertical incidence. Not lowest, not SNR - just highest refracted frequency.
记忆口诀
Critical frequency = highest frequency refracted back to Earth. Think 'C'ritical 'F'requency = 'C'utoff 'F'requency. Highest frequency that can be refracted back to Earth at vertical incidence. Indicates maximum layer ionization.
实际应用示例
The F2 layer has a critical frequency of 8 MHz. This means 8 MHz is the highest frequency that can be refracted back to Earth at vertical incidence. Frequencies above 8 MHz would pass through the layer. Critical frequency indicates the maximum ionization of the layer.
错误选项分析
Option B: Incorrect. Critical frequency isn't the lowest frequency - it's the highest frequency that can be refracted. Lowest usable frequency is LUF, not critical frequency. Option C: Incorrect. Critical frequency isn't about signal-to-noise ratio approaching unity - that's not what it measures. It's about frequency that can be refracted. Option D: Incorrect. Critical frequency isn't about 6 dB signal-to-noise ratio - that's not what it measures. It's about the highest frequency that can be refracted.
知识点
Critical frequency, Highest frequency, Refracted back, Vertical incidence
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.