What does the term “critical angle” mean, as applied to radio wave propagation?
The correct answer is D: The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions. What the term 'critical angle' means, as applied to radio wave propagation is the highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions. Critical angle is the maximum angle at which a signal can be launched and still be refracted back to Earth. For amateur radio operators, this helps explain antenna elevation requirements. Understanding this helps when planning antenna systems.
Exam Tip
Critical angle = highest takeoff angle that returns to Earth. Think 'C'ritical 'A'ngle = 'C'utoff 'A'ngle (highest that works). Highest takeoff angle that will return a radio wave to Earth under specific conditions. Not azimuth, not lowest - just highest takeoff angle.
Memory Aid
"Critical angle = highest takeoff angle that returns to Earth. Think 'C'ritical 'A'ngle = 'C'utoff 'A'ngle. Highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions. Determines maximum antenna elevation."
Real-World Application
Under current ionospheric conditions, the critical angle is 30 degrees. This means signals launched at angles up to 30 degrees will be refracted back to Earth. Signals launched at higher angles (e.g., 45 degrees) will pass through the ionosphere. Critical angle determines the maximum usable elevation for your antenna.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Critical angle isn't about long-path azimuth - that's a different concept. Critical angle is about takeoff angle, not azimuth direction.
Option B: Incorrect. Critical angle isn't about short-path azimuth - that's a different concept. Critical angle is about takeoff angle, not azimuth direction.
Option C: Incorrect. Critical angle isn't the lowest takeoff angle - it's the highest takeoff angle that will return to Earth. Lower angles also work, but critical angle is the maximum.
题目解析
The correct answer is D: The highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions. What the term 'critical angle' means, as applied to radio wave propagation is the highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions. Critical angle is the maximum angle at which a signal can be launched and still be refracted back to Earth. For amateur radio operators, this helps explain antenna elevation requirements. Understanding this helps when planning antenna systems.
考试技巧
Critical angle = highest takeoff angle that returns to Earth. Think 'C'ritical 'A'ngle = 'C'utoff 'A'ngle (highest that works). Highest takeoff angle that will return a radio wave to Earth under specific conditions. Not azimuth, not lowest - just highest takeoff angle.
记忆口诀
Critical angle = highest takeoff angle that returns to Earth. Think 'C'ritical 'A'ngle = 'C'utoff 'A'ngle. Highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions. Determines maximum antenna elevation.
实际应用示例
Under current ionospheric conditions, the critical angle is 30 degrees. This means signals launched at angles up to 30 degrees will be refracted back to Earth. Signals launched at higher angles (e.g., 45 degrees) will pass through the ionosphere. Critical angle determines the maximum usable elevation for your antenna.
错误选项分析
Option A: Incorrect. Critical angle isn't about long-path azimuth - that's a different concept. Critical angle is about takeoff angle, not azimuth direction. Option B: Incorrect. Critical angle isn't about short-path azimuth - that's a different concept. Critical angle is about takeoff angle, not azimuth direction. Option C: Incorrect. Critical angle isn't the lowest takeoff angle - it's the highest takeoff angle that will return to Earth. Lower angles also work, but critical angle is the maximum.
知识点
Critical angle, Highest takeoff angle, Return to Earth, Antenna elevation
Verified Content
Question from official FCC General Class question pool. Explanation reviewed by licensed amateur radio operators.